Phytoplankton readily assimilate dissolved inorganic phosphorus (phosphate; Pi) into essential biomolecules, making it their preferred form of phosphorus (P). In low Pi marine environments, phytoplankton also utilize dissolved organic P (DOP) compounds to sustain primary productivity. Phosphonates comprise up to 10% of the DOP pool and have been shown to be an important source of P in the nutrition of marine prokaryotic phytoplankton. To date, the ability of eukaryotic phytoplankton to supplement growth with natural phosphonates remains unknown. Here, we show the growth rates of two eukaryotic phytoplankton species grown on phosphonate(s) as the sole source of P were significantly greater than growth rates under Pi-deficient conditions. Phosphonate did not support growth of a third species, indicating utilization is not universal among eukaryotic phytoplankton. Taken together, these results indicate that direct uptake and utilization of extracellular phosphonates may no longer be restricted to the realm of prokaryotes.