Abstracts
For many decades, polymorphism and its consequences have only been studied from the taxonomic point of view. Presently, interest has switched to the environmental causes of morphological variation and its consequences in the form and essence of the species. This study aimed at evaluating desmids morphological modifications of Staurastrum rotula Nordstedt during inter-annual succession patterns in two warm monomitic tropical lakes: Dom Helvécio (19°45′- 19°48′45″S, 42°33′45″W) and Carioca (19°45′20″S, 42°37′12″W). The effect of thermal stability and light and nutrients availability was based on samples collected monthly from January 2002 to December 2006 compared the morphological modifications. Results indicated that morphological variation, asexual reproduction, theratological forms, mucilaginous envelope and fungal infection were highest in Lake Dom Helvécio and coincided with the biomass increase of species with complex morphology between September and March (stratification period). The Zmix oscillation, wind and rainfall occurring at the end of the mixing period and beginning of the stratification were suggested as autochthonous and allochthonous disturbance agents, respectively, identified as inducers of asexual reproduction and consequently of the morphological variation. It was suggested that incidence of parasitism may act as a potential controlling agent for the Staurastrum rotula population size. It was concluded that morphological variation represents accidents in the original form, i.e. in the desmid species essence, promoting the existence of ecoforms, not of new infraspecific taxa.
desmid; intermediate disturbance; morphology; temperature
Por muitas décadas, o polimorfismo e suas conseqüências foram estudados apenas do ponto de vista taxonômico. Atualmente, o interesse se volta para as causas ambientais das variações morfológicas e suas consequencias na forma e essência da espécie. O estudo teve como objetivo avaliar as modificações morfológicas de Staurastrum rotula Nordstedt durante padrões inter-anuais de sucessão em dois lagos tropicais monomíticos-quentes: Dom Helvécio (19°45′-19°48′45″S, 42°33′45″W) e Carioca (19°45′20°″S, 42°37′12″W). O efeito da estabilidade térmica e potenciais distúrbios sobre as modificações morfológicas foi baseado em amostras coletadas mensalmente de janeiro de 2002 a dezembro de 2006. Os resultados indicaram que a variação morfológica, a reprodução assexuada, as formas teratológicas, envelope mucilaginoso e infecção fúngica foram maiores no Lago Dom Helvécio e coincidiu com o aumento da biomassa de espécies com morfologia complexa entre setembro e março (período de estratificação). A oscilação Zmix, vento e chuva ocorrentes entre o final do período de mistura e início da estratificação foram sugeridos como agentes perturbadores autóctones e alóctones, respectivamente, identificados como indutores de reprodução assexuada e, consequentemente, de variação morfológica. Foi sugerido que a incidência de parasitismo atua no controle do tamanho da população Staurastrum rotula. A variação morfológica, portanto, representa acidentes na forma original, ou seja, na essência das desmídias, promovendo a existência de ecoformas, não de novos táxons infraespecíficos.
desmídias; distúrbio intermediário; morfologia; temperature
1. Introduction
Considered an important tool for aquatic ecosystems conservation and
management processes (Coesel et al.,
1978COESEL, PFM., KWAKKESTEIN, R. and VERSCHOOR, A., 1978.
Oligotrophication and eutrophication tendencies in some Dutch moorland pools, as
reflected in their desmid flora. Hydrobiologia, vol. 61, no. 1, p. 21-31.
http://dx.doi.org/10.1007/BF00019021.
http://dx.doi.org/10.1007/BF00019021...
; Coesel, 2001COESEL, PFM., 2001. A method for quantifying conservation value in
lentic freshwater habitats using desmids as indicator organisms. Biodiversity
and Conservation, vol. 10, no. 2, p. 177-187.
http://dx.doi.org/10.1023/A:1008985018197.
http://dx.doi.org/10.1023/A:100898501819...
), desmids
have cosmopolitan distribution, enormous environmental plasticity (Brook, 1981BROOK, AJ., 1981. The biology of desmids. Oxford: Blackwell
Scientific Publications. 276 p.) and a high degree of
polymorphism (Bicudo and Sormus,
1972BICUDO, CEM. and SORMUS, L., 1972. Polymorphism in the desmid
Micrasterias laticeps and its taxonomical implications. Journal of Phycology,
vol. 8, no. 3, p. 237-242.
http://dx.doi.org/10.1111/j.0022-3646.1972.00237.x.
http://dx.doi.org/10.1111/j.0022-3646.19...
). Playfair (1908)PLAYFAIR, GI., 1908. Some Sidney desmids. Proceedings of the Linnean
Society of New South Wales, vol. 33, no. 3, p. 603-628.
had already mentioned that desmids are polymorphic and that 90-95% of all at
that time described species would be nothing but morphological expressions of the
remaining 5-10%. According to Bicudo
(1988)BICUDO, CEM., 1988. Polymorphism in the desmid Cosmarium abbreviatum
var. minus (Zygnemaphyceae) and its taxonomical implications. Acta
Botanica Brasílica, vol. 2, no. 1-2, p. 1-6.
http://dx.doi.org/10.1590/S0102-33061988000100001.
http://dx.doi.org/10.1590/S0102-33061988...
