Effects of Microcystis on heterocyst initiation in Anabaena

Background/Question/Methods

Although it is incapable of nitrogen fixation, Microcystis is often well-represented and sometimes dominant in lakes where N:P ratios are low and nitrogen-fixing cyanobacteria, such as Anabaena are present. Factors contributing to dominance of Microcystis include rapid uptake of bioavailable nitrogen and allelochemical inhibition of photosynthesis by other cyanobacteria. It is unknown whether these allelochemicals also inhibit nitrogen fixation. To address this hypothesis, Anabaena flos-aquae was cultivated in high and low nitrogen media, with and without Microcystis aeruginosa. Light and temperature conditions were selected to promote rapid population growth. Heterocysts and vegetative Anabaena cells were enumerated periodically over the length of the experiment (30 days). To address the mechanism behind the interactions, results from treatments containing living Microcystis and will be compared to spent medium from which Microcystis cells have been removed by filtration.

Results/Conclusions

In the high nitrogen treatments, A. flos-aquae produced few or no heterocysts regardless of whether Microcystis was present. The average ratio of heterocysts to vegetative cells was 0.009.  In contrast, A. flos-aquae growing alone in low nitrogen medium consistently produced a 0.033 ratio of heterocysts to vegetative cells. For Anabaena in low nitrogen medium with Microcystis present, the average ratio was 0.029; variability among replicates was high ranging from 0.012 to 0.053. When ratios were high, vegetative cells were similar in shape and color to those in high nitrogen treatments, whereas when ratios were lower than 0.02, vegetative cells were paler, more transparent, and less robust. The interaction between Microcystis and Anabaenais multi-faceted, including effects mediated by allelochemicals and those that impact resource availability. Controlled laboratory experiments provide an approach for assessing potential mechanisms underlying the interactions among cyanobacteria which can then be extended into natural populations in the field with additional study.