engleski

Picoplankton in the South Adriatic Sea: glimpse of winter diversity in oligotrophic ecosystem

Marine picoplankton includes organisms with cell sizes up to 3 µm, dominant in marine ecosystems in both abundance and biomass, embedded in three different domains - Archaea, Bacteria, Eukarya (protists and fungi) and viruses. The diversity of South Adriatic Sea picoplankton has received limited attention, although it is crucial as the base of the food web (Cerino et al. 2012). During the winter of 2016, apparent oxygen utilization indicated that respiration resulting from heterotrophic bacteria was main process in the southern Adriatic Sea (Babić et al., submitted). This study investigates this smallest plankton fraction in both prokaryotic and eukaryotic perspective considering within the euphotic layer (from surface up to 200m of depth) from coastal to open sea during mixed winter conditions. The samples were collected at total of 8 different depths at three stations in the South Adriatic Sea, during the late winter BIOTA (BIO-Tracing Adriatic water masses) cruise conducted in 2016. Using high-throughput sequencing of 16S and 18S rRNA genes along with flow-cytometry counts of bacteria, cyanobacteria and photosynthetic picoeukaryotes, we managed to define picoplanktonic community in the oligotrophic ecosystem. Prokaryotic community was dominated by Alphaproteobacteria (>40%), SAR11 clade (90%), followed by Marinimicrobia (18%), mainly represented by clade SAR406 (8%). Picoeukaryotes, although important primary producers in oligotrophic systems, in this study were dominated in general by heterotrophic counterparts: Syndiniophyceae, parasitic dinoflagellates (79%), Dinophyceae (5%), and distincitvely on two depths Collodaria (24%), Spumellaria (4%) and Chaunocanthida (2%) were significant in number. Total heterotrophic bacteria cell numbers ranged from 1.8×105 to 8.8×105 cells mL-1, while Cyanobacteria (Synechococcus + Prochlorococcus) cell numbers were higher only in surface layers (0-50 m) of offshore stations where they did not exceed 5.0×104 cells mL-1. Photosynthetic picoeukaryotes were most abundant at offshore stations with 9.0×103 cells mL-1 (Babić et al., submitted).

picoplankton, high throughput sequencing, Adriatic Sea

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