, Playfair's
(1908)PLAYFAIR, GI., 1908. Some Sidney desmids. Proceedings of the Linnean
Society of New South Wales, vol. 33, no. 3, p. 603-628. numbers are obviously exaggerated, but the idea of a
considerable percentage of all described desmid species be nothing but morphological
expressions is absolutely valid.
The most important factor towards a correct taxonomical identification of
algae is the careful detailed knowledge of its morphological variability, much more
when desmids are concerned since the species concept in the group is presently so
very bad defined (Kouwets, 2008KOUWETS, F., 2008. The species concept in desmids: the problem of
variability, infraspecific taxa and the monothetic species definition. Biologia,
vol. 63, no. 6, p. 881-883.
http://dx.doi.org/10.2478/s11756-008-0135-7.
http://dx.doi.org/10.2478/s11756-008-013...
).
‘Species’ identification clearly depends on its external cell
morphology and, somewhat occasionally, also on the plastid characteristics and the
zygospore morphology (Brook,
1981BROOK, AJ., 1981. The biology of desmids. Oxford: Blackwell
Scientific Publications. 276 p.).
Description of any species is not inert. On the contrary, it changes
always a new variety is described (Kouwets,
2008KOUWETS, F., 2008. The species concept in desmids: the problem of
variability, infraspecific taxa and the monothetic species definition. Biologia,
vol. 63, no. 6, p. 881-883.
http://dx.doi.org/10.2478/s11756-008-0135-7.
http://dx.doi.org/10.2478/s11756-008-013...
). The question is, however, up to what extent morphological
variation induced by environmental changes allows identification of new species,
varieties or taxonomic forms? For Aristotle, ‘essence’ includes, on
the one hand, everything that characterises the individual creature substratum, i.e.
everything that the individual creature is by itself and that defines its identity.
‘Accident’ includes, on the other hand, every characteristic that is
added to the individual creature, but that despite being part of its constitution
does not define it, being simply considered a secondary aspect of its existence. It
may or not exist, independent of the individual creature be what it really is.
Consequently, a species shall be defined just and only by characteristics that are
inherent to their ‘essence’. Morphological characteristics subjected
to constant modification need to be carefully studied and identified, and
morphological ‘accidents’ shall not be used for the proposition of new
species and infraspecific taxa in desmids.
Morphological and physiological modifications of planktonic algae are
nothing but adaptation mechanisms for its life in suspension (Reynolds, 1984REYNOLDS, CS., 1984. The ecology of freshwater phytoplankton.
Cambridge: Cambridge University Press. 365 p.), protection against
herbivory (Wiltshire et al., 2003WILTSHIRE, K., BOERSMA, M. and MEYER, B., 2003. Grazer-indiced
changes in desmid Staurastrum. Hydrobiologia, vol. 491, no. 1-3, p. 255-260.
http://dx.doi.org/10.1023/A:1024474827107.
http://dx.doi.org/10.1023/A:102447482710...
)
or response to environment contrasts (Morales
et al., 2002MORALES, EA., TRAINOR, FR. and SCHLICHTING, CD., 2002. Evolutionary
and ecological implications of plastic responses of algae. Constancea, vol. 83,
no. 4, p. 1-24.
http://ucjeps.berkeley.edu/constancea/83/morales_etal/plasticity.html
http://ucjeps.berkeley.edu/constancea/83...
). Desmid's morphological complexity is induced by
evolution processes (Kouwets,
2008KOUWETS, F., 2008. The species concept in desmids: the problem of
variability, infraspecific taxa and the monothetic species definition. Biologia,
vol. 63, no. 6, p. 881-883.
http://dx.doi.org/10.2478/s11756-008-0135-7.
http://dx.doi.org/10.2478/s11756-008-013...
). Furthermore, it was observed that changes in growth conditions
may result in greater or lesser morphological differences, even between mother and
daughter semicells, that will misrepresent the original cell symmetry (Kouwets, 2008KOUWETS, F., 2008. The species concept in desmids: the problem of
variability, infraspecific taxa and the monothetic species definition. Biologia,
vol. 63, no. 6, p. 881-883.
http://dx.doi.org/10.2478/s11756-008-0135-7.
http://dx.doi.org/10.2478/s11756-008-013...
) resulting in great
polymorphism, even if a single species is considered. Consequently, the life history
at population level must be constructed by observing the transitional forms and the
relationship among the morphological expressions (Playfair, 1910PLAYFAIR, GI., 1910. Polymorphism and life history in the
Desmidiaceae. Proceedings of the Linnean Society of New South Wales, vol. 35,
no. 2, p. 459-495.).
Present study aimed at testing the hypothesis that desmids present morphological variation dependent on the environment abiotic conditions mainly associated to temperature, and that such structural alterations constitute, with high frequency, ‘accidents’ and do not identify new species, varieties or taxonomic forms, but simply phenotypic modifications or ecoforms.
2. Material and Methods
2.1 Study area
The Rio Doce State Park is located about in the centre of the Rio Doce
Lake District, southeast Brazil (19°29′S;
42°28′W) (De Meis and
Tundisi, 1997DE MEIS, MRM. and TUNDISI, JG., 1997. Geomorphological and
limnological process as a basis for lake tipology. The middle Rio Doce lake
system. In TUNDISI, JG. and SAIJO, Y. Limnological studies in the Rio Doce
Valley lakes. São Carlos: Academia Brasileira de Ciências. p.
