No abstract

The giant sea anemone Condylactis gigantea associates with members of two clades of the dinoflagellate alga Symbiodinium, either singly or in mixed infection, as revealed by clade-specific quantitative polymerase chain reaction of large subunit ribosomal DNA. To explore the functional significance of this molecular variation, the fate of photosynthetically fixed carbon was investigated by (14)C radiotracer experiments. Symbioses with algae of clades A and B released ca. 30-40% of fixed carbon to the animal tissues. Incorporation into the lipid fraction and the low molecular weight fraction dominated by amino acids was significantly higher in symbioses with algae of clade A than of clade B, suggesting that the genetically different algae in C. gigantea are not functionally equivalent. Symbioses with mixed infections yielded intermediate values, such that this functional trait of the symbiosis can be predicted from the traits of the contributing algae. Coral and sea anemone symbioses with Symbiodinium break down at elevated temperature, a process known as 'coral bleaching'. The functional response of the C. gigantea symbiosis to heat stress varied between the algae of clades A and B, with particularly depressed incorporation of photosynthetic carbon into lipid of the clade B algae, which are more susceptible to high temperature than the algae of clade A. This study provides a first exploration of how the core symbiotic function of photosynthate transfer to the host varies with the genotype of Symbiodinium, an algal symbiont which underpins corals and, hence, coral reef ecosystems.

To investigate the molecular diversity of symbiotic algae at the latitudinal limits of their distribution, the ribosomal RNA gene sequences (rDNA) of the dinoflagellate Symbiodinium in benthic Cnidaria (corals, sea anemones etc.) on Bermuda (32°N) and in the Mediterranean and NE Atlantic (35 to 53°N) were analysed. The algae in Bermudian Cnidaria were identified as Symbiodinium of Phylotypes A, B and C, as previously described for benthic Cnidaria in the Caribbean (12 to 27°N). The algae in every sample of sea anemones (Anemonia spp. and Cereus pedunculatus) in the NE Atlantic and Mediterranean were a previously undescribed group within Phylotype A, possibly endemic to this high latitude region.

Bleaching of corals by loss of symbiotic dinoflagellate algae and/or photosynthetic pigments is commonly triggered by elevated temperatures coupled with high irradiance, and is a first-order threat to coral reef communities. In this study, a high-resolution high-performance liquid chromatography method integrated with mass spectrometry was applied to obtain the first definitive identification of chlorophyll and carotenoid pigments of three clades of symbiotic dinoflagellate algae ( Symbiodinium ) in corals, and their response to experimentally elevated temperature and irradiance. The carotenoids peridinin, dinoxanthin, diadinoxanthin (Dn), diatoxanthin (Dt) and b -carotene were detected, together with chlorophylls a and c 2 , and phaeophytin a , in all three algal clades in unstressed corals. On exposure to elevated temperature and irradiance, three coral species ( Montastrea franksi and Favia fragum with clade B algae, and Montastrea cavernosa with clade C) bleached by loss of 50-80% of their algal cells, with no significant impact to chlorophyll a or c 2 , or peridinin in retained algal cells. One species ( Agaricia sp. with clade C) showed no significant reduction in algal cells at elevated temperature and irradiance, but lost substantial amounts of chlorophyll a and carotenoid pigments, presumably through photo-oxidative processes. Two coral species ( Porites astreoides and Porites porites both bearing clade A algae) did not bleach. The impact of elevated temperature and irradiance on the levels of the photoprotective xanthophylls (Dn + Dt) and b -carotene varied among the corals, both in pool size and xanthophyll cycling, and was not correlated to coral bleaching resistance.