Phosphorus (P) is often the limiting nutrient for phytoplankton growth in the Great Lakes, and competition for P sources can influence phytoplankton diversity. P is most readily available to phytoplankton as ortho-phosphate (Pi), but if Pi is limiting, cells can use Pi from monophosphate esters through cleavage by alkaline phosphatase activity (APA). Recent research suggests marine cyanobacteria can also use phosphonates, a P form previously considered unavailable to phytoplankton. Phosphonates are ingredients in common agricultural and domestic herbicides which can contaminate freshwater ecosystems. We posed the question: Can Lake Michigan phytoplankton communities use phosphonates as a P source? To address this question, we conducted growth trials using isolated cyanobacterial strains and natural Lake Michigan communities supplying a range of P compounds including Pi, glycerol-Pi, and phosphonates (1-aminoethylphosphonic acid, phosphonoacetic acid, glyphosate). Cyanobacterial growth was monitored over 2 weeks by chl a fluorescence changes; Pi concentration and APA were also measured. Growth bioassays with natural Lake Michigan phytoplankton supplied with different P sources were carried out over 13 days and growth measured as extracted chlorophyll and APA was examined.

Results/Conclusions

All five cyanobacterial strains tested showed a marked ability to cleave Pi from the monophosphate ester glycerol-Pi via APA. Growth was fastest when cells were supplied with Pi. However, growth rate ranges were 14-36% with glycerol-Pi and 17-35% with phosphonates, relative to 100% when supplied with Pi. One recent Lake Erie isolate, Synechococcus ARC21, grew as fast on glycerol-Pi as when supplied with Pi, showing a well developed capacity for organic P use via APA. Lake Michigan phytoplankton also showed preference for Pi but grew 43% and 5% as fast when supplied with glycerol-Pi and phosphonates, respectively, while there was no growth when no P was added. Phytoplankton and cyanobacteria found in Lake Michigan can utilize monophosphate esters and phosphonates as P sources to support growth. In these chronically P-limited aquatic ecosystems, this has important implications for runoff of agricultural and domestic P compounds into lake catchments.