Growth irradiance curves of three nitrogen fixing cyanobacteria blooming species.

DE TEZANOS PINTO, PAULA; LITCHMAN, ELENA

Ramot

Workshop; 15th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP).; 2008

IAP

Blooms of heterocystous nitrogen fixing cyanobacteria (Nostocales) were recurrently observed in open waters of a RAMSAR floodplain wetland (34°14’S, 58°50’W) throughout the year 2006. Such blooms were not observed before and may generate ecosystem disruption if they become persistent (EDHABs). To get insight about EDHABs eco-physiology, three filamentous nitrogen fixing species (Aphanizomenon gracile, Anabaena minderi and Anabaena torques-reginae) were isolated into cultures from wetland bloom samples. Growth irradiance (GI) curves were performed for linking the species’ optimum light growing conditions with ecosystem performance and persistence. Cultures were exposed to twelve light levels, from darkness/limiting to over saturating irradiances, for 4 days. GI curves were obtained from daily in vivo fluorescence and algae quantification. The species with the highest growth rate was Ap. gracile (0.6 d-1), followed by An. minderi (0.5 d-1) and An. torques-reginae (0.1 d-1), well mimicking their relative contributions in the natural blooms. Growth rates were comparable either at low or high irradiances (e.g. 12 or 70 mmol photons m-2 s-1, respectively): Ap. gracile 0.6 vs. 0.7, An. minderi 0.5 vs. 0.5, and An. torques-reginae 0.1 vs. 0.3). Notwithstanding akinets (resting cells) viability in Ap. gracile and An. torques-reginae were affected by light. Akinets lost their integrity (and thus viability) after persistent low light intensities, whereas they were abundant at high irradiances. This suggests that in ecosystems where sediments are persistently dark, the permanence of Nostocales would not be favored. Regardless of irradiance, akinets in An. minderi were rarely observed, suggesting a lower persistence of this species in the ecosystem.