Shai Einbinder

Changes in scleractinian coral Seriatopora hystrix morphology and its endocellular Symbiodinium characteristics along a bathymetric gradient from shallow to mesophotic reef

The algae living endosymbiotically within coral are thought to increase algal pigmentation with increasing depth to capture the diminishing light. Here, we follow distribution of the hermatypic coral Seriatopora hystrix along a 60-m bathymetric gradient in the Gulf of Eilat, Red Sea, to study coral ecophysiology and response to light regimes. Combining work on coral morphology, pigment content and genotyping of the photosymbiont, we found that total chlorophyll concentration per zooxanthellae cell and the dark- and light-acclimated quantum yield of photosystem II did not vary significantly along the 60-m gradient. However, the chlorophyll a/c ratio increased with depth. This suggests that the symbiotic algae in S. hystrix possess a mechanism for acclimatization or adaptation that differs from previously described pathways. The accepted photoacclimatory process involves an increase in chlorophyll content per alga as light intensity decreases. Based on corallite and branch morphology, this research suggests that S. hystrix has two depth-dependent ecophenotypes. Above 10 m depth, S. hystrix exhibits sturdier colony configurations with thick branches, while below 30 m depth, colonies are characterized by thin branches and the presence of a larger polyp area. Between 10 and 30 m depth, both ecophenotypes are present, suggesting that corallite morphology may act as another axis of photoacclimation with depth.

A beneficial association between a polychaete worm and a scleractinian coral

Fig 1 Cyphastrea chalcidicum: a View of part of colony showingSpirobranchus giganteus burrows (arrowed), b close-up of burrow,c same burrow 1 year later. Scale bars=10 mmThe polychaete worm Spirobranchus giganteus(Serpulidae) is abundant in coral reefs, beingembedded in both stony corals and hydrozoans suchas Millepora sp. In a study at Eilat (Gulf of Aqaba,Red Sea), we observed a colony of the faviid coralCyphastrea chalcidicum which was almost com-pletely dead and covered with turf algae, apart fromthree small areas of living coral tissue which sur-rounded S. giganteus tubes (Fig. 1). After 1 year,one area of coral tissue grew rapidly to produce sixconcentric circles of living polyps while the otherareas died. In other examples, involving bleaching inFavia favus and predator damage to the coral colonyof Favia laxa, areas of coral tissue immediatelysurrounding the polychaete again showed no dam-age and the colonies quickly recovered. It is sug-gested that there may be some benefits to be gainedfor the coral from its association with polychaeteworms that live within the coral skeleton. These mayinvolve improved water circulation close to the coralsurface which would decrease susceptibility tobleaching (Nakamura et al. 2003), improved dis-persal of waste products of the coral host, andincreased availability of nutrients from waste mate-rials excreted by the associated fauna (Mokady et al.1998).ReferencesMokady O, Loya Y, Lazar B (1998) Ammonium contribution from boring bivalves to their coral host—a mutualisticsymbiosis? Mar Ecol Prog Ser 169:295–301Nakamura T, Yamasaki H, Van Woesik R (2003) Water flow facilitates recovery from bleaching in the coral Stylophorapistillata. Mar Ecol Prog Ser 256:287–291O. Ben-Tzvi (

Novel Adaptive Photosynthetic Characteristics of Mesophotic Symbiotic Microalgae within the Reef-Building Coral, Stylophora pistillata

Photosynthetic coral reef structures extend from the shallow sundrenched waters to the dimly lit, “twilight” mesophotic depths. For their resident endosymbiotic dinoflagellates, primarily from the genus Symbiodinium spp., this represents a photic environment that varies ~15 fold in intensity and also differs in spectral composition. We examined photosynthesis in the scleractinian coral Stylophora pistillata in shallow (3 m) and mesophotic settings (65m) in the northern Red Sea. Symbiodinium spp. in corals originating from the mesophotic environment consistently performed below their photosynthetic compensation point and also exhibited distinct light harvesting antenna organization. In addition, the non-photochemical quenching activity of Symbiodinium spp. from mesophotic corals was shown to be considerably lower than those found in shallow corals, showing they have fewer defenses to high-light settings. Over a period of almost four years, we extensively utilized closed circuit Trimix rebreather diving to perform the study. Phylogenetic analysis showed that shallow corals (3m) transplanted to a deep reef environment (65 m) maintained their initial Symbiodinium spp. community (clade A), rather than taking on deep low-light clades (clade C), demonstrating that shallow S. pistillata acclimate to low-light mesophotic environments while maintaining their shallow photosynthetic traits. Mesophotic corals exhibited static depth-related chlorophyll content per cell, a decrease in PSI activity and enhanced sigmoidal fluorescence rise kinetics. The sigmoidal fluorescence rise kinetics we observed in mesophotic corals is an indication of energy transfer between photosynthetic units. We postulate that at mesophotic depths, a community of adapted Symbiodinium spp. utilize a unique adaptation to lower light conditions by shifting their light harvesting to a PSII based system, where PSII is structured near PSI, with additional PCP soluble antenna also trapping light that is funneled to the PSI reaction center. In this study, we provide evidence that mesophotic Symbiodinium spp. have developed novel adaptive low-light characteristics consisting of a cooperative system for excitation energy transfer between photosynthetic units that maximizes light utilization.