Picophytoplankton of Andine deep ultraoligotrophic lakes: a comparison with the oligo- mesotrophic subalpine Lake Maggiore.

CALLIERI, C.; MODENUTTI, BE; CARAVATI, E.; CORNO, G.; ESTEBAN GABRIEL BALSEIRO, ESTEBAN G. BALSEIRO, BALSEIRO, E.G; BERTONI, R.

PAlermo, Italia

Congreso; SEFS-5; 2007

Picophytoplankton (PicoPhy: 0.2-2 µm) has been recognized as an important component within the pelagic communities in both, freshwater and marine environments. Compared  with larger-sized  phytoplankton, PicoPhy do best in resource-poor habitats. Importance of PicoPhy was observed to relatively rise with decreasing phosphorus concentrations. In the Andean-Patagonian region (around 41ºS), many lakes are ultraoligotrophic and could be considered, therefore, as suitable environment for PicoPhy success. In these lakes total dissolved phosphorus (TDP) concentration ranges between 1.46-2.27 µg L-1. They are deep (90 - 460 m max depth range), very clear (kd PAR 0.09 ? 0.16 m-1) with a pronounced deep chlorophyll maximum (DCM) which forms below the thermocline and is caused by communities of mixotrophic ciliates, nanoflagellates, dinoflagellates and PicoPhy.  Lake Maggiore is an oligo- mesotrophic subalpine lake (Northern Italy, around 45°N); it is deep (372 m) and clear (kd PAR 0.22 m-1). Lake Maggiore has been selected as a comparative site in the study for the PicoPhy of Andine lakes.  PicoPhy in Andine lakes is mainly composed by very small picocyanobacteria (rods 0.96x0.58µm, cocci 0.78x0.64µm) but in some of these lakes also elongated forms are present (11x 0.99µm). In Lake Nahuel Huapi  small red fluorescing cells (probably eukaryotic) were observed but not yet classified. Picocyanobacteria from Lake Maggiore (rods 1.57x0.91µm, cocci 1.24x1.08µm) are mainly phycoerytrin-rich cells and a minority of undetermined eukaryots.  During summer stratification the range of daily PicoPhy production was 45-95 mgC m-2 d-1 in Andine lakes versus 34-266 mgC m-2 d-1  in Lake Maggiore.  In this presentation we will discuss a comparative study of PicoPhy ecological characteristics: distribution along the water column, photosynthetic efficiency, P/E curves, phosphorus limitation. Furthermore we applied molecular techniques in order to assess the biodiversity of picocyanobacterial communities from Andine lakes and Lake Maggiore environmental samples of different depths. DNA fragments by denaturing gradient gel electrophoresis (DGGE bands, identifying for operational taxonomic units, OTU) after polymerase chain reaction (PCR) amplification of the 16S rRNA gene were used. Richness and relative composition of PicoPhy communities was compared with a cluster analysis of recognized OTUs.