Abstract
Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii) can produce cylindrospermopsin (CYN) which is of great concern due to its considerable toxicity to human and animals. Its CYN-producing (toxic) and non-CYN-producing (non-toxic) strains co-exist commonly in natural water bodies, while how their relative dominance is regulated has not been addressed. In this study, we combined field investigation with laboratory experiments to assessed the relationship between toxic and non-toxic R. raciborskii abundances under different nutrient levels. The rpoC1- and cyrJ-based qPCR was applied for quantifying total and toxic R. raciborskii abundances, respectively. The field survey showed that toxic R. raciborskii was detected in 97 of 115 reservoirs where its proportion ranged from 0.3% to 39.7% within the R. raciborskii population. Both total and toxic R. raciborskii abundances increased significantly with trophic level of these reservoirs, consistent with our monoculture and co-culture experiments showing in an increase in R. raciborskii growth with increasing nitrogen (N) or phosphorus (P) concentrations. In the monoculture experiments, growth rates of non-toxic and toxic strains from Australia or China were not significantly different under the same culture conditions. On the other hand, in the co-culture experiments, the toxic strains displayed a significantly faster growth than non-toxic strains under nutrient-replete conditions, resulting in an obvious shift toward the dominance by toxic strains from day 3 to the end of the experiments, regardless of the strain originating from Australia or China. The reverse was found under N- or P-limited conditions. Our results indicated that the toxic strains of R. raciborskii have a competitive advantage relative to the non-toxic strains in a more eutrophic world. In parallel to an increase in dominance, both toxic strains grown in the mixed population significantly increased CYN production under nutrient-replete conditions as compared to nutrient-limited conditions, suggesting that CYN may be of significance for ecological advantage of toxic R. raciborskii. These results highlight the importance of nutrient availability in regulating abundances and strain dominance of two genotypes of R. raciborskii. Our findings demonstrated that elevated nutrients would favor the growth of CYN-producing R. raciborskii and CYN production, leading to more blooms with higher toxicity at global scale.
Highlights
Cyanobacterial blooms are a global environmental concern due to their potential to produce a variety of cyanotoxins, namely microcystins (MC), nodularins, cylindrospermopsin (CYN) and neurotoxins (Buratti et al, 2017)
Toxic strains had a relative abundance below 10% in nearly half of reservoirs and only 9 samples had a relatively higher proportion of toxic R. raciborskii (> 30%, Figure 2A), indicating a dominance of non-toxic strains in thesetropical reservoirs
The first attempt to combine field investigation with laboratory experiments revealed that nutrient availability can regulate the relative dominance of CYN-producing and non-CYN-producing R. raciborskii
Summary
Cyanobacterial blooms are a global environmental concern due to their potential to produce a variety of cyanotoxins, namely microcystins (MC), nodularins, cylindrospermopsin (CYN) and neurotoxins (Buratti et al, 2017). Nitrogen (N) and phosphorus (P) concentrations was found to individually or concurrently make toxic strains outcompete non-toxic ones for the same cyanobacteria species (Yoshida et al, 2007; Davis et al, 2009, 2010; Ha et al, 2009; Dziallas and Grossart, 2011; Conradie and Barnard, 2012; Lei et al, 2015; Wang et al, 2018) These studies predict that climate change and further eutrophication will alter the community composition and promote blooms dominated by toxic cyanobacterial strains. How the environmental factors affect the dynamics of toxic and non-toxic cyanobacterial strains remain to be answered, due to their inconsistencies
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