Vitamin B₁₂ and cobalt cycling among diatoms and bacteria in Antarctic sea ice microbial communities

Within McMurdo Sound's annual sea ice, assimilation and concentrations of vitamin B₁₂ (cobalamin), microbial community productivity, and biomass were examined among three 100-m² quadrats where light penetration was manipulated by snow cover during austral summer. From late October through December, B₁₂ concentrations (6-32 pmol L ^-1) and assimilation rates (17-780 pmol m^-2 d ^-1) in congelation ice covaried with primary productivity (0.0001-250 mmol C m^-2 d^-1) and chlorophyll a (0.6-36 mg m ^-2). Within ice core samples, incorporation of ⁵⁷Co-B ₁₂ into the >1.0-μm size fraction (mostly diatoms) was almost 100 times faster than into 0.2-1.0-μm particles (mostly heterotrophic bacteria) and was dependent on light and active transport across cell membranes. Microalgal B₁₂ :C cell quotas in field communities varied widely (2.6-150,000 nmol B₁₂ (mol C)^-1; x̄ = 500) and generally exceeded those of cultured temperate diatoms (0.4-55 nmol B ₁₂ (mol C)^-1; x̄ = 4.1). Comparisons of dissolved B₁₂ pools in the ice and their turnover (0.02-0.6 d^-1) with underlying seawater suggest that this vitamin is produced in situ rather than delivered from waters below. Production and uptake of B₁₂ and uptake of cobalt, required for B₁₂ synthesis, were then examined among bacteria isolated from these communities. Only 23% of 78 bacterial isolates were incapable of B₁₂ uptake, but these clones assimilated dissolved cobalt. Intracellular B₁₂ production was evident in 9 of the 11 isolates screened and their cell quotas varied widely, 0.6-6,800 nmol B₁₂ (mol C)^-1. Mass balance analyses and published kinetics data independently suggest that microalgal growth in sea ice was not limited by vitamin B₁₂ in most of our field observations and that in situ bacterial B₁₂ production could potentially meet microalgal demands. Similar analyses, however, suggest that cobalt supply from underlying waters may have limited community growth and B₁₂ production.