Marine communities of bacteria and protists, their biodiversity and interactions

There are more than 1029 bacteria on our planet which represent the most abundant living form and that act as key players within biogeochemical cycles and in the ecosystem functioning. The photo-/autotrophic prokaryotes are one of the major primary producers in the oceans fueling pelagic food webs especially in oligotrophic conditions, while the heterotrophic fraction represents the principal consumers of the dissolved organic matter (that represent 14 to 20 times the amount of terrestrial organic carbon) contributing to the remineralization of nutrients. Inside the microbial loop, biomass of prokaryotes is continuously recycled via viral lysis while grazing of heterotrophic nanoflagellates and small ciliates channel it towards upper trophic levels. The relevance of prokaryotic biomass as principal source of carbon supporting pelagic food webs in the Mediterranean Sea have been demonstrated among different trophic conditions. The ingestion of prokaryotes by grazers was assessed according with the dilution technique that allows to determine whenever prey communities are efficiently controlled by predators. Bacterivory resulted to be the major pathway for carbon flux in oligo-, meso- and eutrophic conditions although even in eutrophicated situations, when herbivory prevailed, the carbon flux generated by ingestion on prokaryotes was still relevant. Also at the meso- bathypelagic layers the prokaryotes’ consumption still represented an efficient pathway for carbon transfer. Prokaryote communities have been demonstrated to change in abundance and composition over time due to modification of environmental conditions (such as temperature, salinity, resource availability) - bottom-up control - and to mortality mediated by viruses and/or grazers. Predation activity – top-down control – was often identified as the main cause of prokaryotic loss. Some experimental evidences support that feeding process tends to be more selective in regard to more active cells although selection might depend on the composition of grazers’ community. Bacterivores are mainly protists and different taxa display different feeding strategies (e.g. filter-feeding, sedimentation, interception feeding, raptorial, hosmotroph). Several protist communities have been analyzed from water samples taken during the XXIX Italian expedition in Antarctica - Ross Sea; the analysis of their biodiversity was based on molecular approach though Next Generation Sequencing (NGS) technique and samples were sequenced with Ion Torrent PGM platform. The results highlighted as protist communities were shaped accordingly with the history of the water masses: young-newly formed high salinity shelf waters (HSSW) presented protist communities characterized by high relative abundance of autotrophic organisms - which typically bloom at the surface - down at several hundred meters of depths. On the contrary, older circumpolar deep waters (CDW) shown higher abundance of taxa as Radiolaria, Discoba and Diplonema (the latter two belong to the Supergroup of Excavata, typically bacterivore). The same NGS approach was adopted in the experimental design to test the consequences of Aurelia aurita ephyrae appearance in a pelagic community of the Gulf of Trieste. Both protist and prokaryote community compositions were analyzed and the study provides a new insight in jellyfish-microbes interactions; it underlines how jellyfish tend to selectively prey on more motile groups of protists while they impact on prokaryote community and favors the blooming of copiotrophic taxa.