CONICET | Buscador de Institutos y Recursos Humanos

Very small organisms are fundamental components of marine planktonic systems, often accounting for a significant fraction of the biomass and activity. Marine phytoplanktons are responsible for about half of the global carbon fixation. A significant proportion of this fixed carbon derives from the photosynthetic picoeukaryotes (PPEs), component < 3 μm in size. Compared with the prokaryotic component of the picophytoplankton, the PPEs community is highly diverse, comprising members from virtually every algal class. Yet despite the evident ecological significance of the PPEs, relatively little is known of their diversity in the marine environment. Their identity, has remained elusive which has been attributed mainly to difficulties in identification by light microscopy and/or in culturing them in the laboratory. Only recently, with the advent of molecular techniques picophytoplankton diversity has begun to be revealed. To date little is known about the picophytoplankton diversity in the South Atlantic Ocean. The aim of this work was to identify and molecular characterize the components of the picophytoplankton using both microscopic and molecular approaches. We determined the diversity of marine picoeukaryotes (PEs) by sequence analysis from 18S rDNA from four environmental genetic libraries, from samples collected from Argentinean Sea surface waters. Such studies, using universal 18S rDNA gene primers (which target both photo-autotrophic and heterotrophic organisms) have shown the presence of the PPEs affiliated with many different algal classes including the Prasinophyceae, Bacillariophyceae, Dictyochophyceae, Haptophyceae, Cryptophyceae, Dinophyceae, Pelagophyceae y Picobiliphyta. Additionally sequences were associated with heterotrophic forms for Alveolados, Stramenopilos, Cercozoa, Apuzoa and Katablepharidae. In spite of the highly diverse organisms detected at molecular level some groups of picoeukaryotes that were abundant by microscopic observations (Silva & Negri, personal communication) could not be identify in the genetic libraries with the 18S rDNA primer set. Significant differences in 18S rDNA gene copy number of some components of PPEs could be responsible of these differences between both analyses. Use of Chloroplastida (green algae) primers instead of universal primers has been a good approach for increase the occurrence of PPEs species with low 18S rDNA copy number in environmental samples. PCR biased to green linage with specific primers may be significant due to selective amplification of phototrophic over heterotrophic organisms.