Polyphosphate in Trichodesmium from the low‐phosphorus Sargasso Sea

Abstract

Polyphosphate (polyP) is often considered to be the product of luxury uptake in areas of excess phosphorus (P), but can also accumulate in P‐depleted cells in response to P resupply. To test the hypothesis that polyP is present in phytoplankton from oligotrophic systems, the marine diazotroph Trichodesmium was collected from the low‐P surface waters of the Sargasso Sea and assayed with solid‐state 31P nuclear magnetic resonance spectroscopy. Up to 25% of Trichodesmium cellular P was characterized as polyP, despite physiological data that indicated the colonies were P deplete. This was consistent with culture studies where there were high percentages of polyP under P‐deplete conditions. All Trichodesmium species examined had the genetic machinery to produce and degrade polyP. Trends in the amount of Trichodesmium polyP along the cruise transect showed that allocation of P to polyP was consistently high, and that the ratio of polyP : carbon varied with changes in temperature and mixed‐layer depth. It may be that Trichodesmium was taking advantage of pulses in P supply, and that polyP is a physiological fingerprint of this variability. Additionally, if polyP formation is a common trait in phytoplankton, polyP released from cells could be an additional bioavailable component of the dissolved organic P pool. Taken together, this study highlights the importance of polyP to P cycling and cellular P allocation even in oligotrophic regions.