Abstract
The cosmopolitan and increasing distribution of Cylindrospermopsis raciborskii can be attributed to its ecophysiological plasticity and tolerance to changing environmental factors in water bodies. In reservoirs in the semi-arid region of Brazil, the presence and common dominance of C. raciborskii have been described in waters that are considered hard. We investigated the response of a Brazilian C. raciborskii strain to water hardness by evaluating its growth and saxitoxin production. Based on environmental data, a concentration of 5 mM of different carbonate salts was tested. These conditions affected growth either positively (MgCO3) or negatively (CaCO3 and Na2CO3). As a control for the addition of cations, MgCl2, CaCl2 and NaCl were tested at 5 or 10 mM, and MgCl2 stimulated growth, NaCl slowed but sustained growth, and CaCl2 inhibited growth. Most of the tested treatments increased the saxitoxin (STX) cell quota after six days of exposure. After 12 days, STX production returned to concentrations similar to that of the control, indicating an adaptation to the altered water conditions. In the short term, cell exposure to most of the tested conditions favored STX production over neoSTX production. These results support the noted plasticity of C. raciborskii and highlight its potential to thrive in hard waters. Additionally, the observed relationship between saxitoxin production and water ion concentrations characteristic of the natural environments can be important for understanding toxin content variation in other harmful algae that produce STX.
Highlights
Cylindrospermopsis raciborskii is a filamentous diazotrophic cyanobacterium that is globally distributed in freshwater environments [1]
Toxic strains isolated from Brazilian water supplies produce neurotoxins associated with paralytic shellfish poisoning: saxitoxin (STX), neosaxitoxin, gonyautoxins (GTX1-4) and other STX analogues as well as other unidentified toxins [7,8,9]
The lower conductivity increase observed for ASM-1 with CaCO3 in comparison to the control resulted from a partial precipitation of this salt
Summary
Cylindrospermopsis raciborskii is a filamentous diazotrophic cyanobacterium that is globally distributed in freshwater environments [1]. C. raciborskii blooms are increasing in occurrence and frequency, which is attributed to their invasive capacity [2,3,4,5] This circumstance brings additional concern because some strains are toxic to water organisms and humans. Toxic strains isolated from Brazilian water supplies produce neurotoxins associated with paralytic shellfish poisoning: saxitoxin (STX), neosaxitoxin (neoSTX), gonyautoxins (GTX1-4) and other STX analogues as well as other unidentified toxins [7,8,9]. These secondary metabolites are produced by freshwater prokaryotic cyanobacteria as well as by marine eukaryotic dinoflagellates
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