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 by 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 remineralization with uptake rates up to 1.2 ± 1.93 fmol N h-1 cell -1, but were also indicative for fixation of di-nitrogen.