Phytoplankton diversity and co-dependency in a stratified oligotrophic ecosystem in the Mediterranean Sea

The southern Adriatic Sea is an oligotrophic ecosystem with thermal stratification in summer affecting nutrient supply to phytoplankton and bacterioplankton communities. Diversity and co-dependency of phytoplankton and bacterioplankton were studied during a period of water column stratification (12th to 21st July 2021) at Lastovo island in the open waters of southern Adriatic combining a set of interdisciplinary methods (sensors and CTD casts, water-column chemistry and biology, i.e. microscopy, flow-cytometry and high-throughput sequencing (HTS)). A total of 95 taxa were observed, of which: 58 diatoms, 27 dinoflagellates, 6 coccolithophores, and 4 other autotrophs which included chlorophyceae, chrysophyceae and cryptophytes. The prokaryotic plankton community as revealed by HTS was dominated by Proteobacteria (41-73%), Bacteroidota (9.5-27%) and Cyanobacteria (1-10%), while the eukaryotic plankton community was composed of parasitic Syndiniales (45- 80%), Ochrophyta (2-18%), Ciliophora (2-21%), Chlorophytes (2-4%), Haptophytes (1-4%), Bacillariophyta (1-13%), Pelagophyta (0.5-12%) and Chrysophyta (0.5-3%). Differences between fractions (pico-, nano- and micro-) were most prominent between the pico- and nano-fractions versus micro-fraction that was generally less diverse. Microscopy revealed nanophytoplankton and microphytoplankton were dominated by dinoflagellates, and diatoms, respectively, while the most abundant groups belong to nano-scale dinoflagellates, coccolithophorids and cryptophytes. Dominant diatoms were Pseudo-nitzschia delicatissima, Proboscia alata, Rhizosolenia imbricata, and Thalassionema frauenfeldii, also indicators of nutrient-enriched areas. Cyanobacteria favored different niches- Prochlorococcus was more abundant in deep chlorophyl maximum (DCM), and Synechococcus dominated surface and thermocline layers. Heterotrophic bacteria were most active in surface and thermocline layers. Nutrients and temperature were the main environmental factors determining the phytoplankton and bacterioplankton community abundance, and extreme nutrient values at the beginning of the investigating period indicated a possible nutrient flux. Surface, thermocline, and DCM layers were distinct considering community diversity, temperature and nutrient correlations.