New record of Trichodesmium thiebautii Gomont ex Gomont ( Oscillatoriales-Cyanophyta ) for the continental shelf of northeastern Brazil

This study reports the first occurrence of Trichodesmiumthiebautii Gomont ex Gomont on the coast of northeastern Brazil. Samples were gathered during the rainy season (July 2005) and the dry season (November 2006) at six stations distributed along two perpendicular profiles of the coast of Pernambuco (8o18'S - 34o56'W and 8o32'S - 35o00'W). T. thiebautii was found during both periods of the year and at the six collection stations, forming colonies in the form of clusters or more rarely, bundles, with straight cylindrical trichomes, quadratic or slightly longer cells, with widths between 5-10 µm and heights of 3-15 µm. The hydrological variables presented small variations, contributing to wide distribution of the species.


Introduction
The genus Trichodesmium was described by Ehrenberg (1830) and later included by Kützing (1843) in the genus Oscillatoria (Vaucher ex Gomont).Montagne (1844) reestablished the genus Trichodesmium Ehrenberg, based on the formation of its typical and very different colonies of isolated Oscillatoria trichomes, with the genus being defi nitively accepted as a valid taxonomic entity (Komárek & Anagnostides 2005).The genus was recently confi rmed using molecular methods (Orcutt et al. 2002).
These cyanophytes may occur as simple fi laments, but they are normally found forming aggregate colonies with between 50 and 300 fi laments/colony.They are characterized by their straight and slightly curved trichomes without sheaths, constricted or not, 6-22 μm in width, and straight, rounded or slightly capitate cell (Komárek & Anagnostides 2005).
Based on morphological studies, Komárek & Anagnostides (2005) recognized 11 species of Trichodesmium, nine of which are from marine environments and only two species have been reported as freshwater.The marine species are widely distributed in tropical and subtropical seas where they play an important ecological role not only as primary producers, but also due to their ability to fi x atmospheric nitrogen and form massive growths, often with toxic characteristics (Capone et al. 1997).
The fi rst time the genus Trichodesmium was mentioned in Brazilian waters was by Darwin (1834) during his expedition along the coast of Bahia, when he described the growth of T. erythraeum Ehrenberg.Years later, the same species was cited along the coast of Pernambuco, with the description of growth in areas near reef formations, and with the presence of T. erythraeum associated with the human disease called "Tamandaré Fever" (Sato et al. 1963).
Currently in Brazil, the genus is represented by the species T. erythraeum, T. hildebrandtii Gomont and T. thiebautii, of which T. hildebrandtii and T. erythraeum have been considered the most frequent and abundant.The occurrence of T. thiebautii was only reported along the southern coast of Brazil, where there are growths with toxic connotations as seen in bioassays with mollusks (Rörig et al. 1998;Gianesella-Galvão et al. 1995).
The aim of this study is to point out the occurrence and describe the morphological characteristics of T. thiebautii in northeastern Brazil, based on records from continental shelf waters in the state of Pernambuco.

Material and methods
Description of the area -the coast of the state of Pernambuco is one of the least extensive in Brazil, only 187 km long, stretching from 7 o 32' -8 o 56' S to 34 o 49' -35 o 11' W. The shelf is characterized by its reduced width, ranging from 35 to 42 km, low depth, with a break in the shelf at around 50 to 60 m, a mild drop of around 1.5 m/km, almost entirely covered with carbonate sediments, warm waters with an average temperature of 27 o C and high salinity, between 35 and 37. Low ammonia, nitrite and phosphorus values are observed in both periods, the opposite of nitrate, which varies from 0.02 μmol.L -1 at the surface to 1.36 μmol.L -1 at a depth of 30 m (Macedo et al. 2004).
The area is located in a region of hot, humid climate, type As', according to the Köppen classifi cation.Rainfall ranges from 1800 mm to 2364 mm per year, characterizing two annual periods: a rainy season from March to August, with monthly rainfall over 100 mm, and a dry season from September to February, with rainfall less than 100 mm.The continental fl ow does not extend more than 4 miles from the coast and it is more effi cient during the rainy season.Areas further away from nutrient supplies depend on coastal currents (Macedo et al. 2004).
The results underwent analysis of variance (ANOVA) to detect signifi cant differences between the biological variables analyzed.The Student t test was applied to locate differences between collection points and rainy and dry seasons.

