It has been over 10 years since the phenomenon of extensive coral bleaching was first described. In most cases bleaching has been attributed to elevated temperature, but other instances involving high solar irradiance, and sometimes disease, have also been documented. It is timely, in view of our concern about worldwide reef condition, to review knowledge of physical and biological factors involved in bleaching, the mechanisms of zooxanthellae and pigment loss, and the ecological consequences for coral communities. Here we evaluate recently acquired data on temperature and irradiance-induced bleaching, including long-term data sets which suggest that repeated bleaching events may be the consequence of a steadily rising background sea temperature that will in the future expose corals to an increasingly hostile environment. Cellular mechanisms of bleaching involve a variety of processes that include the degeneration of zooxanthellae in situ, release of zooxanthellae from mesenterial filaments and release of algae within host cells which become detached from the endoderm. Photo-protective defences (particularly carotenoid pigments) in zooxanthellae are likely to play an important role in limiting the bleaching response which is probably elicited by a combination of elevated temperature and irradiance in the field. The ability of corals to respond adaptively to recurrent bleaching episodes is not known, but preliminary evidence suggests that phenotypic responses of both corals and zooxanthellae may be significant.

/pdf/coral-bleaching-causes-and-consequences-5ewp5zv1gs.pdf

Coral bleaching: causes and consequences

Cnidarian bleaching is a breakdown in the mutualistic symbiosis between host Cnidarians, such as reef building corals, and their unicellular photosynthetic dinoflagellate symbionts. Bleaching is caused by a variety of environmental stressors, most notably elevated temperatures associated with global climate change in conjunction with high solar radiation, and it is a major contributor to coral death and reef degradation. This review examines the underlying cellular events that lead to symbiosis dysfunction and cause bleaching, emphasizing that, to date, we have only some pieces of a complex cellular jigsaw puzzle. Reactive oxygen species (ROS), generated by damage to both photosynthetic and mitochondrial membranes, is shown to play a central role in both injury to the partners and to inter-partner communication of a stress response. Evidence is presented that suggests that bleaching is a host innate immune response to a compromised symbiont, much like innate immune responses in other host-microbe interactions. Finally, the elimination or exit of the symbiont from host tissues is described through a variety of mechanisms including exocytosis, host cell detachment and host cell apoptosis.

/pdf/cellular-mechanisms-of-cnidarian-bleaching-stress-causes-the-foaxvldk2k.pdf

Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis

SUMMARY The symbiosis between cnidarians (e.g., corals or sea anemones) and intracellular dinoflagellate algae of the genus Symbiodinium is of immense ecological importance. In particular, this symbiosis promotes the growth and survival of reef corals in nutrient-poor tropical waters; indeed, coral reefs could not exist without this symbiosis. However, our fundamental understanding of the cnidarian-dinoflagellate symbiosis and of its links to coral calcification remains poor. Here we review what we currently know about the cell biology of cnidarian-dinoflagellate symbiosis. In doing so, we aim to refocus attention on fundamental cellular aspects that have been somewhat neglected since the early to mid-1980s, when a more ecological approach began to dominate. We review the four major processes that we believe underlie the various phases of establishment and persistence in the cnidarian/coral-dinoflagellate symbiosis: (i) recognition and phagocytosis, (ii) regulation of host-symbiont biomass, (iii) metabolic exchange and nutrient trafficking, and (iv) calcification. Where appropriate, we draw upon examples from a range of cnidarian-alga symbioses, including the symbiosis between green Hydra and its intracellular chlorophyte symbiont, which has considerable potential to inform our understanding of the cnidarian-dinoflagellate symbiosis. Ultimately, we provide a comprehensive overview of the history of the field, its current status, and where it should be going in the future.

https://ir.library.oregonstate.edu/downloads/2z10wr382

Cell Biology of Cnidarian-Dinoflagellate Symbiosis

During the past decade, acute and chronic bleaching of tropical reef corals has occurred with in- creasing frequency and scale. Bleaching, i.e., the loss of pigment and the decrease in population density of sym- biotic dinoflagellates (zooxanthellae), is often correlated with an increase or decrease in sea surface temperature. Because little is known of the cellular events concomitant with thermal bleaching, we have investigated the mech- anism of release of zooxanthellae by the tropical sea ane- mone Aiptasia pulchella and the reef coral Pocillopora damicornis in response to cold and heat stress. Both spe- cies released intact host endoderm cells containing zoox- anthellae. The majority of the released host cells were viable, but they soon disintegrated in the seawater leaving behind isolated zooxanthellae. The detachment and re- lease of intact host cells suggests that thermal stress causes host cell adhesion dysfunction in these cnidarians. Knowledge of the cellular entity released by the host dur- ing bleaching provides insight into both the underlying release mechanism and the way in which natural envi- ronmental stresses evoke a bleaching response.

Temperature Stress Causes Host Cell Detachment in Symbiotic Cnidarians: Implications for Coral Bleaching

The declining health of coral reefs worldwide is likely to intensify in response to continued anthropogenic disturbance from coastal development, pollution, and climate change. In response to these stresses, reef-building corals may exhibit bleaching, which marks the breakdown in symbiosis between coral and zooxanthellae. Mass coral bleaching due to elevated water temperature can devastate coral reefs on a large geographical scale. In order to understand the molecular and cellular basis of bleaching in corals, we have measured gene expression changes associated with thermal stress and bleaching using a complementary DNA microarray containing 1310 genes of the Caribbean coral Montastraea faveolata. In a first experiment, we identified differentially expressed genes by comparing experimentally bleached M. faveolata fragments to control non-heat-stressed fragments. In a second experiment, we identified differentially expressed genes during a time course experiment with four time points across 9 days. Results suggest that thermal stress and bleaching in M. faveolata affect the following processes: oxidative stress, Ca2+ homeostasis, cytoskeletal organization, cell death, calcification, metabolism, protein synthesis, heat shock protein activity, and transposon activity. These results represent the first medium-scale transcriptomic study focused on revealing the cellular foundation of thermal stress-induced coral bleaching. We postulate that oxidative stress in thermal-stressed corals causes a disruption of Ca2+ homeostasis, which in turn leads to cytoskeletal and cell adhesion changes,

http://medinalab.org/new/wp-content/uploads/2013/10/desalvo_etal2008_MolEcol.pdf

Differential gene expression during thermal stress and bleaching in the Caribbean coral Montastraea faveolata.

