LAUSR

In situ dissolved B12 concentration in marine ecosystems is controlled by the balance between rates of release of B12 by prokaryotes, uptake by prokaryotes and eukaryotes, and abiotic degradation. We used chemostats at a range of specific growth rates (μ, d−1; 0.1 to 1) with natural communities of prokaryotes and monospecific cultures of a B12 producer, Dinoroseobacter shibae. We measured the dissolved B12 concentration produced in the culture (B12-d), the B12 in the particulate fraction (B12-p), cell concentration, respiration rate, particulate organic carbon and nitrogen (POC, PON), and the 16S amplicon composition. Total dissolved B12 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 μ 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 B12 cell quota (900 to 3300 molecules cell−1). In multispecies and D. shibae cultures, B12 production rates per cell in creased with respiration rates (volumetric or per cell), and with rates of cellular organic carbon and nitrogen production. Rates increased with μ, but not the concentrations of B12-d or of B12-p. To understand the physiological and ecological dynamics of B12, concentrations alone are insufficient since they do not provide rates, which are important in understanding the dynamics between producers and consumers.