The composition and new records of micro-and mesophytoplankton near the Vitória-Trindade Seamount Chain

Abstract: In spite of the length of the Brazilian coastline, studies of marine phytoplankton species in offshore areas have been largely neglected. Regarding phytoplankton species composition, the aim was to assess biodiversity status, species frequency, and the similarities at different sites along the Vitoria-Trindade Seamount Chain. Phytoplankton net samples were analyzed. One hundred and seventy five infrageneric taxa were identified. They represented four phyla, viz., Cyanobacteria, Bacillariophyta, Dinophyta, and Ochrophyta. Dinophyta was the most diverse, and its genus Tripos the largest contributor. This genus also represented more than half of the most common taxa in the sampled area. Thirty-five new infrageneric taxa were recorded, 12 of which are new reports for Brazil. Neritic stations assemblages were markedly different from those of seamounts and deep offshore. The high species diversity, especially among dinoflagellates can be considered typical of tropical oligotrophic waters. The new records demonstrate the gap in knowledge of phytoplankton biodiversity in Brazil.


Introduction
In spite of the long extent of the Brazilian coastline, the study of phytoplankton in offshore areas is incipient, notwithstanding the important role of this marine community as the source of more than 45% of primary production worldwide (Field 1998).Regarding group diversity, a high range in phyla is found, since many organisms are not descended from the same lineage.The greatest species-diversity is found among diatoms (13.776 species) and dinoflagellates (3.281) (Guiry & Guiry 2015), although other groups such as haptophytes, prasinophytes and cyanobacteria, are also significant.According to Barton et al. (2010), phytoplankton composition is driven by a latitudinal gradient of species richness, whereby tropical and subtropical zones prove to be the richest.
The Vitória-Trindade Seamount Chain is located off the central coast of Brazil.Starting 175km off the coast of Espírito Santo State and extending for 950km eastward, the seamounts are disposed almost linearly at 20° and 21°S (Almeida 2006), and situated between the parallels 28° and 38°W.The chain emerged on the seafloor during the Cenozoic, simultaneously with the westward movement of the South America Plate (Almeida 2006).Trindade Island and Martin Vaz Archipelago are located at the eastern end, the farthest location from the coast (Almeida 2006, Motoki et al. 2012).The last 17 submarine banks rise higher than 2.5km from the seafloor, with more than half reaching the euphotic zone (Motoki et al. 2012).
The Brazilian current, originating from the South Equatorial Current at 10°S (Silveira et al. 2000), passes through the region transporting an oligotrophic, warm and highly saline water mass (Brandini et al. 1997, Gaeta et al. 1999).Due to low nutrient availability, these environmental conditions favor phytoplankton species that are mixotrophic, heterotrophic or diazotrophic.
The only two studies available of phytoplankton close to the Vitória-Trindade Seamount Chain, both as a part of the REVIZEE program (Tenenbaum et al. 2006(Tenenbaum et al. , 2007)), were published in Portuguese, thereby precluding access by the international scientific community.Other organisms from this region, such as cetaceans (Wedekin et al. 2014), fishes (Pinheiro et al. 2009(Pinheiro et al. , 2015)), barnacles (Young 1999), rhodolites (Pereira-Filho et al. 2012), etc, received more attention.
On considering the extreme importance of investigating the fundamental role of phytoplankton community in marine ecosystems, their high biodiversity in tropical waters, and the few studies currently available, the aims of this study were to assess: (i) biodiversity status, (ii) frequency of occurrence of each taxon, and (iii) station similarity, as regards taxonomic composition of micro-and mesophytoplankton communities, close to the Vitória-Trindade Seamount Chain.

