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B12 production by marine microbial communities and Dinoroseobacter shibae continuous cultures under different growth and respiration rates

Josué Villegas-Mendoza*, Ramón Cajal-Medrano, Helmut Maske

*Corresponding author:

ABSTRACT: In situ dissolved B12 concentration is controlled by the balance of 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. The total dissolved B12 concentrations (0.92 to 4.90 pmol l-1) were comparable to those found in the surface ocean. B12-p showed 6 to 35 times higher concentrations than B12-d. B12-d and B12-p or the community composition showed no relation with µ of 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 per cell). In multi-species and D. shibae cultures the B12 production rates per cell increased with respiration rates, volumetric or per cell, and with the 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, we suggest that production rates have to be known as knowledge of concentrations alone does not provide rates that are important to understand the dynamics between producers and consumers.