Tracing Cobalt Uptake and Allocation in Prochlorococcus with Metalloproteomics

Cobalt is scarce throughout the Pacific Ocean, but its status as a limiting nutrient is uncertain. Because marine cyanobacteria require cobalt for metabolism, their growth may be affected by the <10 pM concentrations present in the euphotic zone. Through growth experiments in trace metal buffered media, we confirmed a cobalt requirement for Prochlorococcus strain MIT9215 – originally isolated from the Equatorial Pacific – and investigated intracellular allocation with metalloproteomic analyses. Uptake by Co limited cells was far slower than that predicted by the diffusion of Co²⁺ ion, implying that transporter binding kinetics for Co, like iron, are rate-limiting. These results contrast with zinc-limited diatoms and coccolithophores, which acquire Co near the maximum rate allowed by diffusion and populate the same enzymes with either metal. However, cyanobacterial Co usage is distinct from Zn. HPLC separation of the Prochlorococcus proteome under native, non-denaturing conditions shows that Co-binding proteins do not bind Zn in vivo, nor does Zn addition relieve Co limited growth. Minimal Co quotas for this strain match the magnitude of particulate Co in the tropical Pacific and suggest that resident Prochlorococcus are not far from a Co limitation threshold.