Material and Methods
Samples were collected during November of the austral spring of 2003, aboard R.V. Antares of the Brazilian Navy.Collection was by vertical trawling up to 100m deep, using a specific net (20µm mesh size).After sample concentration, a formaldehyde solution was added (final concentration 2%).The samples were then incorporated into the Phytoplankton Collection of the Federal University of Espírito Santo.Vertical profiles of salinity, temperature, and dissolved oxygen were obtained with a CTD and oximeter coupled to a rosette.Sixteen sampling stations were arranged in two transects (Figure 1), the first comprising nine stations located over the seamount chain (19°S), and the second, seven, most of which close to individual seamounts (20°S).Distances between stations varied from 150 to 200km.One station was located at the southern end of the Abrolhos Bank (E26), another two on the continental slope (E23 and E27), and two more over the Vitória and Montague seamounts (E30 and E32, respectively).A further three were relatively close to the Jasur, Dogaressa and Columbia seamounts (E34, E36, and E38, respectively), and just one station close to Trindade Island (E40).The last seven were situated in deep offshore areas (E14, E15, E16, E17, E18, E19 and E20).The Geographical Information System (GIS) approach with Diva-Gis program v7.5 (http://www.diva-gis.org/),as well as a shape file from the Brazilian Institute of Geography and Statistics (IBGE) (http://mapas.ibge.gov.br/en/interativos/arquivos/downloads), were applied to assessing submarine bathymetry at the sampled sites.
Hierarchical clustering methods are useful for evaluating species-composition correlations between stations, thereby revealing biogeographical connections with subjacent processes (Kreft & Jetz 2010).The pvclust package v1.32 (Suzuki & Shimodaira 2006), available in R program (http://www.r-project.org/), was applied in the present case.The Ward agglomerative method, based on the binary distance of taxonomic composition among stations, was employed for inferring hierarchical clustering.Statistical confidence of dendrogram nodes was defined by approximately unbiased (AU) support values that are less biased than the traditional bootstrap (Suzuki & Shimodaira 2006).
Thirty five new records of micro-and mesophytoplankton infrageneric taxa were found in the Vitória-Trindade Seamount Chain and continental shelf of Espírito Santo State, this including 12 new reports for Brazil (Table 3 and Figure 4).At a depth of 100 meters, and among stations, there was no significant change in environmental variables.Salinity varied between 37.39 and 37.055 (average 37.25) and temperatures 25.55 and 24.01°C (average 24.7ºC), thereby showing the predominance of tropical water in surface layers.Average dissolved oxygen was 4.64mg/l, this varing from 4.95 to 4.32mg/l.Cluster analysis, by revealing differences in taxonomic composition among the various stations (Figure 5), facilitated their division into three main groups (Figure 1 and 5).The first, marked in green, and located closer to the continental shelf, were the most distant (71% AU value), the second, in red, were near the Vitória-Trindade Seamount Chain (81% AU), and the third, in blue, were in deep offshore areas (81% AU).

