Characterizing Cobalamin Cycling by Antarctic Marine Microbes across Multiple Scales

Deepa Rao, Ph.D., 2020
Mick Follows, Advisor

Marine microbes are the foundation of highly productive coastal Antarctic ecosystems and help regulate Earth’s climate via the biological carbon pump, but the non-trophic interactions underlying their dynamics are unconstrained. Regional phytoplankton growth is primarily limited by light and iron (Fe) and co-limited by vitamin B12, a cobalt-containing molecule only synthesized by bacterioplankton. This thesis investigates nutritional dependencies at multiple scales in the plankton community to characterize microbe-driven B12 cycling in Antarctic seas. In lab, I identified key proteome responses in Phaeocystis antarctica (CCMP 1871) morphotypes to altered Fe-B12 availability, demonstrating that it is not B12-auxotrophic. Integrating cobalt and B12 uptake rates, measured in radiotracer incubations at sea, with hydrographic and pigment data, I found significant correlations indicative of diatom-driven uptake in warmer, fresher surface waters. Incorporating these insights into an ecosystem model, I demonstrated how cross-feeding interactions can enable coexistence and the observed microbial seasonal succession patterns from P. antarctica, to bacterioplankton, and diatoms. This dissertation highlights the micronutrient-driven ecology of coastal Antarctic marine microbes by connecting the Fe-B12 ecophysiology of keystone phytoplankton, environmental conditions for B12 uptake, and the role of trace metals and vitamins in microbial oceanography.