25-50.). Located in a humid tropical forest area, 20 m
below the river Doce level (De Meis and
Tundisi, 1997DE MEIS, MRM. and TUNDISI, JG., 1997. Geomorphological and
limnological process as a basis for lake tipology. The middle Rio Doce lake
system. In TUNDISI, JG. and SAIJO, Y. Limnological studies in the Rio Doce
Valley lakes. São Carlos: Academia Brasileira de Ciências. p.
25-50.), the ecology of the two lakes is closely related
to the geomorphological processes that gave origin to them (De Meis and Tundisi, 1986DE MEIS, MRM. and TUNDISI, JG., 1986. Geomorphological and
limnological process as a basis for lake tipology: the middle Rio Doce lake
system. Anais da Academia Brasileira de Ciências, vol. 58, no. 1. p.
103-120.). A dominant
form among the Park lakes is the dendritic (identification by aerial
photograph), the circular form of Lake Carioca being less common
(Tundisi and Mussarra,
1986TUNDISI, JG. and MUSSARRA, ML., 1986. Morphometry of four lakes in
the Rio Doce Valley System and its relationship with primary production of
phytoplankton. Revista Brasileira de Biologia =. Brazilian Journal of
Biology, vol. 46, no. 1, p. 159-171.). Among the lakes and ponds of the Park lacustrine system, Lake
Carioca is a shallow, more or less round-shaped, rich in nutrients system
(Barbosa and Tundisi,
1980BARBOSA, FAR. and TUNDISI, JG., 1980. Primary production of
phytoplankton and environmental characteristics of a shallow Quaternary lake at
Eastern Brazil. Archiv fuer Hydrobiologie, vol. 90, no. 2, p.
139-161.), with a surface area of 0.14 km2 and maximum depth
of 11.8 m. Lake Dom Helvécio is, in contrast, a system that is deep,
oligotrophic, dendritic in shape, with a surface area of 5.27 km2 and
maximum depth of 39.2 m (Bezerra-Neto and
Pinto-Coelho, 2008BEZERRA-NETO, JF. and PINTO-COELHO, RM., 2008. Morphometric study of
Lake Dom Helvécio, Parque Estadual do Rio Doce (PERD), Minas
Gerais, Brazil: a re-evaluation. Acta Limnologica Brasiliensia, vol. 28, no. 2,
p. 161-167.). Both are monomitic, warm, stratified
systems between September and April, and basically isothermal from May to August
(Henry and Barbosa,
1989HENRY, R. and BARBOSA, FAR., 1989. Thermal structure, heat content
and stability of two lakes in the National Park of Rio Doce Valley (Minas
Gerais, Brazil). Hydrobiologia, vol. 171, no. 3, p. 189-199.
http://dx.doi.org/10.1007/BF00008142.
http://dx.doi.org/10.1007/BF00008142...
).
2.2 Laboratory analysis
Water temperature, dissolved oxygen, electric conductivity and pH
profiles were measured ‘in situ’ with a
multiparameter probe (Horiba sensor model U-22) at regular 0.50 m
intervals. The mixing zone (Zmix) was identified according
to Reynolds (1984)REYNOLDS, CS., 1984. The ecology of freshwater phytoplankton.
Cambridge: Cambridge University Press. 365 p.. The
euphotic zone (Zeu):mixing zone (Zmix)
ratio was used as an index for light availability in the mixing zone (Jensen et al., 1994JENSEN, P., JEPPESEN, E., OLRIK, K. and KRISTENSEN, P., 1994. Impact
of nutrients and physical factors on the shift from cyanobacterial to
chlorophyte dominance in shallow Danish lakes. Canadian Journal of Fisheries and
Aquatic Sciences, vol. 51, no. 8, p. 1692-1699.
http://dx.doi.org/10.1139/f94-170.
http://dx.doi.org/10.1139/f94-170...
). Stability
(S) was calculated using Idso's
(1973)IDSO, S., 1973. On the concept of lake stability. Limnology and
Oceanography, vol. 18, no. 4, p. 681-683.
http://dx.doi.org/10.4319/lo.1973.18.4.0681.
http://dx.doi.org/10.4319/lo.1973.18.4.0...
equation. The mixing zone
(Zmix) was calculated on the basis of the water temperature
differences, considering the layer above in which the temperature gradient first
exceeded 0.2 °C (Coche,
1974COCHE, AG., 1974. Limnological study of a tropical reservoir. In
BALON, EK. and COCHE, AG. Lake Kariba: a man-made tropical ecosystem in central
Africa. The Hague: Dr. W. Junk Publishers. p. 7-247.).
Desmids were collected both at the littoral and limnetic region of the Dom Helvécio and Carioca lakes. Monthly samples were gathered during a five-year period (2002-2006), with emphasis on the months of February, May, July and September. These months were chosen according to the thermal regimen of the two lakes, to include stratification (February), intermediate between stratification and mixing (May and September) and mixing (July) periods. Samples were immediately fixed after collection and preserved in 3-4% formalin water solution (Bicudo, 1990BICUDO, DC., 1990. Considerações sobre metodologias de contagem de algas do perifíton. Acta Limnologica Brasiliensia, vol. 3, p. 459-475.).
