B-12 Production By Marine Microbial Communities And Dinoroseobacter Shibae Continuous Cultures Under Different Growth And Respiration Rates

In situ dissolved B-12 concentration in marine ecosystems is controlled by the balance between rates of release of B-12 by prokaryotes, uptake by prokaryotes and eukaryotes, and abiotic degradation. We used chemostats at a range of specific growth rates (mu, d(-1); 0.1 to 1) with natural communities of prokaryotes and monospecific cultures of a B-12 producer, Dinoroseobacter shibae. We measured the dissolved B-12 concentration produced in the culture (B12-d), the B-12 in the particulate fraction (B12-p), cell concentration, respiration rate, particulate organic carbon and nitrogen (POC, PON), and the 16S amplicon composition. Total dissolved B-12 concentrations (0.92 to 4.90 pmol l(-1)) were comparable to those found in the surface ocean. B12-p concentration was 6 to 35 times higher than B12-d. B12-d, B12-p, and community composition showed no relation to p. for either natural populations or D. shibae. The chemostats allowed calculation of the rates of production: B12-d (0.34 +/- 0.28 pmol l(-1) d(-1)) and B12-p (5.65 +/- 2.34 pmol l(-1) d(-1)), and the B-12 cell quota (900 to 3300 molecules cell(-1)). In multispecies and D. shibae cultures, B-12 production rates per cell increased with respiration rates (volumetric or per cell), and with rates of cellular organic carbon and nitrogen production. Rates increased with II, but not the concentrations of B12-d or of B12-p. To understand the physiological and ecological dynamics of B-12, concentrations alone are insufficient since they do not provide rates, which are important in understanding the dynamics between producers and consumers.