Darknessevokes expulsion ofzooxanthellae from the sea anemone Aiptasia pu! chella, but brief exposure to low temperature(4Â°C, 4 h) increasesthe expulsion rate four-fold Ninety-eight percent of the zooxanthellae are expelled within four days Low temperature incubation had no detectable effect on host animal survival or be havior, but the effect on zooxanthellae was profound Low temperaturereduced the rateofphotosynthesis, increasedthe rateofrelease offixed carbon, reducedthe num ber of viable cells and decreased the rate of cell division Electron micrographsre vealed that low temperatureelicited the appearanceofa thickened cell envelope and novel electron-denseinclusions, tentatively identified as crystallizedlipoprotein Im mediately after low temperature incubation zooxanthellae move towards the apex of the host cells and are releasedto the coelenteron by exocytosis The mechanisms by which low temperaturemay evoke exocytosis ofzooxanthellae (eg, disassembly of host cell microtubules; increased concentration of cytosolic calcium ions) are dis cussed

https://www.jstor.org/stable/pdfplus/1541797.pdf

Low temperature evokes rapid exocytosis of symbiotic algae by a sea anemone

Sea anemones (Aiptasia pulchella) fed 35S methionine labeled food translocated labeled material to symbiotic zooxanthellae. This uptake of host-derived organic materials by the symbiotic algae is interpreted as evidence for heterotrophy. It is calculated that 2.8-6.4% of the zooxanthellae growth requirement for protein was satisfied by heterotrophy under a 12 hour light:12 hour dark photoperiod. The cost of algal heterotrophy to the animal host was demonstrated by exposing aposymbiotic anemones to cultured zooxanthellae. Aposymbiotic animals rapidly re-established a stable symbiosis, but the symbiotic animals had a protein biomass significantly lower than control aposymbionts, suggesting that the reinfection of the symbiotic animals by zooxanthellae caused a decrease in animal biomass. Zooxanthellae spontaneously appeared and grew in the control aposymbionts. These repopulating algal cells were apparently derived from a residual heterotrophic population of zooxanthellae maintained by anemones after four y...

Evidence for heterotrophy by zooxanthellae in symbiosis with aiptasia pulchella

We tested the hypothesis that some zoanthids are able to meet a portion of their daily respiratory carbon requirement with photosynthetic carbon from symbiotic algal cells (= zooxanthellae). A daily budget was constructed for carbon (C) photosyn thetically fixed by zooxanthellae of the Bermuda zoanthids Zoanthus sociatus and Palythoa variabilis. Zooxanthellae have an average net photosynthetic C fixation of 7.48 and 15.56 @igC . polyp' . day' for Z. sociatus and P. variabilis respectively.The C-specificgrowth rate (zr) was 0.215 @ day' for Z. sociatus and 0.152 â€¢?� day' for P. variabiis. The specific growth rate (it) of zooxanthellae in the zoanthids was measured to be 0.011 and 0.017 . day' for Z. sociatus and P. variabilis zooxantheHaerespectively. Z. sociatus zooxanthellae translocated 95. 1% of the C assimilated in photosynthesis, while P. variabilis zooxanthellae translocated 88.8% of their fixed C. As the animal tissue of a polypofZ. sociatusrequired14.75 @zgC @ daf' forrespiration,and one ofP. variabiis required 105.54 ;sgC â€˜?� day', the contribution of zooxanthellae to animal respiration (CZAR) was 48.2% for Z. sociatus and 13.1% for P. variabilis. Ir@rritoDUcrioN Zoanthids (Cnidaria:Zoanthidea) are conspicuous and abundant members of shal low intertidalreef flat communities where scleractiniancoral growthis not extensive (Sebens, 1982). Zoanthids contain dense populations of symbiotic algae (=zooxan thellae) which can fix carbon and translocatephotosynthetic productsto the animal host (Von Holt and Von Holt, 1968; Trench, 1974). Sebens (1977) has recently described the natural diet ofthe zoanthids Zoanthus sociatus and Palythoa variabiis at study sites in Panama and Brazil. Although the species exist in close proximity to each other, P. variabiis fed extensively on demersal zooplankton while Z. sociatus fed sparinglyon smalldetritalparticles.Sebens(1977) also demonstratedthat although z. sociatus lost weight when starved, significantly less weight was lost when starved in light sufficient for photosynthesis by symbiotic zooxanthellae. Because Z. sociatus fed on fewer, smaller food items than P. variabilis, Sebens (1977) conjectured that z. sociatus polyps acquired alarger proportion oftheir daily carbon requirement from zooxanthellae than did P. variabilispolyps.

R. Grant Steen

Papers

Low temperature evokes rapid exocytosis of symbiotic algae by a sea anemone

Evidence for heterotrophy by zooxanthellae in symbiosis with aiptasia pulchella

DAILY BUDGETS OF PHOTOSYNTHETICALLY FIXED CARBON IN SYMBIOTIC ZOANThIDS