Roth-Rosenberg, D. | Search bioRxiv

bioRxiv2020

Microbial communities in an ultra-oligotrophic sea are more affected by season than by distancefrom shore

Marine microbial communities vary seasonally and spatially, but these two factors are rarely addressed together. We studied temporal and spatial patterns of the microbial community structure and activity along a coast to offshore transect from the Israeli coast of the Eastern Mediterranean Sea (EMS) over six cruises, in three seasons of two consecutive years. The ultra-oligotrophic status of the South Eastern Mediterranean Sea was reflected in the microbial community composition that was dominated by oligotrophic microbial groups such as SAR11 throughout the year, even at the most coastal station sampled. Seasons affected microbial communities much more than distance from shore explaining about half of the observed variability in the microbial community, compared to only about 6% that was explained by station. However, the most coastal site differed significantly in community structure and activity from the three further offshore stations in early winter and summer, but not in spring. Our data on the microbial community composition and its seasonality from a transect into the South Eastern Levantine basin support the notion that the EMS behaves similar to open gyres rather than an inland sea.

ecology↗

bioRxiv2019

Prochlorococcus rely on microbial interactions rather than on chlorotic resting stages to survive long-term stress

Many microorganisms produce resting cells with very low metabolic activity that allow them to survive phases of prolonged nutrient or energy stress. In cyanobacteria and some eukaryotic phytoplankton, the production of resting stages is accompanied by a loss of photosynthetic pigments, a process termed chlorosis. Here, we show that a chlorosis-like process occurs under multiple stress conditions in axenic laboratory cultures of Prochlorococcus, the dominant phytoplankton linage in large regions of the oligotrophic ocean and a global key player in ocean biogeochemical cycles. In Prochlorococcus strain MIT9313, chlorotic cells show reduced metabolic activity, measured as C and N uptake by NanoSIMS. However, unlike many other cyanobacteria, chlorotic Prochlorococcus cells are not viable and do not re-grow under axenic conditions when transferred to new media. Nevertheless, co-cultures with a heterotrophic bacterium, Alteromonas macleodii HOT1A3, allowed Prochlorococcus to survive nutrient starvation for months. We propose that reliance on co-occurring heterotrophic bacteria, rather than the ability to survive extended starvation as resting cells, underlies the ecological success of Prochlorococcus. ImportanceThe ability of microorganisms to withstand long periods of nutrient starvation is key to their survival and success under highly fluctuating conditions as is common in nature. Therefore, one would expect this trait to be prevalent among organisms in the nutrient-poor open ocean. Here, we show that this is not the case for Prochlorococcus, a globally abundant and ecologically impactful marine cyanobacterium. Instead, Prochlorococcus rely on co-occurring heterotrophic bacteria to survive extended phases of nutrient and light starvation. Our results highlight the power of microbial interactions to drive major biogeochemical cycles in the ocean and elsewhere with consequences at the global scale.

microbiology↗