NITRATE UTILIZATION BY PHYTOPLANKTON IN LAKE SUPERIOR IS IMPAIRED BY LOW NUTRIENT (P, Fe) AVAILABILITY AND SEASONAL LIGHT LIMITATION — A CYANOBACTERIAL BIOREPORTER STUDY¹

Abstract

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.

Lake Superior Supports Novel Clusters of Cyanobacterial Picoplankton

“…25% lower than the total. A temporal comparison clearly showed higher efficiency for those samples collected during March (␣ ϭ 5.9 ϫ 10 Ϫ2 ) compared to August (␣ ϭ 2.4 ϫ 10 Ϫ2 to 3.2 ϫ 10 Ϫ2 ), consistent with deep mixing and the resultant light-limiting conditions experienced by phytoplankton prior to the onset of thermal stratification at this site in late June to early July (17). Also consistent with the light-limiting conditions synonymous with vernal holomixis were depressed values of E k , a parameter derived from P m Chl /␣ Chl to estimate the irradiance at which photosynthesis becomes light saturated.…”

“…Instead, the picocyanobacterial community at PD appeared more diverse than the pelagic community and consisted of members of the C. gracile cluster, group B (subalpine cluster I), the Lake Biwa cluster sensu Ernst et al (7), group H, as well as two new minor PD-specific clusters, PDI and PDII. Pelagic Lake Superior is an extremely oligotrophic system in which the combination of chemical and physical factors may create a unique environment in which phytoplankton are severely limited by low concentrations of phosphorus and iron, as well as constrained by light and temperature regimes unfavorable for photosynthesis (17,24,26,34). The evident differences in the APP community between the pelagic western arm of Lake Superior and the nearshore station PD located in a region subject to high coastal influence allow us to suggest the existence of novel ecotypes of Synechococcus spp.…”

Nutrients drive transcriptional changes that maintain metabolic homeostasis but alter genome architecture in Microcystis

“…3.1.3.1.) activity measurement was performed in biological triplicate across all treatments with the procedure adapted for cyanobacterial culture assays from Ivanikova et al (2007). A TD-700 fluorometer was used to make measurements (Turner Designs).…”

Cyanobacterial Bioreporters as Sensors of Nutrient Availability

“…PCC 7002 [17]. Despite the fact that these strains are not particularly ecologically relevant with respect to most oligotrophic aquatic environments, these prototype strains afforded an opportunity to demonstrate the feasibility in using reporters in both freshwater [9,18,35,38,41,[44][45][46] and marine samples [17].…”

“…Whereas seasonal drawdown of nitrate is demonstrated, it is modest (2-4 mmol L À1 ) compared to the lower lakes in the Great Lakes system [101]. To gain insight into the factors that constrain nitrate consumption in Lake Superior, the positive induction of the AND100 luminescent response was used to measure the onset of nitrate use as light levels were manipulated and nutrients (P, Fe) were amended to samples collected from the lake [8,45]. These data suggest that P-and Fe-limited cyanobacteria are deficient in their ability to assimilate nitrate in Lake Superior (Fig.…”