Due to the high incidence of polymorphism, taxonomic identification of algae was based on population sample analysis, i.e. on the examination of about 100 individual specimens of each species, variety or taxonomic form in the community or on as many specimens was needed to not find any different morphological expression in the preparation. Eventual differences in the shape of individual specimens identified (shape of semicell, proportion between length and maximum breadth linear cell dimensions, and angular processes length) were considered during the taxonomic identification process. In brief, all diacritic, metric and meristic morphological characteristics of the vegetative part of life history were considered.
Among the characteristics studied, cell and semicell shape, cell maximum length and semicell breadth (with and without angular processes), angular processes length, cell wall facial decoration and texture deserved special attention. Observation of asexual reproduction (cell division), morphological expressions, presence of mucilaginous envelope and parasitism by fungi was made from a 10 preparations average of each sampling unit, which was examined under a Zeiss (Axioskop 2) regular microscope, with a camera lucida and digital metric ocular adapted to the optical system of microscope. Frequency of occurrence of ecoforms followed the criterion below: Constant (CT): ecoform present in at least 80% of preparations examined; Common (CM): ecoform present in 50-80% of preparations examined, and Rare (R): ecoform present in less than 50% of preparations examined. The size was evaluated from the values of the maximum linear dimension (MLD): Class 1 (< 10 mm), class 2 (between 11 and 20 mm), class 3: between 21 and 50 mm, and class 4: > 50 mm.
Analysis of environmental factors of the two lakes, Zmix
and Zmix / Zeu ratio led to selection of descriptors,
i.e. of some robust variables that would indicate changes
and fluctuations in the morphology of the algae over the five years series of
the present study (Naselli-Flores and
Barone, 2000NASELLI-FLORES, L. and BARONE, R., 2000. Phytoplankton dynamics and
structure: a comparative analysis in natural and man-made water bodies of
different trophic state. Hydrobiologia, vol. 438, no. 1/3, p. 65-74.
http://dx.doi.org/10.1023/A:1004109912119.
http://dx.doi.org/10.1023/A:100410991211...
; Naselli-Flores and
Barone, 2007NASELLI-FLORES, L. and BARONE, R., 2007. Pluriannual morphological
variability of phytoplankton in a highly productive Mediterranean reservoir
(Lake Arancio, Southwestern Sicily). Hydrobiologia, vol. 578, no. 1,
p. 87-95.
http://dx.doi.org/10.1007/s10750-006-0436-1.
http://dx.doi.org/10.1007/s10750-006-043...
).
Desmids were studied according to their morphological complexity and divided into simple (forms without angular processes or spines: Cosmarium and Euastrum), complex (forms with angular processes or spines: Staurastrum and Staurodesmus) and cylindrical attenuate forms (elongate and/or filamentous forms: Closterium and Pleurotaenium). In the case of the simple forms, both unicelular (Cosmarium) and isodiametric fragments morphologically similar to Cosmarium were considered. The origin of such fragments is associated to the dissociation of filaments such as those of Teilingia and Spondylosium.
3. Results
3.1. Physical and chemical variables: spatial segregation and recurrent seasonal differences
Winds measured for the area were classified from breeze (< 11 m s–1) to tempest (maximum values 27.5 m s–1) that occurred towards the end of the mixing period and during the stratification one (September-April). Wind speed measured at the surface of systems varied between 0.3 and 8.5 m s–1 at Lake Carioca and between 0.2 and 8.9 m s–1 at Lake Dom Helvécio (minimum and maximum values, respectively). The greatest precipitation values (mm) were measured between November and March, identifying a rainy period, the greatest values always documented in December, except for 2003 when it was measured in November (219 mm) (Table 1). Epilimnion extension or Zmix varied during the stratification period from 1.5 a 6.5 m at Lake Carioca and from 5 to 12 m at Dom Helvécio, and progressively increased during the transition rainy-dry period. During the mixing period (May-September), an isothermal episode was identified for both lakes. Thermocline varied between 4 and 14 m deep at Lake Dom Helvécio and 2 and 8 m at the Carioca.
Climatic features provided by the Meteorological Station of the Instituto Mineiro de Gestão das Águas (10 km from the Rio Doce State Park), for the period 2002-2006.
Dissolved oxygen (DO) patterns had its temporal distribution dynamics affected by the thermal regimen in both lakes. The first phase corresponded to the thermal stratification (January-April and October-December), with DO super saturation being observed at the epilimnion (> 100%) and sub saturation (< 50%) below the thermocline. Hypolimnetic anoxia also occurred, a characteristic of the period. The second phase corresponded to the mixing period (May-September), when an orthograd DO profile was identified.
Greatest water transparency values (variation range 2-4 m) and those of least light attenuation (variation range 0.56-0.68) were measured at Lake Dom Helvécio. Lake Carioca presented, in turn, the least water transparency values (variation range 0.5-2.5 m) and the greatest light attenuation ones (variation range 0.85-3.4). Regarding the optical structure, two distinct phases were observed associated to the thermal regimen of the two lakes: (1) a phase with greater light availability in January-April and September-December and (2) another one with less water transparency and consequent numerical reduction of Zeu / Zmix in May-August. Carioca and Dom Helvécio lakes showed the same variation range for their Zeu / Zmix ratio, i.e. 0.9 < Zeu / Zmix < 1.8.
