Harmful cyanobacterial blooms negatively affect aquatic communities and ecosystems. Research aimed at understanding the environmental factors that promote such blooms has revealed the importance of direct and indirect effects of temperature. Despite the recent attention on how climate warming influences thermal characteristics of aquatic ecosystems (e.g., surface temperatures, depth profiles, and temporal patterns of variation), little is known about the ways in which specific temperature patterns influence cyanobacterial growth. Pivotal to this is appreciating how organisms cope with thermal variability over relatively short time-scales, often exemplified through short-term reversible phenotypic responses (i.e. acclimation). Our study aims to uncover such acclimation effects by identifying how different thermal histories influence the present performance (as measured by exponential growth rate) of four common cyanobacteria species that have known ecological impacts on freshwater communities. To accomplish this, we experimentally measured population-level performance in response to variable temperature, ranging from 16-45°C, and the availability of both nitrogen and phosphorus under controlled laboratory conditions. We subsequently analyzed toxin production (as measured by microcystin content) in Microcystis aeruginosa, a widespread and potent producer of microcystins, following a temperature perturbation using an enzyme-linked immunosorbent assay (ELISA).
Results/Conclusions
The pattern of environmental temperature change (e.g., transition from cool to warm environments versus warm to cool environments) impacted the growth rates of all four important species of freshwater cyanobacteria over growth assays lasting 40 hours. This held true in both low nutrient and nutrient-replete conditions, indicated by populations in fluctuating environments achieving meaningfully different growth rates than populations maintained in the same ultimate environment over the long-term (fully-acclimated). Cold-acclimated populations achieved higher growth rates than fully-acclimated populations across 65% of instances, while populations acclimated to a hot environment underperformed relative to fully-acclimated populations in 75% of cases. This trend also persisted with regard to microcystin production, exemplified by a 2.4-fold increase in microcystin over a five-day period for M. aeruginosa acclimated to cold conditions relative to hot-acclimated populations, when exposed to the same environmental temperature. These results suggest that the patterning of environmental temperatures in aquatic environments is a major, yet currently unappreciated, factor in determining the ability of cyanobacteria to reach high levels of biomass and toxin production for reasons related to thermal acclimation. As such, incorporating acclimation into modeling efforts that utilize robust temperature datasets can provide a crucial bridge between ecological forecasting and ecosystem management.