Discussion
The high infrageneric diversity among the micro and mesophytoplankton observed corroborates previous findings for tropical oligotrophic regions (Hallegraeff & Jeffrey 1984, Balech 1988).The high diversity of dinoflagellates collected in this area seems to be correlated with its preference for warm oligotrophic oceanic zones (Taylor et al 2007).Recently, the marine species of the genus Ceratium F.Schrank were renamed Tripos (Gómez et al. 2010, Gómez 2013) marine species of Ceratium (Dinophyceae, Alveolata.This presents, by far, the highest number of infrageneric taxa.Incidentally, it was the most common genus in the samples collected.Furthermore, besides being more specious in tropical regions (Dodge 1993, Dodge & Marshall 1994, Okolodkov & Dodge 1996, Tunin-Ley & Lemée 2013), it is usually the most diverse of the dinoflagellates (Taylor et al. 2007).
In an environment with low nutrient availability, nitrogen fixation is an important feature for ensuring fitness.Hence, the prevalence of the Cyanobacteria Trichodesmium can be understood.It is a diazotrophic organism that has been well documented both in the marine waters of Brazil (Satô et al. 1963, Brandini et al. 1997, Carvalho et al. 2008, Proença et al. 2009, Monteiro et al. 2010) and worldwide (Sellner 1997, Janson et al. 1999).
Several micro-and mesophytoplankton organisms accompany cyanobacteria nitrogen fixers.Richelia intracellularis J.Schmidt in Ostenfeld & Schmidt is a diazotrophic cyanobacteria found in symbiotic relationship with certain diatoms, such as Rhizosolenia Brightwell (Padmakumaret al. 2010), Hemiaulus Heiberg (Kimor et al. 1978, Villareal 1994) and Chaetoceros Ehrenberg (Gómez et al. 2005).Furthermore, some oceanic dinoflagellates abide in symbiosis with unicellular diazotrophic bacteria, commonly found in certain genera of the non-photosynthetic Dinophysiales order, such as Histioneis, Ornithocercus Stein and Amphisolenia Stein (Foster et al. 2006, Farnelid et al. 2010).This corroborates our results on the high diversity of the above cited genera in the region of the Vitória-Trindade Seamount Chain.
When considering the lack of research on phytoplankton assemblages in Brazilian offshore waters, novel reports on infrageneric taxa can be expected.Other propitious factors could be (i) the fluctuation of phytoplankton composition due to environmental changes, such as  seasonality, (ii) misidentification of species, and (iii) very low density of some species, thereby precluding their perception in previous studies.
There was little variation in environmental data.In temperature, although the highest range, the values varied between 1.5°C.Considering the variables obtained, the conditions in the study area could be considered stable.Therefore, it was impossible to associate the differences in phytoplankton composition with the data obtained.
Maybe, quantifying and qualifying nutrients could give a better indication for the findings.
Cluster analysis indicated how close the stations were in terms of taxonomic composition, and also that geographic distances between the stations were correlated with species composition: the closer the stations to one another, the more species they shared in common.The same association was found in a biogeographic study of Tripos (Dodge 1993).Furthermore, the grouping of stations into three different clusters could be justified by the high number of uncommon and rare taxa (101 taxa, 57.5%), thereby indicating that many species are not extensively disposed.It was noteworthy that the three clusters seemed to be correlated to continental closeness and seafloor bathymetry.This is in accordance with previous findings, whereby neritic species assemblage differs from the oceanic (Dodge 1993, Raine et al. 2002, Taylor et al. 2007), and seamounts can influence species composition and density (Genin & Boehlert 1985, Pitcher et al. 2007).
The high micro-and mesophytoplankton species diversity is apparently related to tropical oligotrophic oceanic zones.Several of the organisms identified were either mixotrophic, or maintained a symbiotic association with diazotrophic bacteria.The results are important in defining micro-and mesophytoplankton diversity, especially among the dinoflagellates.The numerous new infrageneric taxa reported for the marine waters off Espírito Santo State, and Brazil as a whole, indicate the importance of inventory surveys, and the lack of studies of phytoplankton assemblages.Bathymetric dissemblance in the area studied could be associated with differences in species composition.Neritic stations formed a distinct cluster from the oceanic and seamount.Furthermore, geographic distances between stations possibly exert an influence on species distribution.Further studies of phytoplankton should be extended to the area, especially those of other phytoplankton size-fractions, such as pico-and nanoplankton.The application of other methodologies, such as cultures, metabarcoding and metagenomics, would be useful for enhance the knowledge about the community.

Figure 1 .
Figure 1.Map of the study area and sampling station sites.The colors of the stations are in accordance with Figure 5. Green stations are neritic, red, those closer to the Vitória-Trindade Seamount Chain, and bluethose located over deep offshore oceanic waters.The grey lines are isobaths (100-2000 deep meters). www.scielo.br/bnhttp://dx.doi.org/10.1590/1676-0611-BN-2016-0164 occurence for Brazil; * New occurence for the state of Espírito Santo.Continued Table 1.

Figure 2 .
Figure 2. Genera biodiversity in the proximity of the Vitória-Trindade Seamount Chain.Four different phyla were identified.The number of species found for each genus is represented by bars.

Table 3 .
New infrageneric taxa reports.New records for Brazil.(*) New records for waters of the Espírito Santo continental shelf and near the Vitória-Trindade seamount chain.