Nitric oxide production rather than oxidative stress and cell death is associated with the onset of coral bleaching in Pocillopora acuta

Abstract

Elevated seawater temperatures associated with climate change lead to coral bleaching. While the ultimate causes of bleaching are well understood, the proximate physiological mechanisms underlying the bleaching response are not as well defined. Here we measured nitric oxide synthase activity, oxidative stress, and cell death in algal symbionts (Symbiodinaceae) freshly isolated from the reef-building coral Pocillopora acuta collected in the field under natural non-bleaching conditions and from corals experimentally exposed to elevated temperatures. Nitric oxide synthase activity in the algal symbionts was >3 orders of magnitude higher than that of the host and increased dramatically with increasing temperature and time of exposure (up to 72 h), consistent with the onset of bleaching for these corals. Oxidative stress and cell death among the algal symbionts were highest in coral holobionts exposed to intermediate as opposed to maximal temperatures, suggesting that these mechanisms are not proximal triggers for bleaching in this species. Our results point to nitric oxide production by the algal symbionts, rather than symbiont dysfunction, as a more important driver of coral bleaching under acute thermal stress in this coral.

Keywords: Climate change; Coral bleaching; Coral-algal symbiosis; Nitric oxide; Oxidative stress.

Figure 1. Plot of data from field sampling shown according to sampling date (scale is June 1, 2006–September 1, 2006).

Percentage of symbionts from each coral sample positively stained with Sytox green (cell death) shown as black bars in (A), percentage stained with H₂DCF (oxidative stress) shown as black bars in (B). Staining data only for corals collected mid-Aug to early-Sep. Nitric oxide synthase activity shown as black bars in (C) and temperature shown as black line in (D). NOS activity data for corals collected early-June to mid-August. Bars are mean ± sd, n = 1–3 corals per time point.

Figure 2. Barplot of data from the thermal stress experiment shown according to temperature treatment (28.1, 28.6, 29.9 °C) and sampling time (24, 72 h).

Sytox green staining (cell death) shown in (A) and H₂DCF staining (oxidative stress) shown in (B). Bars are mean ± sd, n = 6–9 coral nubbins per treatment. Nitric oxide synthase activity shown in (C). Bars are mean ± sd, n = 2–3 trials per treatment. Groups sharing letters are not statistically different based on Tukey HSD post hoc results.