Results
Trichodesmium thiebautii Gomont ex Gomont was found at the six collection stations and during both periods of the year, showing that it is a common species along the coast of Pernambuco.It had homocyte trichomes, forming cluster-shaped, and rarely twisted bundle shaped colonies.Cylindrical, straight trichomes, without constriction, on average 48 per colony, were more abundant in colonies collected at Serrambi (Station 5, dry season), where colonies with up to 100 trichomes were observed.Trichome length ranged from 110 to 2,110 μm, being longer during the dry season.Quadratic or slightly longer cells, with widths between 5-10 μm and heights between 3 -15 μm (Fig. 1).
Significant differences (<0.001, highly significant) between the height and width of cells and the size of trichomes collected at different collection points were revealed.These differences were not significant when trichomes from the same colony were considered.Highly signifi cant differences were also detected when comparing trichomes between the two annual periods.

Discussion
In northeastern Brazil, the genus Trichodesmium was only represented by T. erythraeum, which has been found with dense growths that are at times harmful to the environment (Sato et al. 1963), and thus the occurrence of T. thiebautii represents an expansion of the genus into northeastern Brazil.The two species are easily separated by their morphological characteristics.While T. erythraeum has a constricted trichome on its outer wall, and a visibly capitate apical cell, in T. thiebautii the trichome has no constriction and the apical cell is rounded or slightly capitate.Furthermore, bundle-shaped colonies in T. erythraeum have trichomes arranged parallelly, whereas in T. thiebautii these colonies are slightly twisted.
The morphometric characteristics of the T. thiebautii specimens found on the Pernambuco shelf confi rm Janson et al. (1995) hypothesis, showing that trichome morphology can vary from colony to colony, but the trichomes within a colony are very uniform in size and in morphology, suggesting that specimens in the same colonies are clonal.
Trichodesmium thiebautii is characterized by high productivity and capacity to assimilate atmospheric nitrogen, which is why it is one of the main producers in oligotrophic seas.Its presence and ecological importance have been confi rmed in other tropical regions, and it is considered responsible for the increase in planktonic biomass (Gallon et al. 1996).The species' presence in the oligotrophic waters of northeastern Brazil confi rms its preference for high-temperature sites.Small morphological variations in T. thiebautii colonies found in diverse tropical environments have been related to annual environmental conditions, characterized by small variations in water temperature.According to Lugomela et al. (2002), although the species is found year round, its occurrence along the coast of Tanzania is mainly related to the rainy season, when there is a greater concentration of nutrients.According to Carpenter et al. (2004), in the tropical North Atlantic Ocean, T. thiebautii is the most common species of Trichodesmium and populations have trichomes with approximately 100 cells, and average diameters of 10 μm.In the Bay of Aqaba (Red Sea), Post et al. (2002) indicated the presence of T. thiebautii throughout the year, including growth on the surface of the water.In this bay, 45 trichomes were found per colony.
These data indicate that some environmental variables contribute more than others to the morphological variation of these specimens.Lugomela et al. (2002) reported morphological variation in species of Trichodesmium linked to high nutrient concentrations.In the areas currently under analysis, differences are more signifi cant in cell size and number of trichomes in different colonies.There have been more robust fi laments at sites farther from the coast, regardless of the two periods of the year, denoting that in the tropical waters of northeastern Brazil, Trichodesmium thiebautii specimens are better adapted to sites with less movement and lower nutrient salt content.