The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach

Abstract

Bleaching of reef corals is a phenomenon linked to temperature stress which involves loss of the symbiotic algae of the coral, which are known as zooxanthellae, and/or loss of algal pigments. The photosynthetic efficiency of zooxanthellae within the corals Montastrea annularis, Agaricia lamarki, Agaricia agaricites and Siderastrea radians was examined by pulse‐amplitude modulation fluorometry (PAM) during exposure to elevated temperatures (30–36°C). Zooxanthellae within M. annularis and A. lamarki were found to be more sensitive to elevated temperature, virtually complete disruption of photosynthesis being noted during exposure to temperatures of 32 and 34°C. The photosynthetic efficiency of zooxanthellae within S. radians and A. agaricites decreased to a lesser extent. Differences in the loss of algal cells on an aerial basis and in the cellular chlorophyll concentration were also found between these species. By combining the non‐invasive PAM technique with whole‐cell fluorescence of freshly isolated zooxanthellae, we have identified fundamental differences in the physiology of the symbionts within different species of coral. Zooxanthellae within M. annularis appear to be more susceptible to heat‐induced damage at or near the reaction centre of Photosystem II, while zooxanthellae living in S. radians remain capable of dissipating excess excitation energy through non‐photochemical pathways, thereby protecting the photosystem from damage during heat exposure.

Different thermal sensitivity of the repair of photodamaged photosynthetic machinery in cultured Symbiodinium species

“…Moderate heat stress accelerates photoinhibition of symbiotic Symbiodinium cells within corals, and the sensitivity to heat stress can vary among host coral species (3). It has been demonstrated that the variation in the sensitivity of photoinhibition to heat stress among corals is associated with sensitivity of the repair of photodamaged PSII (21).…”

“…In Symbiodinium isolated from the jellyfish Cassiopeia xamachana, photosynthesis is inhibited at temperatures above 30°C and completely disrupted at 34-36°C (7). Subsequent studies have shown that reduced photosynthesis caused by thermal stress in Symbiodinium can be attributed to photoinhibition of PSII (3,4). It has also been proposed that the Calvin cycle is the primal site of heat stress, and interruption of the Calvin cycle causes acceleration of photoinhibition (8).…”

“…Moderate heat stress causes photoinhibition of photosystem II (PSII), and this decreases the efficiency of the photosynthesis. Because coral bleaching follows severe photoinhibition, impairment of the photosynthetic machinery has been proposed to be a trigger of coral bleaching (3,4). Coral bleaching, caused by ongoing global warming, has become more frequent and has resulted in extensive coral mortality and the destruction of diverse coral ecosystems worldwide.…”

Host–symbiont recombination versus natural selection in the response of coral–dinoflagellate symbioses to environmental disturbance

“…Analysis of active chlorophyll fluorescence by PAM fluorometry indicated no significant change in the maximal quantum yield of photosystem II (Fv/Fm) (Warner et al 1996) in bleached and unbleached colonies relative to measurements taken during the same season in preceding years (data not shown), indicating that the symbionts in bleached corals had recovered physiologically. One hundred per cent of colonies harbouring Symbiodinium C1b-c were bleached, with Symbiodinium numbers significantly lower relative to densities calculated before bleaching and to colonies harbouring D1 symbionts (figure 1b,c; p , 0.01; Kruskal-Wallis test).…”

Bioindicators of changes in water quality on coral reefs: review and recommendations for monitoring programmes

“…Symbionts acclimatised to high irradiances are characterised by high PS max and E k , with low a, but the opposite occurs for symbionts acclimatised to low-irradiance as the symbionts attempt to optimise their light capture and utilisation capability (Anthony and Hoegh-Guldberg 2003;Cooper and Ulstrup 2009). However, these parameters are influenced by variation in seawater temperature (Coles and Jokiel 1977;Warner et al 1996;Fitt et al 2001), flow regime (Nakamura et al 2005), diurnal changes in benthic irradiance (Jones and Hoegh-Guldberg 2001;Lesser and Gorbunov 2001) and symbiont genotype (Frade et al 2008;Hennige et al 2009). Thus, these factors must be accounted for when using symbiont photophysiology to infer changes in water quality.…”