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Vitamin B 12 and cobalt cycling among diatoms and bacteria in Antarctic sea ice microbial communities
Author(s) -
Taylor Gordon T.,
Sullivan Cornelius W.
Publication year - 2008
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2008.53.5.1862
Subject(s) - algae , biogeochemical cycle , chlorophyll a , environmental chemistry , seawater , bacteria , microbial population biology , chemistry , biology , botany , oceanography , ecology , genetics , geology
Within McMurdo SoundÂs 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 57 Co‐B 12 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 B 12 :C cell quotas in field communities varied widely (2.6‐150,000 nmol B 12 (mol C) −1 ; x̄x = 500) and generally exceeded those of cultured temperate diatoms (0.4‐55 nmol B 12 (mol C) −1 ; x̄x = 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 1 2 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.