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

Abstract

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 21 ) and assimilation rates (17-780 pmol m 22 d 21 ) in congelation ice covaried with primary productivity (0.0001-250 mmol C m 22 d 21 ) and chlorophyll a (0.6-36 mg m 22 ). 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) 21 ; x 5 500) and generally exceeded those of cultured temperate diatoms (0.4-55 nmol B 12 (mol C) 21 ; x 5 4.1). Comparisons of dissolved B 12 pools in the ice and their turnover (0.02-0.6 d 21 ) 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) 21 . 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.

The Role of B Vitamins in Marine Biogeochemistry

“…However, the similar dissolved B 12 concentrations measured using the bioassay technique in the open-ocean waters of different marine systems in the 1950s and 1960s (ranging from undetectable to ∼7 pM) along with the levels measured more recently using direct techniques (Table 4) suggest that, at least for vitamin B 12 , the bioassays and the direct quantifications produce similar results. The same applies to B 12 measurements in the Antarctic, where bioassay-based measurements of dissolved B 12 concentrations (ranging from 0.5 to 2.4 pM; Carlucci & Cuhel 1977, Taylor & Sullivan 2008 were similar to those measured with the direct technique (ranging from 0.5 to 3.5 pM; Panzeca et al 2009). The similar range of concentrations as well as the similar depth profiles obtained by the two methodologies (Figure 5a) in water samples collected in the Pacific Ocean almost 50 years apart suggest that both techniques produce coherent results.…”

“…In the field, Ohwada & Taga (1972) found that particulate B 1 and B 7 concentrations in surface waters of the North Pacific were approximately 1% of the dissolved concentrations, whereas in coastal waters, particulate B 1 and B 7 concentrations were approximately 150% and 50% of the dissolved concentrations, respectively. The rapid turnover rates calculated for B 12 with respect to biological assimilation (on the order of a few hours; Koch et al 2011, Taylor & Sullivan 2008 suggest that water column vitamin depletion due to biological scavenging is feasible in certain environments.…”

Two distinct pools of B12analogs reveal community interdependencies in the ocean

“…Laboratory cultures of Alphaproteobacteria and Procholorococcus strains had lower amounts of cobalamin or pseudocobalamin (less than 1,200 nmol cobalamin analog per mole carbon) than Synechococcus and Thaumarchaeota isolates (1,480-11,600 nmol cobalamin analog per mole carbon). Published values (17) for sea ice bacterial isolates estimated using a bioassay were highly variable (0.6-6,800 nmol cobalamin analog per mole carbon). In our environmental samples, we observed an average stoichiometry of 87 nmol pseudocobalamin per mole cyanobacterial carbon, lower than the cyanobacterial isolates (Fig.…”

Microbial processes in the weathering crust aquifer of a temperate glacier

“…The positive correlation of the cell and ATP data with Al 3+ and K + likely reflect contributions from aeolian transport to the ice 20 surface and subsequent weathering, but the data for Co 2+ (Fig. 3A) are interesting when juxtaposed with the results of Taylor and Sullivan (2008). This study showed that uptake of Co 2+ and vitamin B 12 synthesis by bacteria in Antarctic sea ice supplied this vital micronutrient to algae auxotrophic for B 12 .…”