Abstract
Cyanobacteria populate most water environments, and their ability to effectively exploit light and nutrients provide them with a competitive advantage over other life forms. In particular conditions, cyanobacteria may experience considerable growth and give rise to the so-called harmful algal blooms (HABs). HABs are often characterized by the production of cyanotoxins, which cause adverse effects to both aquatic organisms and humans and even threaten drinking water supplies. The concentration of cyanotoxins in surface waters results from the budget between production by cyanobacteria and transformation, including photodegradation under sunlight exposure. Climate change will likely provide favorable conditions for HABs, which are expected to increase in frequency over both space and time. Moreover, climate change could modify the ability of some surface waters to induce phototransformation reactions. Photochemical modeling is here carried out for two cyanotoxins of known photoreaction kinetics (microcystin-LR and cylindrospermopsin), which follow different phototransformation pathways and for particular freshwater scenarios (summertime stratification in lakes, water browning, and evaporative water concentration). On this basis, it is possible to quantitatively predict that the expected changes in water-column conditions under a changing climate would enhance photodegradation of those cyanotoxins that are significantly transformed by reaction with the triplet states of chromophoric dissolved organic matter (3CDOM*). This is known to be the case for microcystin-LR, for which faster photodegradation in some environments would at least partially offset enhanced occurrence. Unfortunately, very few data are currently available for the role of 3CDOM* in the degradation of other cyanotoxins, which is a major knowledge gap in understanding the link between cyanotoxin photodegradation and changing climate.
Highlights
Cyanobacteria are organisms that trace their origins to billions of years into the past, and they are ubiquitous in all aquatic environments
Different species of cyanobacteria are able to exploit to their advantages to the variable conditions that can be found in surface waters
Some cyanobacteria grow at the water surface where radiation is intense, and they tend to bloom during summer stratification of lake water,[4−6] while others prefer conditions of low radiation intensity that can be found in turbid waters, for example, during lake overturn.[7]
Summary
Cyanobacteria are organisms that trace their origins to billions of years into the past, and they are ubiquitous in all aquatic environments. Cyanobacteria can undergo rapid growth under favorable conditions, which may result in what is called harmful algal blooms (HABs) events These events, with frequencies that are impacted by both anthropogenic activities (e.g., enhanced nutrient inputs into reservoirs) and climatic factors,[1] result in numerous concerns, from damage to aquatic organisms to potential impacts to human health and animals, via exposure through recreation or potentially through potable water.[2]. Warmer water during summer and a longer period of thermal stratification in lakes can favor the blooms of some organisms (e.g., Microcystis), as well as their likelihood to induce HABs.[4,5,12,13] Climate change is increasingly characterized by alternations of drought periods and floods that deeply alter the hydrology of water basins and enhance the mobilization of nutrients.[14−17] As mentioned above, the availabilities of nitrogen and phosphorus, and an intermittent supply for the latter, are conditions that either prove favorable to the growth of cyanobacteria or provide these species with a competitive advantage over other living organisms. Model predictions enable the identification of possible future trends in photodegradation kinetics and of key knowledge gaps, which prevent a clear understanding of how the fate of other cyanotoxins may be impacted by a warming future
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