Strong Zonal Gradients in States, Rates and Proteomics provide New Insight into Trace Metal Control on Phosphorus Acquisition in the subtropical Atlantic

Phosphate is chronically deplete in the subtropical North Atlantic (stNA) and thus dissolved organic phosphorus (DOP) becomes a crucial phosphorus (P) source for microbes. Alkaline phosphatases (AP) are a key hydrolytic enzyme group that facilitate microbial DOP-ester utilisation. APs PhoA and PhoX have zinc (Zn) or iron (Fe) cofactors, respectively, and therefore are susceptible to trace metal limitation. This gives rise to the hypothesis that P-Fe or P-Zn co-limits biological activity in P-deplete ocean regions.

We explored coupled P-trace metal co-limitation in summer 2017 along a zonal transect in the stNA. Here, we compare two contrasting regions; the west, with low phosphate and DOP (<10 nM and 60 nM) but high Zn (0.21 nM) and Fe (1.4 nM) and high AP activity rates (2000 nmol P µg Chl-a­^-1 d^-1) and the east, with relatively high phosphate and DOP (20 nM and 160 nM) but relatively low Zn (0.1 nM) and Fe (0.4 nM) and lower AP activity rates (200 nmol P µg Chl-a­^-1 d^-1). Synechococcus abundance was highest in the west, while Prochlorococcus abundance was highest in the east. There were strong zonal gradients in their DOP acquisition proteins. Prochlorococcus and Synechococcus PhoA concentrations were both highest in the west, whereas Prochlorococcus PhoX concentrations were highest in the west while Synechococcus PhoX concentrations were highest in the east. Overall, PhoX concentrations were 15 to 18-fold higher than PhoA. PhoA and PhoX concentrations were similar for both Prochlorococcus and Synechococcus despite Prochlorococcus being more abundant over the entire transect.

We explain our observations by a combination of hydrography, dust deposition, community structure and extent of microbial P-stress. Furthermore, the influence of trace metal availability on DOP acquisition strategy appeared to vary both with metal (Zn or Fe) and species, indicating niche differentiation. Our study highlights the strength in combining states, rates and proteomics to gain novel understanding of microbial nutrient acquisition strategies.