No abstract

Community dynamics and physiology of Symbiodinium associated with Orbicella (5 Montastraea) faveolata were examined before, during, and after a thermally induced coral bleaching event in Puerto Morelos, Mexico. We combined microsampling molecular genotyping with in situ pulse-amplitude modulated fluorometry to correlate colony variability of Symbiodinium population identities and the phenomena of partial coral bleaching. Pigmented nonbleached portions of O. (5M.) faveolata were compared with bleached portions of the same colony. During bleaching, maximum quantum yield of photosystem II (PSII; F v : F m ) was significantly lower and highly variable (range 0.110 to 0.680) compared with previous summers in which coral bleaching was absent (range 0.516 to 0.661) and recovery (range 0.480 to 0.716). Differential susceptibility to environmental perturbation of F v : F m corresponded to distinct genetic identities of Symbiodinium. Analysis of ribosomal deoxyribonucleic acid (rDNA) internal transcribed spacer 2 (ITS2) revealed regions of the coral colonies that had phylotype A3 prior to bleaching were more resistant to the bleaching perturbation than adjacent bleaching-prone patches that harbored phylotypes B17 and C7. During environmental perturbation, regions of the colonies containing predominantly Symbiodinium phylotypes A3 or D1a retained significantly higher F v : F m values than adjacent regions with phylotypes B17 and C7. Following bleaching, rapid recovery of symbiotic algal densities greatly exceeded normal seasonal oscillations. During recovery we document shifts in Symbiodinium populations and increase prevalence of Symbiodinium types A3 and D1a, phylotypes known to have enhanced thermal tolerances. Thermal tolerance of Symbiodinium spp. influences the changes of coral-Symbiodinium communities during disturbance events and the dynamics of coral-Symbiodinium repopulation.

Coral reefs of the Florida Keys typically experience seasonal temperatures of 20-31°C. Deviation outside this range causes physiological impairment of reef-building corals, potentially leading to coral colony death. In January and February 2010, two closely spaced cold fronts, possibly driven by an unusually extreme Arctic Oscillation, caused sudden and severe seawater temperature declines in the Florida Keys. Inshore coral reefs [e.g., Admiral Reef (ADM)] experienced lower sustained temperatures (i.e., < 12°C) than those further offshore [e.g., Little Grecian Reef (LG), minimum temperature = 17.2°C]. During February and March 2010, we surveyed ADM and observed a mass die-off of reef-building corals, whereas 12 km away LG did not exhibit coral mortality. We subsequently measured the physiological effects of low-temperature stress on three common reef-building corals (i.e., Montastraea faveolata, Porites astreoides, and Siderastrea siderea) over a range of temperatures that replicated the inshore cold-water anomaly (i.e., from 20 to 16 to 12°C and back to 20°C). Throughout the temperature modulations, coral respiration as well as endosymbiont gross photosynthesis and maximum quantum efficiency of photosystem II were measured. In addition, Symbiodinium genotypic identity, cell densities, and chlorophyll a content were determined at the beginning and conclusion of the experiment. All corals were significantly affected at 12°C, but species-specific physiological responses were found indicating different coral and/or Symbiodinium cold tolerances. Montastraea faveolata and P. astreoides appeared to be most negatively impacted because, upon return to 20°C, significant reductions in gross photosynthesis and dark respiration persisted. Siderastrea siderea, however, readily recovered to pre-treatment rates of dark respiration and gross photosynthesis. Visual surveys of inshore reefs corroborated these results, with S. siderea being minimally affected by the cold-water anomaly, whereas M. faveolata and P. astreoides exhibited nearly 100% mortality. This study highlights the importance of understanding the physiological attributes of genotypically distinct coral-Symbiodinium symbioses that contribute to tolerance, recovery, and consequences to an environmental perturbation. These data also document effects of a rarely studied environmental stressor, possibly initiated by remote global climate events, on coral-Symbiodinium symbioses and coral reef communities.

Increased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994–2007), eleven years in the Exuma Cays, Bahamas (1995–2006), and four years in Puerto Morelos, Mexico (2003–2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1–4 m) compared to deeper-dwelling conspecifics (12–15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels.

High sea surface temperatures often lead to coral bleaching wherein reef-building corals lose significant numbers of their endosymbiotic dinoflagellates (Symbiodiniaceae). These increasingly frequent bleaching events often result in large scale coral mortality, thereby devasting reef systems throughout the world. The reef habitats surrounding Palau are ideal for investigating coral responses to climate perturbation, where many inshore bays are subject to higher water temperature as compared with offshore barrier reefs. We examined fourteen physiological traits in response to high temperature across various symbiotic dinoflagellates in four common Pacific coral species, Acropora muricata , Coelastrea aspera , Cyphastrea chalcidicum and Pachyseris rugosa found in both offshore and inshore habitats. Inshore corals were dominated by a single homogenous population of the stress tolerant symbiont Durusdinium trenchii , yet symbiont thermal response and physiology differed significantly across coral species. In contrast, offshore corals harbored specific species of Cladocopium spp. (ITS2 rDNA type-C) yet all experienced similar patterns of photoinactivation and symbiont loss when heated. Additionally, cell volume and light absorption properties increased in heated Cladocopium spp., leading to a greater loss in photo-regulation. While inshore coral temperature response was consistently muted relative to their offshore counterparts, high physiological variability in D . trenchii across inshore corals suggests that bleaching resilience among even the most stress tolerant symbionts is still heavily influenced by their host environment.