Eigemann, F. | Search bioRxiv

bioRxiv2019

Distinctive tasks of different cyanobacteria and associated bacteria in carbon as well as nitrogen fixation and cycling in a late stage Baltic Sea bloom

Cyanobacteria and associated heterotrophic bacteria hold key roles in carbon as well as nitrogen fixation and cycling in the Baltic Sea due to massive cyanobacterial blooms each summer. The species specific activities of different cyanobacterial species as well as the N- and C-exchange of associated heterotrophic bacteria in these processes, however, are widely unknown. Within one time series experiment we tested the cycling in a natural, late stage cyanobacterial bloom by adding 13C bi-carbonate and 15N2, and performed sampling after 10 min, 30 min, 1 h, 6 h and 24 h in order to determine the fixing species as well as the fate of the fixed carbon and nitrogen in the associations. Uptake of 15N and 13C isotopes by the most abundant cyanobacterial species as well as the most abundant associated heterotrophic bacterial groups was then analysed with a NanoSIMS. Overall, the filamentous, heterocystous species Dolichospermum sp., Nodularia sp., and Aphanizomenon sp. revealed no or erratic uptake of carbon and nitrogen, indicating mostly inactive cells. In contrary, non-heterocystous Pseudanabaena sp. dominated the nitrogen and carbon fixation, with uptake rates up to 1.49 {+/-} 0.47 nmol N h-1 l-1 and 2.55 {+/-} 0.91 nmol C h-1 l-1. Associated heterotrophic bacteria dominated the subsequent nitrogen cycling with uptake rates up to 1.2 {+/-} 1.93 fmol N h-1 cell -1, but were also indicative for fixation of di-nitrogen.

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↗