Evidence of Phosphonate Utilization by the Eukaryotic Phytoplankton, Isochrysis galbana

Comprising up to 10% of the marine dissolved organic phosphorus (DOP) pool, phosphonates have been shown to be a dynamic P pool both being assimilated and produced by marine cyanobacteria. It’s thought that eukaryotic phytoplankton do not have the ability to supplement growth with phosphonates; however, this tenet is being challenged. We, and others, have shown at least some eukaryotic phytoplankton are able to use phosphonates as the sole source of P to support growth. In this study, we investigated changes in cell yield and cellular P in the haptophyte, Isochrysis galbana, when given a phosphonate as the sole source of P and compared to cells grown under phosphate (Pi) deficient conditions. The cell yield of I. galbana supplemented with the phosphonate was 30% greater than the Pi deficient treatment. The cellular P, however, was significantly reduced in the cells given the phosphonate as the sole source of P. This suggests cells grown on phosphonates may use this P source to support growth, while Pi deficient cells use available P for storage. This work is timely given the prediction that future oceans will become more stratified which could increase the importance of DOP, including phosphonates, as a nutrient source. Species able to utilize phosphonates may have an advantage over those that cannot, thereby influencing community composition and carbon cycling in future oceans.