Adaptation of the picoplankton community to changes in environmental conditions

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Abstract
This two-year study investigated the spatial and temporal dynamics of picoplankton communities in the oligotrophic central Adriatic Sea, focusing on their responses and adaptation to environmental variability at two stations differing in trophic status, and identifying the key environmental factors shaping their community structure. The results indicated that chlorophyll a concentration, salinity, and nitrite concentration were the main environmental drivers of community picoplankton dynamics and structure, highlighting the combined influence of trophic status and hydrographic conditions. In addition, salinity was negatively correlated with chlorophyll a concentration. The picoplankton community responded to changes in these environmental conditions either through an increase in abundance, particularly under elevated chlorophyll a concentration, or a reduction in the relative contribution and abundance of most community members in response to increased salinity. In contrast, the cyanobacterial genus Prochlorococcus did not exhibit a decline in abundance with increasing salinity. Thus, the contrasting responses may reflect niche partitioning along the salinity gradient rather than direct physiological effects of salinity. The bacterial community structure was dominated by low nucleic acid bacteria, while an increase in bacterial biomass accompanied by reduced bacterial cell production was observed. In addition, a decline in heterotrophic nanoflagellates abundance may indicated reduced grazing pressure on bacterial populations. Overall, the findings suggest a shift toward more efficient, low-activity microbial communities under changing environmental conditions, with important implications for microbial-mediated carbon cycling in the Adriatic Sea.

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