The trophic status of the two lakes, with maximum values were detected during the period May-December, which were situated at the border line between mesotrophic and eutrophic (44 < IET < 54) for both lakes.
The mechanisms governing chemical dynamics and thermal stability of
the reservoir in the studied period were analysed in detail in Barbosa et al. (2012a)BARBOSA, LG., BARBOSA. and BICUDO, CEM., 2012a. Inter-annual
chemical stratification in Brazilian natural lakes: meromixis and hypolimnetic
memory. Acta Limnologica Brasiliensia, vol. 24, no. 2, p. 127-139.
http://dx.doi.org/10.1590/S2179-975X2012005000032.
http://dx.doi.org/10.1590/S2179-975X2012...
.
3.2. Biological characteristics: mucilaginous sheath, fungi infestation and asexual reproduction
Presence of mucilaginous sheath was registered (Figure 7), however, rarely and with a less fibrous texture than that of the simple forms (Cosmarium). Despite its presence during the entire study period, the greatest number of species with mucilaginous sheath was registered between September and December. At Lake Carioca, however, the presence of species with sheath was rare and restricted to Cosmarium specimens.
Staurastrum rotula Nordstedt. 1. Morphotype 1; a. typical form; b. lateral view with 8 processes; c. form undergoing cell division. 2. Morphotype 2; a. typical form; b. lateral view with 6 processes; c. form in the final stage of cell division; d. form with 8 processes. 3. Morphotype 3; a. typical form; b. lateral view with 5 processes; c. form undergoing cell division. Bar = 20 mm.
Staurastrum rotula Nordstedt, different morphological expressions from Lake Dom Helvécio.
Staurastrum rotula was very much subjected to infestation by chitridiacean fungi (Chytridiomycetes), the development of epidemic conditions (> 90%) identified during the months of maximum development of the species at Lake Dom Helvécio, between September and February (Figures 4-8). Maximum epidemics was recurrently registered in September. Specimens with greater MLD presented greater infestation by fungi incidence than those with lesser MLD (< 40 mm). At Lake Carioca, there was no register of epidemic infestation and the incidence of infection by fungi was much lower.
The greatest occurrence of asexual reproduction (cell division) was observed during the September-April period, with the maximum detected in September at Lake Dom Helvécio (Figure 5). Great asexual reproduction rates induced formation of aberrant or theratological forms in great proportions (Figures 8, 16, 17, 22). Acceleration of the cell division process led to not yet totally morphologically developed new semicells to start a new division, giving origin to forms that deviate to what is more or less an extension from the original pattern, which are presently named aberrant.
3.3. Morphological variation
Cell length 21.5-41.5 mm, cell breadth 25-50.8 mm, isthmus breadth 9-9.2 mm; processes length 10.9-28.9 mm, processes breadth 3-6.6 mm.
At Lake Dom Helvécio (Figures 4-15), the great majority of S. rotula populations presented a wide morphological variation, in the number of angular processes, shape of semicells, shape of the semicell apical margin and length of angular processes and that of the semicell apical view being the most relevant one. Distinct morphotypes were observed for some of these presented semicells with 8 (seldom 7) long, horizontally disposed processes (morphotype 1, Figures 1a-c); some others presented semicells with 6 short, convergent processes (morphotype 2, Figures 2a-d); still others presented semicells with 5 short, slightly convergent processes (morphotype 3, Figures 3a-c); and finally others presented semicells with 6 long, horizontally disposed processes (morphotype 4, Figures 25, 26). Differences in the length of processes were observed even in one single individual specimen.
All four morphotypes were documented throughout the entire study period, especially in September. During all other months, however, morphotype 2 prevailed, all other ones occurring just sporadically, classified as constant (CT). Forms with 5-8 angles in the apical view were recorded, the rarest one being the 7-angled. Protuberances, undulations and differences in the process length were registered for all morphotypes. It was also observed that some morphotypes showed some overlapping. Separation was mainly based on the number and size of angular processes. The first group included individual specimens with semicells with (7-)8 long processes together with others with 6 equally long processes (morphotypes 1 and 4). The second group included individual specimens with semicells with 6 comparatively shorter processes together with others with equally short 5 processes (morphotypes 2 and 3) (Figure 27).
Alometric relationships of Staurastrum rotula Nordstedt. (a) cell length x cell breadth, (b) cell length:cell breadth x cell breadth and (c) cell length x cell breadth and cell breadth. Highlighted,for morphotypes 1 and 2 populations.
At Lake Carioca (Figures 16-26), only two morphotypes were identified, with prevalence of that with 6 comparatively long processes (morphotype 4) (Figures 25, 26). The other morphotype showed 6 short processes (morphotype 2) (Figures 16-24), and both were considered rare (R).
Figure 28 summarises S. tetracerum (Kützing) Ralfs ex Ralfs var. tetracerum f. tetracerum, S. laeve Ralfs and Staurastrum rotula to environment characteristics and ecological ranges that allowed their coexistence in the Lake Dom Helvécio.
