Abstract
Balanophyllia europaea is a shallow water solitary zooxanthellate coral, endemic to the Mediterranean Sea. Extensive field studies across a latitudinal temperature gradient highlight detrimental effects of rising temperatures on its growth, demography, and skeletal characteristics, suggesting that depression of photosynthesis at high temperatures might cause these negative effects. Here we test this hypothesis by analyzing, by means of pulse amplitude modulation fluorometry, the photosynthetic efficiency of B. europaea specimens exposed in aquaria to the annual range of temperatures experienced in the field (13, 18, and 28°C), and two extreme temperatures expected for 2100 as a consequence of global warming (29 and 32°C). The indicators of photosynthetic performance analyzed (maximum and effective quantum yield) showed that maximum efficiency was reached at 20.0–21.6°C, slightly higher than the annual mean temperature in the field (18°C). Photosynthetic efficiency decreased from 20.0 to 13°C and even more strongly from 21.6 to 32°C. An unusual form of bleaching was observed, with a maximum zooxanthellae density at 18°C that strongly decreased from 18 to 32°C. Chlorophyll a concentration per zooxanthellae cell showed an opposite trend as it was minimal at 18°C and increased from 18 to 32°C. Since the areal chlorophyll concentration is the product of the zooxanthellae density and its cellular content, these trends resulted in a homogeneous chlorophyll concentration per coral surface across temperature treatments. This confirms that B. europaea photosynthesis is progressively depressed at temperatures >21.6°C, supporting previous hypotheses raised by the studies on growth and demography of this species. This study also confirms the threats posed to this species by the ongoing seawater warming.
Highlights
Reef-building corals are highly dependent on their symbiotic relationship with photosynthetic dinoflagellates of the genus Symbiodinium, commonly referred to as zooxanthellae (Brandt, 1881)
The loss of zooxanthellae, due to environmental stresses, impacts coral energy and carbon budget, and may result in death if the stress is severe and prolonged (Glynn, 1996; Lesser, 2011), unless the symbiosis is re-established from remaining zooxanthellae (Koren et al, 2008)
Despite the mass of studies on tropical corals documenting both positive (Jacques et al, 1983) and negative (Jones et al, 2000; Nakamura et al, 2003) effects of elevated temperatures on photosynthesis, few studies have investigated the response of temperate coral symbionts (Jacques et al, 1983; Ben-Haim et al, 1999; Jones et al, 2000; Nakamura et al, 2003; Rodolfo-Metalpa et al, 2006)
Summary
Reef-building corals are highly dependent on their symbiotic relationship with photosynthetic dinoflagellates of the genus Symbiodinium, commonly referred to as zooxanthellae (Brandt, 1881). Along the Italian coastline, its net calcification rate is negatively correlated with sea surface temperature (SST; Goffredo et al, 2009), resulting in a progressive decrease of skeletal bulk density (Goffredo et al, 2007) and an increase in skeletal porosity (Caroselli et al, 2011), especially of larger sized pores (Fantazzini et al, 2013). This determines a decrease of the resistance of the skeleton to mechanical stress (Goffredo et al, 2015). Given the threats for the survival of this species in light of global temperature increase (Goffredo et al, 2008, 2009; Caroselli et al, 2011), the present study aims at analyzing for the first time the response of photosynthesis of the symbionts of B. europaea to the whole range of temperatures naturally experienced in the field and projected for the future, to verify if photosynthesis is depressed at high temperatures
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