Cobalt limitation and uptake in Prochlorococcus

Processes that enable marine phytoplankton to acquire trace metals are fundamental to our understanding of primary productivity and global carbon biogeochemical cycling. Here we show that the abundant marine cyanobacterium, Prochlorococcus strain MED4-Ax, has an absolute cobalt requirement and that zinc cannot substitute for cobalt in the growth medium, as is the case in some other phytoplankton species. When resuspended into fresh medium, uptake of cobalt into the cell occurs as free cobalt (Co21 ). In contrast, cultures augmented with conditioned medium assimilated cobalt significantly faster than those in fresh medium, leading to the hypothesis that Prochlorococcus produced organic cobalt ligands in the conditioned medium. This work suggests that the availability of cobalt might influence the composition of phytoplankton assemblages in the open ocean. Analytical methods for measuring trace metals in seawater have improved significantly over the past two decades, contributing to important advances in our understanding of oceanic biogeochemistry. The findings that major regions of the oceans are iron limited (Martin and Fitzwater 1988) and that iron can have nutrient-like profiles (Martin and Gordon 1988) have illustrated the importance of trace metals in global carbon cycling. The development of electrochemical iron speciation methods (Rue and Bruland 1995; Wu and Luther 1995) has led to the idea that biogenic ligands may in fact be controlling the residence time of iron in the surface ocean (Johnson et al. 1997). With the discovery that zinc, cobalt, and cadmium can each occupy the active site of carbonic anhydrase enzymes in phytoplankton (Price and Morel 1990; Morel et al. 1994; Roberts et al. 1997) and that potentially growth limiting levels of Co and Zn have been measured in oceanic surface waters (Martin et al. 1989; Sunda and Hunts1 To whom correspondence should be addressed. Current address: Princeton University, Department of Geosciences, Guyot Hall, Princeton, New Jersey 08544-1003 ([email protected]).