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 B-12 (cobalamin), microbial community productivity, and biomass were examined among three 100-m(2) quadrats where light penetration was manipulated by snow cover during austral summer. From late October through December, B-12 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 Co-57-B-12 into the >= 1.0-mu m size fraction (mostly diatoms) was almost 100 times faster than into 0.2-1.0-mu m particles (mostly heterotrophic bacteria) and was dependent on light and active transport across cell membranes. Microalgal B-12 : C cell quotas in field communities varied widely (2.6-150,000 nmol B-12 (mol C)(-1); (x) over bar = 500) and generally exceeded those of cultured temperate diatoms (0.4-55 nmol B-12 (mol C)(-1); (x) over bar = 4.1). Comparisons of dissolved B-12 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 B-12 and uptake of cobalt, required for B-12 synthesis, were then examined among bacteria isolated from these communities. Only 23% of 78 bacterial isolates were incapable of B-12 uptake, but these clones assimilated dissolved cobalt. Intracellular B-12 production was evident in 9 of the 11 isolates screened and their cell quotas varied widely, 0.6-6,800 nmol B-12 (mol C)(-1). Mass balance analyses and published kinetics data independently suggest that microalgal growth in sea ice was not limited by vitamin B-12 in most of our field observations and that in situ bacterial B-12 production could potentially meet microalgal demands. Similar analyses, however, suggest that cobalt supply from underlying waters may have limited community growth and B-12 production.