Polyphosphate metabolism and dynamics in aquatic systems


Polyphosphate (polyP)


Polyphosphate (polyP) is a long-chain polymer of 3 to hundreds of orthophosphate (PO4) units linked by high energy bounds (Fig. 1) . It exists in cells of many living organisms including aquatic primary producers (algae and cyanobacteria; Fig. 2), considered to an important intracellular P reserve to survive low nutrient conditions. However its metabolism responding to nutrient levels and roles in P cycling in aquatic systems remains largely unknown.

Our work in polyP includes lab-controlled experiments on polyP metabolism in cyanobacteria responding to nutrient levels, and polyP dynamics in freshwater lakes and its role in P cycling.

Publications:


- Li, J. and M. Dittrich. 2019. Dynamic polyphosphate metabolism in cyanobacteria responding to phosphorus availability. Environmental Microbiology. 21: 572- 583 (PDF)- Li, J., A. Zastepa, S. Watson, and M. Dittrich. Polyphosphate in picoplankton responses strongly to phosphorus excess and deficiency in the dynamic coastal Lake Ontario (In preparation).
Fig. 1 Structure formula of polyphosphate
Fig. 2 PolyP granules in Synechococcus sp. shown under transmission electron microscope.

PolyP in cyanobacteria responding to P levels

We investigate polyP in cultures of three unicellular cyanobacteria strains, under various nutritional conditions. Our experiments show that the accumulation of polyP in cyanobacteria is strongly dynamic, depending on phosphate levels and growth stages. The following schematic (Fig. 3) summarizes our findings.

(TPP = total particulate P; APase = activity of alkaline phosphatase)
Fig. 3 Schematic plots of cell growth, dissolved phosphate (PO4), activity of alkaline phosphatase (APase), polyP, polyP:cell, and polyP:TPP. All values are in arbitrary units.
  • Increase in the ratios of polyP:cell and polyP:TPP during the lag phase suggests “overplus” uptake of phosphorus leads to the rapid accumulation of polyP.
  • Decrease of polyP ratios during the exponential growth stage is a result of competing “luxury” P uptake and polyP utilization to support rapid cell division.
  • When PO4 concentrations become low, the cells start to experience P stress, as indicated by increased activity of alkaline phosphatase (APase). Increase of polyP:TPP in this period suggests that cyanobacteria is capable of P deficiency responses that preferentially maintain polyP over other cellular P.
  • Eventually preferential utilization of polyP occurs under severe P stress, suggesting the crucial role of polyP as P reserve to support cellular survival.
  • Strong variability was observed among different species of cyanobacteria in their ability to accumulate polyP, and likely in the threshold P levels at which preferential polyP degradation occurs. This suggests that some cyanobacteria may be more adaptive to P- stressed or P-fluctuating conditions.

PolyP dynamics in the eutrophic Hamilton Harbor, nearshore Lake Ontario

We investigate the dynamics of polyP in planktons in coastal Lake Ontario, the Hamilton Harbor (Fig. 4) . Hamilton Harbor is one of the most eutrophic waters in the Great Lakes Regions, designated as an Area of Concern (AOC). It has been suffering from intensive summer algal blooms and hypoxia.

Our results suggest that polyP in the planktons are strongly dynamics responding to concentrations of external P (Fig. 5). Increase of polyP:TPP, polyP:Chl-a, and polyP:PC in the summer may be indicative of P deficiency response, while increase of these ratios in the winter is a result of "overplus" responses triggered by the increase of SRP due to the lost of stratification that mixed the deeper nutrient-rich water up to the surface (Fig. 5).

(TPP = total particulate P; SRP = soluble reactive P; Chl-a = chlorophyll a; PC = phycocyanin)
Fig. 4 Water samples were taken from 2 sites in Hamilton Harbor, Lake Ontario. Particulate samples were collected on filters of 0.2 um and 2 um to separate particles of different size ranges. Particulate samples were analyzed for total particular P, pigments chlorophyll a and phycocyanin, polyP. Filtrates were analyzed for soluble reactive P (SRP).
Fig 5 Seasonal variability at site 9031 shows increase of polyP (polyP:TPP, polyP:Chl-a, polyP:PC) in the summer when SRP was low, indicative of P deficiency responses; polyP and the ratios (polyP:TPP, polyP:Chl-a, polyP:PC) increased in winter, suggesting "overplus" mechanism responding to SRP increase.

The adaptation strategies to survive environments of highly fluctuating nutrient levels were found most prominent in the smaller size picoplankton (< 2 um) (Fig. 6). Preferential liberation of polyP over bulk organic phosphorus strongly suggests an unquantified but important role of polyP in P internal cycling (Fig.7).

Fig. 6 PolyP variability responding to P levels (soluble reactive P concentrations). PolyP dynamics is contributed primarily by picoplankton (< 2 um).
Fig. 7 Total particulate P (TPP) and polyP decreases with depth, indicative of P recycling into the water column while settling. PolyP: TPP decrease with depth, suggesting that polyP is preferentially recycled compared to other cellular P components.

Polyphosphate in the upper Great Lakes (ongoing work)

This is an ongoing project investigating spatial variability of polyP in the water column of the upper Great Lakes (Lakes Superior, Michigan, and Huron). Water samples have been taken from multiple locations covering regions of different primary productivity and nutrient levels, from the very oligotrophic waters of abyssal Lake Superior to the highly eutrophic Green Bay in Lake Michigan (Fig. 8). We aim to investigate how plankton polyP respond to nutrient heterogeneity, and how polyP impact P cycling.

Fig. 8 Sampling locations in the Great Lakes. The red markers indicate sampling sites from our resent cruise (August 2018); the blue markers indicate study sites in coastal Lake Ontario (previous study).
Filtration setup on R/V Blue Heron (August, 2018)