Phosphorus (P) is an essential nutrient that is frequently found in complex organic forms.  Dissolved organic phosphorus (DOP) is increasingly thought to play a critical role in the distribution and activities of marine microbes, and as such DOP can influence primary production and the extent to which ocean microbes called phytoplankton cycle carbon. Despite the importance of DOP, there are analytical challenges that have historically hindered its study. Given the importance of DOP as a phosphorus source to microorganisms and the climatic and ecosystem implications of DOP utilization, here we focused on understanding what phosphorus compounds are produced and consumed by phytoplankton.

Intellectual merit

This project used an innovative combination of methods, and method development, to examine the composition of phosphorus produced by phytoplankton, how this varies under different conditions, and how this is controlled at the molecular level.  Further studies were also performed to evaluate how bioavailable these compounds are.  The research was structured around the following general goals:

1)  Characterize the proportions of the major P compound classes (phosphonates, polyphosphates, and phosphoesters) produced by major phytoplankton functional groups.

2)  Track changes in the proportions of the major P compound classes produced by phytoplankton in different conditions.

3)  Connect patterns in P-related gene expression to phosphorus dynamics in model cultures.

4)  Compare the diversity in P speciation observed in laboratory studies to P biogeochemistry in the ocean.

Research done on this project applied a new method for concentrating DOP and assaying polyphosphate concentration and showed that the cellular dynamics of polyphosphate concentration, and nucleotide metabolism, are modulated by phosphorus physiological ecology in key phytoplankton like diatoms, and that this is controlled at the transcriptional level.  Using a combination of approaches, which leveraged cruises of opportunity we showed that the relative contribution of polyphosphate in the total particulate phosphate pool is modulated as a function of phosphorus physiology and biogeochemistry in both the North Atlantic and the North Pacific.  Culture controls confirmed that phosphorus physiology also controlled release of polyphosphate to the dissolved phase.  Last, polyphosphate bioavailability was shown to vary between even closely related phytoplankton, suggesting that phosphorus form could influence community composition in the phytoplankton. 

Taken together, the overarching finding from this research is that the physiology of the phytoplankton, in particular the extent to which they may be phosphorus deficient, has a large impact on the production and consumption of polyphosphate and DOP.  Key outcomes are as follows:

1)    Phosphorus compound classes (phosphonate, polyphosphate and phosphoester) are produced by major phytoplankton functional groups.  The production of phosphonate is difficult to detect because it has signatures similar to other compounds.  Although there was no clear evidence of phosphonate production in the eukaryotic phytoplankton, phosphonate was produced by some cyanobacteria.

2)    There are changes in the proportions of the major phosphorus compound classes produced by phytoplankton in different conditions, namely as a function of phosphorus deficiency. In some cases (e.g. nucleotides, polyphosphate), this is linked to changes in the transcription of genes in these metabolic pathways.

3)    Patterns in phosphorus biogeochemistry, includin...

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