Habitat model summarising Staurastrum tetracerum (Kützing) Ralfs ex Ralfs var. tetracerum f. tetracerum and S. laeve Ralfs (dotted line) and Staurastrum rotula Nordstedt (continuous line) attributes during the second annual stratification (September-December; n = 80). Minimum values depart from the centre of graph. Habitat models of S. rotula ecoforms overlap. Legends: Temp = Temperature; Zeu = Euphotic zone; Zmix = Mixing zone; Free CO2; SRP = soluble reactive phosphorus; Forms of dissolved nitrogen: NH4 = Ammonium; NO3 = Nitrate.
4. Discussion
4.1. Mucilaginous sheath and fungi infestation
The occurrence of a great number of individual specimens with mucilaginous sheath occurred in September and December, but it is not possible to state that sheath production is an opportunistic event (Reynolds, 2006REYNOLDS, CS., 2006. Ecology of phytoplankton. Cambridge: Cambridge University Press. 535 p.). However, its function towards reducing cell density and propitiating a microclimate for cell division are hypothesis to be considered for both studied systems. The selective permeability of mucilaginous envelopes might provide a defense against the toxic cations uptake in the acidic environment tolerated by some desmids (Coesel, 1994COESEL, PFM., 1994. On the ecological significance of a cellular mucilaginous envelope in planktic desmids. Algological Studies, vol. 73, p. 65-74.).
A possible factor during the host selection by zoosporic fungi is the
cell size, but this possibility was never studied in detail (Kagami et al., 2007KAGAMI, M., BRUIN, A., IBELINGS, BW. and VAN DONK, E., 2007.
Parasitic chytrids: their effects on phytoplankton community and food-web
dynamics. Hydrobiologia, vol. 578, no. 1, p. 113-129.
http://dx.doi.org/10.1007/s10750-006-0438-z.
http://dx.doi.org/10.1007/s10750-006-043...
). However, a
positive correlation between the fungus cell size and that of the host was
already documented (Holfeld,
1998HOLFELD, H., 1998. Fungal infections of the phytoplankton:
seasonality, minimal host density, and specificity in a mesotrophic lake. The
New Phytologist, vol. 138, no. 3, p. 507-517.
http://dx.doi.org/10.1046/j.1469-8137.1998.00126.x.
http://dx.doi.org/10.1046/j.1469-8137.19...
). Staurastrum rotula is a highly sensitive
species to fungi infestation, and the latter always occur at population level
during periods of high algal density, as already registered for
Staurastrum longipes (Nordstedt) Teiling in
Shearwater, England (Sen,
1988SEN, B., 1988. Fungal parasitism of planktonic algae in Shearwater.
VI. Parasitic occurrence of a new chytrid species on the blue-green alga
Microcystis aeruginosa Kuetz emend. Elenkin. Archiv fuer Hydrobiologie, vol. 79,
sup. Suppl, p. 177-184.), with a subsequent decrease of the algal population density.
In the present study, infestation occurred in all four morphotypes, however,
much greater in representative specimens of morphotypes 1 and 4. Larger size
cells are more susceptible to parasites than the smaller size ones due to having
more resources to offer and favour parasite fecundity (Kagami et al., 2007KAGAMI, M., BRUIN, A., IBELINGS, BW. and VAN DONK, E., 2007.
Parasitic chytrids: their effects on phytoplankton community and food-web
dynamics. Hydrobiologia, vol. 578, no. 1, p. 113-129.
http://dx.doi.org/10.1007/s10750-006-0438-z.
http://dx.doi.org/10.1007/s10750-006-043...
). Furthermore, the
relationship between parasite and host is highly specific in desmids (Holfeld, 1998HOLFELD, H., 1998. Fungal infections of the phytoplankton:
seasonality, minimal host density, and specificity in a mesotrophic lake. The
New Phytologist, vol. 138, no. 3, p. 507-517.
http://dx.doi.org/10.1046/j.1469-8137.1998.00126.x.
http://dx.doi.org/10.1046/j.1469-8137.19...
; Holfeld, 2000HOLFELD, H., 2000. Relative abundance, rate of increase, and fungal
infections of freshwater phytoplankton. Journal of Plankton Research, vol. 22,
no. 5, p. 987-995.
http://dx.doi.org/10.1093/plankt/22.5.987.
http://dx.doi.org/10.1093/plankt/22.5.98...
).
Staurastrum rotula population infestation reached
approximately 90% of examined individual specimens in September, and was
recurrent during the entire study period. Effects of such infestation on the
phytoplankton population in natural lakes were collected by Kagami et al. (2007)KAGAMI, M., BRUIN, A., IBELINGS, BW. and VAN DONK, E., 2007.
Parasitic chytrids: their effects on phytoplankton community and food-web
dynamics. Hydrobiologia, vol. 578, no. 1, p. 113-129.
http://dx.doi.org/10.1007/s10750-006-0438-z.
http://dx.doi.org/10.1007/s10750-006-043...
. Decrease in
population size and density, replacement of dominant species by other ones
(Canter and Lund, 1951CANTER, HM. and LUND, JWG., 1951. Studies on plankton parasites, 3:
examples of the interaction between parasitism and other factors determining the
growth of diatoms. Annals of Botany, vol. 15, p. 359-371.) and
competition among species subdominant (Canter and Lund, 1969CANTER, HM. and LUND, JWG., 1969. Parasitism of planktonic desmids
by fungi. Österreichische Botanische Zeitschrift, vol. 116, p.
