Integrating field and satellite monitoring for assessing environmental risk associated with bacteria in recreational waters of a large reservoir.

In a large South American Reservoir (750 km2, limit between Uruguay and Argentina), we characterized the environmental risk posed by cyanobacteria proxies (abundance, toxin concentration, chlorophyll-a) and Escherichia coli abundances, integrating field (six sites, summers 2011-2015) and satellite (750 km2, summers 2011-2017) monitoring. We further assessed how well field cyanobacteria quantitative proxies (abundance, toxin concentration, chlorophyll-a and scum formation) used to build a local risk communication system for recreational (bathing) use of waters named "cyano-traffic-light", ongoing since 2011, reflected its outcome. Cyanobacteria abundance in the field ranged from moderate (>20,000 to <100,000 cells mL-1) to high-risk (>100,000 cells mL-1), and its abundance was positively related to toxin (microcystin) concentration. Mean microcystin concentrations was within the low (≤2 µg L-1, 50% sites) or moderate (>2 < 10 µg L-1, 50% sites) risk categories. On rare occasions, toxin concentration posed a high-risk for human health. E. coli abundance was within the high-risk category (>126 CFU 100 mL-1) for human health, mostly in the northern part of the reservoir. Cyanobacteria proxies (abundance and toxins) and E. coli abundance were, however, unrelated. The predictive model showed that, out of the four cyanobacteria proxies used to construct the cyano-traffic-light only cyanobacteria abundance (p < 0.05) explained the outcome of the reports, yet with low explanatory power (41%). The satellite monitoring allowed delimiting the extent and magnitude of the environmental risk posed by cyanobacteria at landscape scale (highest risk in the meander parts of the Argentinean side of the reservoir) and producing risk maps that can be used by water management agencies. Based upon our results we propose including E. coli abundances and satellite derived cyanobacteria abundances in the building of the cyano-traffic-light, among other modifications.

The role of heterocytes in the physiology and ecology of bloom-forming harmful cyanobacteria.

Dolichospermum flos-aquae and Cylindrospermopsis raciborskii are two cyanobacteria species which cause harmful blooms around the world. Both these species share the capacity to fix atmospheric nitrogen in heterocytes (cell where fixation occurs). While Dolichospermum can express heterocytes at rather regular intervals across the filament, Cylindrospermopsis can only express heterocytes at the end of the filament. The aim of this study was to experimentally assess the role of heterocyte position in the eco-physiological responses of these bloom forming cyanobacteria. Replicated monocultures of each species were grown at different eutrophication scenarios (limiting and sufficient nitrogen and phosphorus concentrations, in factorial design). Dolichospermum reached high biomass regardless of the nitrogen (and phosphorus) provided, suggesting that this species could bloom in situations with and without nitrogen limitation. In contrast, Cylindrospermopsis reached high biomass only when nitrogen supply was high; its biomass was 15-20 times lower when relying on nitrogen fixation. Hence, despite its ability to fix nitrogen, blooms of Cylindrospermopsis would be expected only under high total nitrogen availability. In Dolichospermum heterocytes occurred only in the scenarios without supplied nitrogen while in Cylindrospermopsis heterocytes occurred regardless of nitrogen availability. Yet, in both species nitrogen fixation occurred (heterocytes were functional) only when nitrogen was limiting, and nitrogen fixation increased significantly at higher phosphorus concentration. Finally, in the absence of supplied nitrogen, filament length in Dolichospermum was the longest, while filaments in Cylindrospermopsis were the shortest (up to 13 times shorter than at nitrogen sufficiency). Therefore, heterocyte expression in Dolichospermum, and filament length in Cylindrospermopsis seem good proxies of nitrogen fixation. The eco-physiological responses recorded here help understand the distribution of these species along nutrient gradients in nature.