Vitamin B12 Production by Marine Bacteria in Organic Substrate Limited, Slow Growth Conditions

The conditions and processes governing the B₁₂ vitamin dissemination through planktonic organisms are little understood. It is generally assumed that bacteria produce B₁₂ vitamin and the whole auxotrophic plankton community consumes it. We used natural marine bacteria communities and marine bacteria Dinoroseobacter shibae cultures, growing in substrate-limited continuous cultures at low specific growth rates [0.1 to 1 d^-1] to measure intracellular and dissolved B₁₂ production, bacterial and viral abundance, particulate organic carbon, and nitrogen, bacterial production, oxygen consumption, carbon dioxide production, ETS activity, and taxonomic composition. We find dissolved B₁₂ vitamin at concentrations between 0 to 1.4 pM with no relation to growth or respiration rates. The intracellular B₁₂ vitamin normalized to cell volume ranged between 1x10^-2 to 4.6x10^-2 pmol μm³ showing a significant relationship with growth rate [y=0.02(m)^1.07; r²=0.78; p≤0.05; y=intracellular B₁₂ production, pmol μm³ day^-1; m=specific growth rate, day^-1], and respiration rates [y=2.4ln(x)-2.66; r²=0.87; p≤0.05; x=CO₂ production, μM day^-1]. The vitamin B₁₂ producing bacteria D. shibae, showed a dissolved B₁₂ concentration between 0 and 1.8 pM, whereas intracellular B₁₂ normalized to cell volume varied between 1.1x10^-2 to 1.8x10^-2 pmol μm^-3, responding significantly to growth rate [y=0.01(m)^0.56; r²=0.85; p≤0.05], and to respiration rates [y=3.01ln(x)-7.56, r²=0.97, p≤0.05; x=CO₂ production, μM day^-1]. The lack of correlation of dissolved B₁₂ vitamin with the metabolic activity suggests that the dissolved B₁₂ concentration depends on the interactions among vitamin B₁₂ producers and consumers while the bacterial metabolism is regulating the intracellular production of B₁₂ vitamin.