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

Abstract

Within McMurdo SoundÂs 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‐B12 into the >1.0‐mm size fraction (mostly diatoms) was almost 100 times faster than into 0.2−1.0−mm 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; x̄x = 500) and generally exceeded those of cultured temperate diatoms (0.4‐55 nmol B12 (mol C)−1; x̄x = 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.