Abstract
BackgroundReef corals are heterotrophic coelenterates that achieve high productivity through their photosynthetic dinoflagellate symbionts. Excessive seawater temperature destabilises this symbiosis and causes corals to “bleach,” lowering their photosynthetic capacity. Bleaching poses a serious threat to the persistence of coral reefs on a global scale. Despite expanding research on the causes of bleaching, the mechanisms remain a subject of debate.Methodology/Principal FindingsThis study determined how light and food availability modulate the effects of temperature stress on photosynthesis in two reef coral species. We quantified the activities of Photosystem II, Photosystem I and whole chain electron transport under combinations of normal and stressful growth temperatures, moderate and high light levels and the presence or absence of feeding of the coral hosts. Our results show that PS1 function is comparatively robust against temperature stress in both species, whereas PS2 and whole chain electron transport are susceptible to temperature stress. In the symbiotic dinoflagellates of Stylophora pistillata the contents of chlorophyll and major photosynthetic complexes were primarily affected by food availability. In Turbinaria reniformis growth temperature was the dominant influence on the contents of the photosynthetic complexes. In both species feeding the host significantly protected photosynthetic function from high temperature stress.Conclusions/SignificanceOur findings support the photoinhibition model of coral bleaching and demonstrate that PS1 is not a major site for thermal damage during bleaching events. Feeding mitigates bleaching in two scleractinian corals, so that reef responses to temperature stresses will likely be influenced by the coinciding availabilities of prey for the host.
Highlights
Coral bleaching is a global issue that threatens the persistence of coral reefs, the world’s most diverse marine ecosystems [1,2,3]
Pigment and protein content For Stylophora, chlorophyll a content was mainly affected by food availability: chl was significantly higher in fed nubbins of this species (Fig. 2A, Table 1) than in unfed nubbins maintained at the same (Fig. 2B) or higher light levels (Fig. 2C)
Using simultaneous measurements of the function of photosystem II and photosystem I of coral symbionts, in combination with measures of total colony oxygen production and chlorophyll content and the content of three key proteins within the symbiont photosynthetic apparatus, we demonstrate that photosystem 1 (PS1) is comparatively robust to temperature stress
Summary
Coral bleaching is a global issue that threatens the persistence of coral reefs, the world’s most diverse marine ecosystems [1,2,3]. Over the past 15 years, several major bleaching events have occurred around the globe and caused widespread mortality of corals [4]. Bleaching events are caused by anomalously high sea surface temperatures that exceed the thermal thresholds for corals and their symbionts photosynthetic dinoflagellates from the genus Symbiodinium commonly known as zooxanthellae. In the presence of moderate to high light intensities, excessive temperature damages the photosynthetic machinery of the zooxanthellae, leading to a decrease in the size and/or pigmentation of the symbiont population and causing the characteristic white colour of bleached corals [7]. Bleaching poses a serious threat to the persistence of coral reefs on a global scale. Despite expanding research on the causes of bleaching, the mechanisms remain a subject of debate
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