Abstract
Sea ice supports a unique assemblage of microorganisms that underpin Antarctic coastal food-webs, but reduced ice thickness coupled with increased snow cover will modify energy flow and could lead to photodamage in ice-associated microalgae. In this study, microsensors were used to examine the influence of rapid shifts in irradiance on extracellular oxidative free radicals produced by sea-ice algae. Bottom-ice algal communities were exposed to one of three levels of incident light for 10 days: low (0.5 μmol photons m−2 s−1, 30 cm snow cover), mid-range (5 μmol photons m−2 s−1, 10 cm snow), or high light (13 μmol photons m−2 s−1, no snow). After 10 days, the snow cover was reversed (either removed or added), resulting in a rapid change in irradiance at the ice-water interface. In treatments acclimated to low light, the subsequent exposure to high irradiance resulted in a ~400× increase in the production of hydrogen peroxide (H2O2) and a 10× increase in nitric oxide (NO) concentration after 24 h. The observed increase in oxidative free radicals also resulted in significant changes in photosynthetic electron flow, RNA-oxidative damage, and community structural dynamics. In contrast, there was no significant response in sea-ice algae acclimated to high light and then exposed to a significantly lower irradiance at either 24 or 72 h. Our results demonstrate that microsensors can be used to track real-time in-situ stress in sea-ice microbial communities. Extrapolating to ecologically relevant spatiotemporal scales remains a significant challenge, but this approach offers a fundamentally enhanced level of resolution for quantifying the microbial response to global change.
Highlights
Light is the primary driver of ice-associated phototrophy in polar marine ecosystems and variation in the quantity and quality of in-situ irradiance significantly influences primary production (Arrigo et al, 2008; Petrou et al, 2016)
After 72 h, H2O2 production in LL → HL plots reduced to a mean of 13.23 μmol H2O2 mg chla L−1 h−1, which was not significantly different compared to the other treatments (Figure 3A)
There was no significant difference in H2O2 production in treatments transitioning from high light to low light (30 cm snow; HL → LL) compared to the control
Summary
Light is the primary driver of ice-associated phototrophy in polar marine ecosystems and variation in the quantity and quality of in-situ irradiance significantly influences primary production (Arrigo et al, 2008; Petrou et al, 2016). The extreme variation of PAR in the sea-ice environment has forced many diatom species to evolve mechanisms that enable rapid adjustment of their photosynthetic apparatus to either increase photon capture in low light or minimize damage from excess irradiance (Mock et al, 2017; Kennedy et al, 2019). High interannual variability in irradiance (Turner et al, 2008) and the increased occurrence of extreme climatic events (Massom and Stammerjohn, 2010) in this region will influence both the timing and magnitude of light transmission and subsequently the development of ice-associated microbial communities (Deppeler and Davidson, 2017). Advances in the application of microsensors that measure fluctuations in both the rate of and products relating to photosynthesis [e.g., oxygen (O2), glucose, hydrogen peroxide (H2O2), nitric oxide (NO)], could provide a real-time indication of cell health for effectively mapping the in-situ physiological status of ice-associated microbial communities
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