Abstract
Phytoplankton can accumulate polyphosphate (polyP) to alleviate limitation of essential nutrient phosphorus (P). Yet polyP metabolisms in aquatic systems and their roles in P biogeochemical cycle remain elusive. Previously reported polyP enrichment in low-phosphorus oligotrophic marine waters contradicts the common view of polyP as a luxury P-storage molecule. Here, we show that in a P-rich eutrophic bay of Lake Ontario, planktonic polyP is controlled by multiple mechanisms and responds strongly to seasonal variations. Plankton accumulate polyP as P storage under high-P conditions via luxury uptake and use it under acute P stress. Low phosphorus also triggers enrichment of polyP that can be preferentially recycled to attenuate P lost. We discover that picoplankton, despite their low production rates, are responsible for the dynamic polyP metabolisms. Picoplankton store and liberate polyP to support the high primary productivity of blooming algae. PolyP mechanisms enable efficient P recycling on ecosystem and even larger scales.
Highlights
Phosphorus (P) is an essential element of life
We investigated the water column of a coastal embayment of Lake Ontario, Hamilton Harbour (Fig. S1), which has been suffering from frequent harmful algal blooms and summer anoxia[27,28]
The loss of polyP through particle settling and degrading, calculated as the difference between the surface and the bottom divided by the surface concentrations, averaged 53 ± 13% and 80 ± 13% at sites 9031 and 1001, respectively, higher than the loss of Total particulate phosphorus (TPP) estimated in range of 23 ± 13% and 50 ± 22%, respectively (Table S2; recycling efficiencies of TPP and polyP are significantly different (t-test, p < 0.001for both sites))
Summary
Phosphorus (P) is an essential element of life It limits the primary productivity of most freshwater environments and is frequently scarce in many marine systems[1,2]. PolyP metabolisms in natural environments are scarcely studied, and their potential important roles on ecosystem scales were revealed only recently, including energy reservation, phosphorus recycling and transport[17,18,19,20,21,22,23,24]. Freshwater and the scarce of systematic studies describing polyP dynamics under natural conditions hinder our understanding of the diverse polyP functions[13,14], as well as their roles in phosphorus and carbon biogeochemical cycles on ecosystem and even global scales
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