Episodic Decrease in Temperature Increases mcy Gene Transcription and Cellular Microcystin in Continuous Cultures of Microcystis aeruginosa PCC 7806

Abstract

Microcystins produced during harmful cyanobacterial blooms are a public health concern. Although patterns are emerging, the environmental cues that stimulate production of microcystin remain confusing, hindering our ability to predict fluctuations in bloom toxicity. In earlier work, growth at cool temperatures relative to optimum (18°C vs. 26°C) was confirmed to increase microcystin quota in batch cultures of Microcystis aeruginosa NIES-843. Here, we tested this response in M. aeruginosa PCC 7806 using continuous cultures to examine temporal dynamics and using RNA-sequencing to investigate the physiological nature of the response. A temperature reduction from 26 to 19°C increased microcystin quota ∼2-fold, from an average of ∼464 ag μm–3 cell volume to ∼891 ag μm–3 over a 7–9 d period. Reverting the temperature to 26°C returned the cellular microcystin quota to ∼489 ag μm–3. Long periods (31–42 d) at 19°C did not increase or decrease microcystin quota beyond that observed at 7–9 d. Nitrogen concentration had little effect on the overall response. RNA sequencing indicated that the decrease in temperature to 19°C induced a classic cold-stress response in M. aeruginosa PCC 7806, but this operated on a different timescale than the increased microcystin production. Microcystin quota showed a strong 48- to 72-h time-lag correlation to mcy gene expression, but no correlation to concurrent mcy expression. This work confirms an effect of temperature on microcystin quota and extends our understanding of the physiological nature of the response.

Changes in Microbiome Activity and Sporadic Viral Infection Help Explain Observed Variability in Microcosm Studies

“…Microcystin concentrations were not different between treatments ( p -value > 0.1), nor were there differences in congener profile ( Supplementary Figure 1 ). This is in many ways not surprising as it was recently demonstrated that up-regulation of the genes associated with toxin synthesis in Microcystis can take 48 h to result in a measurable increase in toxin quota ( Martin et al, 2020 ). There was no significant change in chlorophyll a associated with nitrogen amendments, indicating that the phototroph biomass was not constrained by available nitrogen at the time of sampling.…”

Thermal acclimation influences the growth and toxin production of freshwater cyanobacteria

“…We have direct measurements of r by taking ln(final/initial cell density)/5 d. We obtained cell density values using the established linear relationship between fluorescence and particles/ml (from a FlowCam 5000, Flow Imaging Particle Analyzer, Fluid Imaging Technologies, Inc., Scarborough, ME, USA) for each nutrient condition for toxic M. aeruginosa and confirmed these relationships persisted over relevant timescales (Figure S6(a),(b)). Previous research has indicated the importance of temperature for influencing cell size (Martin et al 2020), while not considered in this study, could influence the relationship between density and biovolume. Finally, we assume negligible initial MC concentrations (τ 0 ≈ 0) and a loss rate of l = 0.09/day (given a half-life of 7 days reported by Zastepa et al 2014).…”

Viral infections mediate microbial controls on ecosystem responses to global warming

“…Encounter rates between viruses and hosts depend on virus and host densities (Murray and Jackson 1992), host cell size, and host motility (Wilhelm et al 1998). Host cell sizes (Atkinson, Ciotti and Montagnes 2003;Daufresne, Lengfellner and Sommer 2009;Martin et al 2020) and population densities Bernhardt, Sunday and O'Connor 2018) often decrease while motility increases (Crozier and Federighi 1924;Maeda et al 1976;Savage 2011, 2014;Gibert et al 2016) with temperature. Consequently, warming could have positive or negative effects on virus-host encounter rates, although more studies are needed (Table 1, Fig.…”