Heritable variation and lack of tradeoffs suggest adaptive capacity in Acropora cervicornis despite negative synergism under climate change scenarios

Knowledge of multi-stressor interactions and the potential for tradeoffs among tolerance traits is essential for developing intervention strategies for the conservation and restoration of reef ecosystems in a changing climate. Thermal extremes and acidification are two major co-occurring stresses predicted to limit the recovery of vital Caribbean reef-building corals. Here, we conducted an aquarium-based experiment to quantify the effects of increased water temperatures and pCO2 individually and in concert on 12 genotypes of the endangered branching coral Acropora cervicornis, currently being reared and outplanted for large-scale coral restoration. Quantification of 12 host, symbiont and holobiont traits throughout the two-month-long experiment showed several synergistic negative effects, where the combined stress treatment often caused a greater reduction in physiological function than the individual stressors alone. However, we found significant genetic variation for most traits and positive trait correlations among treatments indicating an apparent lack of tradeoffs, suggesting that adaptive evolution will not be constrained. Our results suggest that it may be possible to incorporate climate-resistant coral genotypes into restoration and selective breeding programmes, potentially accelerating adaptation.

Chlorophyll fluorescence - A tool to assess photosynthetic performance and stress photophysiology in symbiotic marine invertebrates and seaplants

Chlorophyll a fluorescence is increasingly being used as a rapid, non-invasive, sensitive and convenient indicator of photosynthetic performance in marine autotrophs. This review presents the methodology, applications and limitations of chlorophyll fluorescence in marine studies. The various chlorophyll fluorescence tools such as Pulse-Amplitude-Modulated (PAM) and Fast Repetition Rate (FRR) fluorometry used in marine scientific studies are discussed. Various commonly employed chlorophyll fluorescence parameters are elaborated. The application of chlorophyll fluorescence in measuring natural variations, stress, stress tolerance and acclimation/adaptation to changing environment in primary producers such as microalgae, macroalgae, seagrasses and mangroves, and marine symbiotic invertebrates, namely symbiotic sponges, hard corals and sea anemones, kleptoplastic sea slugs and giant clams is critically assessed. Stressors include environmental, biological, physical and chemical ones. The strengths, limitations and future perspectives of the use of chlorophyll fluorescence technique as an assessment tool in symbiotic marine organisms and seaplants are discussed.

Finding of 13(2) , 17(3) -cyclopheophorbide a enol as a degradation product of chlorophyll in shrunk zooxanthellae of the coral Montipora digitata

We examined the morphology and pigment composition of zooxanthellae in corals subjected to normal temperature (27°C) and thermal stress (32°C). We observed several normal and abnormal morphological types of zooxanthellar cells. Normal cells were intact and their chloroplasts were unbroken (healthy); abnormal cells were shrunken and had partially degraded or broken chloroplasts, or they were bleached and without chloroplasts. At 27°C, most healthy zooxanthellar cells were retained in the coral tissue, whereas shrunken zooxanthellae were expelled. Under thermal stress, the abundance of healthy zooxanthellae declined and the proportion of shrunken/abnormal cells increased in coral tissues. The rate of algal cell expulsion was reduced under thermal stress. Within the shrunken cells, we detected the presence of a chl-like pigment that is not ordinarily found in healthy zooxanthellae. Analysis of the absorption spectrum, absorption maxima, and retention time (by HPLC) indicated that this pigment was 13(2) , 17(3) -cyclopheophorbide a enol (cPPB-aE), which is frequently found in marine and lacustrine sediments, and in protozoans that graze on phytoplankton. The production of cPPB-aE in shrunken zooxanthellae suggests that the chls have been degraded to cPPB-aE, a compound that is not fluorescent. The lack of a fluorescence function precludes the formation of reactive oxygen species. We therefore consider the formation of cPPB-aE in shrunken zooxanthellae to be a mechanism for avoiding oxidative stress.