Seasonal variation modulates coral sensibility to heat-stress and explains annual changes in coral productivity.

The potential effects of seasonal acclimatization on coral sensitivity to heat-stress, has received limited attention despite differing bleaching thresholds for summer and winter. In this study, we examined the response of two contrasting phenotypes, termed winter and summer, of four Caribbean reef corals to similar light and heat-stress levels. The four species investigated were categorized into two groups: species with the ability to harbour large number of symbionts, Orbicella annularis and O. faveolata, and species with reduced symbiont density (Montastraea cavernosa and Pseudodiploria strigosa). The first group showed higher capacity to enhance photosynthetic rates per area (Pmax), while Pmax enhancement in the second group was more dependent on Symbiodinium performance (Psym). In summer all four species presented higher productivity, but also higher sensitivity to lose coral photosynthesis under heat-stress. In contrast, corals in winter exhibit symbionts with higher capacity to photoacclimate to the increased levels of light-stress elicited by heat-stress. Overall, our study supports the importance of the acclimatory coral condition in addition to the previous thermal history, to determine the severity of the impact of heat-stress on coral physiology, but also the dependence of this response on the particular structural and functional traits of the species.

Absorptance determinations on multicellular tissues.

The analysis of the variation of the capacity and efficiency of photosynthetic tissues to collect solar energy is fundamental to understand the differences among species in their ability to transform this energy into organic molecules. This analysis may also help to understand natural changes in species distribution and/or abundance, and differences in species ability to colonize contrasting light environments or respond to environmental changes. Unfortunately, the challenge that optical determinations on highly dispersive samples represent has strongly limited the progression of this analysis on multicellular tissues, limiting our knowledge of the role that optical properties of photosynthetic tissues may play in the optimization of photosynthesis and growth of benthonic primary producers. The aim of this study is to stimulate the use of optical tools in marine eco-physiology, offering a succinct description of the more convenient tools and also solutions to resolve the more common technical difficulties that arise while performing optical determinations on highly dispersive samples. Our study focuses on two-dimensional (2D-) parameters: absorptance, transmittance, and reflectance, and illustrates with correct and incorrect examples, specific problems and their respective solutions. We also offer a general view of the broad variation in light absorption shown by photosynthetic structures of marine primary producers, and its low association with pigment content. The ecological and evolutionary functional implications of this variability deserve to be investigated across different taxa, populations, and marine environments.