351-377.) are well known.
Reduction of S. rotula population was observed from
September on in terms of the specimen's density over the 5 years of present
study. The quantitative expression is restricted to biomass (mg
L–1), due to the high biovolume of its representative
specimens (8,532.2 mm3) (Barbosa et al., 2012bBARBOSA, LGB., BARBOSA, FAR. and BICUDO, CEM., 2012b. Adaptive
strategies of desmids in two tropical monomictic lakes in southeast Brazil: do
morphometric differences promote life strategies selection? Hydrobiologia, in
press.). In this regard, the hypothesis
of S. rotula population control by parasites is highly
feasible. Desmids infestation by fungi and its seasonal fluctuation may be
explained by the very same environment factors that favour development of the
alga, such as: temperature increase (> 25 °C), light and
nutrients availability increase and structural modifications in the
Zmix Simultaneous occurrence of environmental optima for
parasites and hosts was registered by Van Donk
and Ringelberg (1983)VAN DONK, E. and RINGELBERG, J., 1983. The effect of fungal
parasitism on the succession of diatoms in Lake Maarsseveen I (The
Netherlands). Freshwater Biology, vol. 13, no. 3, p. 241-251.
http://dx.doi.org/10.1111/j.1365-2427.1983.tb00674.x.
http://dx.doi.org/10.1111/j.1365-2427.19...
in population of diatoms, but never
of desmids.
4.2. Morphological variation in desmids: disturbance or a natural occurrence?
Phenotypic variability is, ‘a priori’,
a response to light and nutrient availability (Stoyneva et al., 2007STOYNEVA, MP., DESCY, JP. and VYVERMAN, W., 2007. Green algae in
Lake Tanganyika: Is morphological variation a response to seasonal changes?
Hydrobiologia, vol. 578, no. 1, p. 7-16.
http://dx.doi.org/10.1007/s10750-006-0428-1.
http://dx.doi.org/10.1007/s10750-006-042...
), as well as to herbivory and
system disturbance (Naselli-Flores and
Barone, 2007NASELLI-FLORES, L. and BARONE, R., 2007. Pluriannual morphological
variability of phytoplankton in a highly productive Mediterranean reservoir
(Lake Arancio, Southwestern Sicily). Hydrobiologia, vol. 578, no. 1,
p. 87-95.
http://dx.doi.org/10.1007/s10750-006-0436-1.
http://dx.doi.org/10.1007/s10750-006-043...
). Special emphasis shall be put to the thermal
regimen seasonal variation as the main determining factor for the distribution
of resources such as light and nutrients in the water column, the morphological
variation of algae being a reflex of the above mentioned variation (e.g.
Naselli-Flores and Barone, 2007NASELLI-FLORES, L. and BARONE, R., 2007. Pluriannual morphological
variability of phytoplankton in a highly productive Mediterranean reservoir
(Lake Arancio, Southwestern Sicily). Hydrobiologia, vol. 578, no. 1,
p. 87-95.
http://dx.doi.org/10.1007/s10750-006-0436-1.
http://dx.doi.org/10.1007/s10750-006-043...
)
and, at a micro scale, also of atelomixis as an intermediate disturbance
agent.
In December 2002 at Lake Dom Helvécio, a high number of
individual specimens were observed undergoing asexual reproduction during the
morning, soon after heavy rain during the dawn. Allochthonous disturbance agents
in the Lake Balaton (Hungary) indicated that sudden changes in the
phytoplankton community structure may be a result of tempests occurring between
five and 15 days after a drought period (Padisák et al., 1990PADISÁK, J., G.-TÓTH, L. and RAJCZY, M., 1990. Stir-up
effect of wind on a more-or-less stratified shallow lake phytoplankton
community, Lake Balaton, Hungary. Hydrobiologia, vol. 191, no. 1, p. 249-254.
http://dx.doi.org/10.1007/BF00026058.
http://dx.doi.org/10.1007/BF00026058...
). Jacobsen and Simonsen (1993)JACOBSEN, BA. and SIMONSEN, P., 1993. Disturbance events affecting
phytoplankton biomass, composition and species diversity in a shallow,
eutrophic, temperate lake. Hydrobiologia, vol. 249, no. 1-3, p. 9-14.
http://dx.doi.org/10.1007/BF00008838.
http://dx.doi.org/10.1007/BF00008838...
observed in a Danish lake
that the incidence of winds above >10 m s–1 and of
precipitations at the end of July increased the algae mortality rate, breaking
the lake stratification after a stagnation period. Thus, heavy rains occurrent
during thermal stratification can be considered disturbances, promoting breaking
of environmental stability and increased replication rate. In this condition,
the cells do not reach maturity before a new morphological reproductive cycle
favouring the morphological differentiation and generation of morphotypes in a
process seasonal.
Small scale alterations verified in the Lake Dom Helvécio thermal structure between the end of the dry period (August-September) and beginning of the stratification one may be interpreted as autochthonous disturbance agents (e.g. atelomixis, Zmix and structural modifications). Furthermore, there are allochthonous intermediate disturbance agents represented by the instant wind strokes > 10 m s–1, which influence the species life history. Consequently, there was a recurrent increase in the asexual reproduction rate as a strategy for the maintenance of the species. Acceleration of the cell division process led to not yet totally morphologically developed new semicells will start a new division, originating forms that deviate to a more or less extension from the original pattern, which are presently named aberrant.
