Nitrate utilization by phytoplankton in Lake Superior is impaired by low nutrient (P, Fe) availability and seasonal light limitation - A cyanobacterial bioreporter study

We previously developed a luminescent Synechococystis sp. strain PCC 6803 cyanobacterial bioreporter that is used as a real-time whole-cell sensor to assess nitrate assimilatory capacity in freshwaters. Applying the bioreporter assay to Lake Superior, a system whose nitrate levels have increased 6-fold since 1900, we investigated factors that constrain nitrate utilization in this oligotrophic system. Clean sampling methods were used to collect water from Lake Superior during spring and summer 2004, and nitrate utilization was measured by monitoring bioreporter luminescence. Bioreporter response was monitored during experiments in which the lake water was amended with nutrients and incubated under light regimes simulating integrated spring and summer mixing depths. These studies demonstrated that nitrate utilization was enhanced at most stations following addition of phosphorus (P). Moreover, at many stations, addition of iron (Fe) enhanced the P effect. Strength-of-effect statistical analysis provided the individual contribution of P and Fe toward stimulating bioreporter response. In general, distance from shore and season were not good predictors of nitrate assimilatory capacity. Manipulation of light flux during bioreporter experiments also showed that light intensities experienced during spring mixing are likely insufficient to saturate the rate of nitrate utilization. Overall, these data suggest that P-limited algae are deficient in their ability to assimilate nitrate in Lake Superior. Furthermore, we suggest that a secondary limitation for Fe may occur that further constrains nitrate drawdown. Lastly, during spring, light fluxes are sufficiently low to prevent maximal nitrate utilization, even in the absence of nutrient limitation.