Phosphorus (P) is an essential element for phytoplankton growth, and in recent years our understanding of P use based on kinetics has been overturned with new knowledge of the complexity of P utilization. However, much of this knowledge is based on culture studies with individual species. Our objective was to measure the effect of dissolved inorganic P (DIP) concentrations on DIP uptake rates by phytoplankton, in the context of seasonal phytoplankton succession in a large monomictic, DIP-depauperate lake. We demonstrated an inverse relationship between surface DIP concentration and DIP uptake rate, with substantially higher uptake rates occurring under thermally stratified, DIP-depauperate conditions. The combination of surface water DIP concentration and water temperature explained 50.3% of the variation in uptake rates. DIP concentration explained the majority of variation, with a concentration of 4.75 µg DIP l −1 appearing to be a transition between low-and high-affinity uptake. Variability below 4.75 µg DIP l −1 was further explained by water temperature. High-affinity DIP uptake was most common when Cyanobacteria dominated the phytoplankton assemblage. We validated our field results by conducting a DIP starvation study on an isolate of a dominant cyanobacterium, Cylindrospermopsis raciborskii. High-affinity uptake was demonstrated as the culture became progressively starved of P. Our findings indicate that rapid DIP scavenging via high-affinity uptake is advantageous under DIP-depauperate conditions during the summer-stratification period. It may also contribute to the switch from diatom/cryptophyte/chlorophyte dominance to cyanobacterial dominance in summer. This study also has implications for phytoplankton-nutrient models, which typically do not incorporate high-affinity P uptake.
David Schindler and his colleagues pioneered studies in the 1970s on the role of phosphorus in stimulating cyanobacterial blooms in North American lakes. Our understanding of the nuances of phosphorus utilization by cyanobacteria has evolved since that time. We review the phosphorus utilization strategies used by cyanobacteria, such as use of organic forms, alternation between passive and active uptake, and luxury storage. While many aspects of physiological responses to phosphorus of cyanobacteria have been measured, our understanding of the critical processes that drive species diversity, adaptation and competition remains limited. We identify persistent critical knowledge gaps, particularly on the adaptation of cyanobacteria to low nutrient concentrations. We propose that traditional discipline-specific studies be adapted and expanded to encompass innovative new methodologies and take advantage of interdisciplinary opportunities among physiologists, molecular biologists, and modellers, to advance our understanding and prediction of toxic cyanobacteria, and ultimately to mitigate the occurrence of blooms.
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