Abstract
In certain regions of the predominantly nitrogen limited ocean, microbes can become co-limited by phosphorus. Within such regions, a proportion of the dissolved organic phosphorus pool can be accessed by microbes employing a variety of alkaline phosphatase (APase) enzymes. In contrast to the PhoA family of APases that utilize zinc as a cofactor, the recent discovery of iron as a cofactor in the more widespread PhoX and PhoD implies the potential for a biochemically dependant interplay between oceanic zinc, iron and phosphorus cycles. Here we demonstrate enhanced natural community APase activity following iron amendment within the low zinc and moderately low iron Western North Atlantic. In contrast we find no evidence for trace metal limitation of APase activity beneath the Saharan dust plume in the Eastern Atlantic. Such intermittent iron limitation of microbial phosphorus acquisition provides an additional facet in the argument for iron controlling the coupling between oceanic nitrogen and phosphorus cycles.
Highlights
In certain regions of the predominantly nitrogen limited ocean, microbes can become co-limited by phosphorus
We find that Fe can become limiting to whole microbial community alkaline phosphatase (APase) activity distal to the Saharan dust plume in the tropical Western North Atlantic
As for previous studies in thetropical North Atlantic, we encountered waters that were depleted in dissolved inorganic phosphorus (DIP) (mean excess DIP (DIP*) 1⁄4 9.9 nmol l À 1, s.d. 1⁄4 16.0 nmol l À 1, n 1⁄4 37) yet host to a relatively large residual dissolved organic P (DOP) pool[2,8]
Summary
In certain regions of the predominantly nitrogen limited ocean, microbes can become co-limited by phosphorus. In the (sub)tropical North Atlantic, supply of dissolved inorganic nitrogen (DIN) through diazotrophic N2 fixation[1,2,3] results in drawdown of dissolved inorganic phosphorus (DIP), to the extent that growth of phytoplankton[1,3,4,5] and heterotrophic bacteria[6] can be enhanced by the simultaneous addition of both nutrients relative to supply of N alone Microbial communities experiencing such a shortage of P upregulate production of phosphatase enzymes that hydrolyse P-esters, a large constituent of the oceanic dissolved organic P (DOP) pool, thereby supplying additional DIP for cell growth[7]. Such a role for Fe in P acquisition, alongside requirements for both photosynthesis and N2 fixation[2,3,4], further underscores the critical role of this element in regulating the productivity of past and future (sub)tropical ocean systems
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