Picoeukaryotic diversity in Argentinean Sea costal site studied by molecular approaches

A. CUMINO; COVACEVICH F; SILVA R. I; NEGRI R.; SALERNO G. L.

Pinamar

Encuentro; XLI Reunión Anual de la Sociedad Argentina de Investigación Bioquímica y de Biología Molecular; 2005

Sociedad Argentina de Investigación Bioquímica y de Biología Molecular

INTRODUCTION - Marine picoeukaryotes (cells < 3µm in size), are found throughout the marine photic zone worldwide at concentrations between 102 and 104 cells ml-1 and play fundamental roles in marine ecosystems. This assemblage is composed of chlorophyll containing cells (phytoplankton) and heterotrophic cells, that have a pivotal role in both primary production and microbial loop, respectively. It is well known the ecological importance of marine picoeukaryotes in the coastal plankton. The taxonomical identification of these tiny cells has been a difficult task due to their lack of morphological distinct feature. Recently, the introduction of molecular techniques in microbial ecology has increased our knowledge on this topic. Molecular techniques based on rRNA genes, such as denaturing gradient gel electrophoresis (DGGE), a fingerprinting technique, offer the best compromise between the number of samples processed and the information obtained, providing both rapid comparison data for many communities and specific phylogenetic information derived from excised bands.Our aim is to assess the temporal variation of the diversity of picoeukaryotes by DGGE at a coastal fixed station EPEA.SOURCE OF THE ORGANISMSPicoplankton were collected at fixed coastal Station EPEA (38º28´S-57º41´W at 20 km South of Mar del Plata, Argentine). Samples were collected monthly during one year (2003-2004) at 5 m of depth. The water samples were pre-filtered through a membrane of 5 µm pore size and then filtered through a membrane of 0.2 µm pore size and immediately frozen in liquid nitrogen. Water samples -May 2004- were brought to the laboratory for unialgal culture (dilution method), and the cells obtained were grown in 0.2-µm filtered F/2 medium prepared on the base of seawater. Cultures of the strain characterized in this study (EPEA-CC08/04-A) were performed under 16/8 day-night light cycles.TEMPORAL AND SEASONAL PICOEUKARYOTIC COMMUNITY.Water temperature showed a marked seasonality (winter -12,7ºC to 10,4ºC- andsummer -19,2ºC to 21,5ºC-) and the smaller phytoplankton fraction (chl a < 5µm) peaked from June to September (0,3 to 0,55 µg/L). A statistical comparison of the picoeukaryoticassemblages of all samples analyzed is shown as a dendrogram (Fig. 6). There was greater similarity between adjacent samples, suggesting replacement of phylotypes along thetemporal factor. This relationship was strongest for the October-November, February-March and July-August samples, though salinity and temperature distributions were similar during sampling periods (data not shown). The picoeukaryotic community exhibited a marked seasonal cycle, with minima of diversity in the summer and early autumn and maxima in midwinter. Direct relationship was found between the number of bands detected and the chlorophyll a in the smaller cellular fraction.