The temperature and seasonal variation range plays an important role
in the production of morphological variation in algae. Brandham (1964)BRANDHAM, PE., 1964. Cytology, sexuality and mating type in culture
of certain desmids. London: University of London. Ph.D. thesis. investigated the effect of
temperature on Staurastrum polymorphum Brébisson and
concluded that the cell symmetry in apical view was intimately dependent on that
factor, so that under low temperature dominated the 4-radiate form and under
high temperature, the 3-radiate one. This identical fact was observed by Reynolds (1940)REYNOLDS, N., 1940. Seasonal variations in Staurastrum paradoxum.
The New Phytologist, vol. 39, no. 1, p. 86-89.
http://dx.doi.org/10.1111/j.1469-8137.1940.tb07124.x.
http://dx.doi.org/10.1111/j.1469-8137.19...
in natural
population of Staurastrum chaetoceras (West et G.S.
West) G.M. Sm., the 3-radiate forms dominating during the summer and the
2-radiate ones in the fall, winter and spring. The increasing temperature in Dom
Helvecio lake at the subsurface and the entire water column reached differences
between 1.6 and 2 °C in August and September.
Structural modifications in the cell morphology occur mainly as a
function of the algal adaptation to life in suspension. Brook (1959)BROOK, AJ., 1959. The status of desmids in the plankton and the
determinations of phytoplankton quotients. Journal of Ecology, vol. 47, no. 2,
p. 429-445.
http://dx.doi.org/10.2307/2257375.
http://dx.doi.org/10.2307/2257375...
registered a possible evolution
trend in some Staurastrum species characterised by the
elongation of the cell and angular processes and reduction in the cell
ornamentation parallel to an increase in the adaptation to flotation.
The subaquatic light regimen may represent an important trigging factor for alterations in desmids. Coesel and Kooijman-Van Blokland (1991)COESEL, PFM. and KOOIJMAN-VAN BLOKLAND, H., 1991. Seasonality of planktonic desmid species in Lake Maarsseveen (The Netherlands) related to experimentally determined growth rates in a temperature-light gradient. Verhandlungen - Internationale Vereinigung für Theoretische und Angewandte Limnologie, vol. 24, p. 763-767. registered in the field and unialgal culturing experiments [Closterium acutum (Kyngbye) Brébisson ex Ralfs var. variable (Lemmermann) Krieger, Closterium aciculare T. West, Staurastrum cingulum (West & West) G. M. Smith e Cosmarium abbreviatum Raciborski] the presence of an optimal development range under experimental conditions using high temperature (25-30 °C) and irradiance (8,22-85 mE m–2 s–1). Interspecific differences were also observed, however, with Closterium acutum presenting high growth rates (more than one duplication per day) under 30 °C temperature and 85 mE m–2 s–1 irradiance, whereas Closterium aciculare presented its optimum growth under comparatively lower temperature (25-28 °C) and high irradiance (22-85 mE m–2 s–1), with theratological forms observed under high temperature (30 °C) conditions.
Considering an evolution approach, it was repeatedly demonstrated
that natural selection favours establishment of plastic genotypes, because the
environment unpredictability somewhat tends to suppress phenotypes (Scheiner, 1998SCHEINER, SM., 1998. The genetics of phenotypic plasticity VII.
Evolution in a spatially-structured environmental. Journal of Evolutive Biology,
vol. 11, no. 3, p. 303-320.
http://dx.doi.org/10.1046/j.1420-9101.1998.11030303.x.
http://dx.doi.org/10.1046/j.1420-9101.19...
). In other words,
plasticity allows the genotype a capacity of exhibiting a great number of
phenotypes in order to face the environment variation (Fordyce, 2006FORDYCE, JA., 2006. The evolutionary consequences of ecological
interactions mediated through phenotypic plasticity. The Journal of Experimental
Biology, vol. 209, no. 12, p. 2377-2383.
http://dx.doi.org/10.1242/jeb.02271.
PMid:16731814
http://dx.doi.org/10.1242/jeb.02271...
). It must be stressed,
however, the importance of the environment variability produced by
stratification for the coexistence of species with distinct morphologies
(Naselli-Flores and Barone,
2007NASELLI-FLORES, L. and BARONE, R., 2007. Pluriannual morphological
variability of phytoplankton in a highly productive Mediterranean reservoir
(Lake Arancio, Southwestern Sicily). Hydrobiologia, vol. 578, no. 1,
p. 87-95.
http://dx.doi.org/10.1007/s10750-006-0436-1.
http://dx.doi.org/10.1007/s10750-006-043...
).
Finally, one must also stress after considering the Aristotelian definitions of ‘essence’ and ‘accident’ that morphological modifications represent, in fact, ‘accidents’ of the original form, since they are phenotypical variations of species with great phenotypic plasticity and, thus, are not enough to define nor subdivide them in new specific and infraspecific taxa. On the contrary, they must be evaluated and explained using the concept of ecoform or morphotype.
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Publication Dates
-
Publication in this collection
May 2014
History
-
Received
22 Nov 2012 -
Accepted
1 Mar 2013