In situ photobiology of corals over large depth ranges: A multivariate analysis on the roles of environment, host, and algal symbiont

Abstract

We applied a multivariate analysis to investigate the roles of host and symbiont on the in situ physiological response of genus Madracis holobionts towards light. Across a large depth gradient (5-40 m) and for four Madracis species and three symbiont genotypes, we assessed several variables by measuring chlorophyll a fluorescence, photosynthetic pigment composition, or symbiont population descriptors. Most of the variation is explained by two major photobiological components: light-use efficiency and symbiont cell densities. Two other minor components emphasize photoprotective pathways and light-harvesting properties such as secondary pigments. Statistics highlight the role of irradiance on coral physiology and reveal mechanisms that are either genetically constrained, such as symbiont cell sizes, or environmentally dependent, such as photochemical efficiencies. Other parameters, such as cellular light-harvesting and photoprotective pigment concentrations, are regulated by host, symbiont, and environment. The interaction between host and environment stresses the role of host properties in adjusting the internal environment available for the endosymbionts. Different holobiont strategies, relating to symbiont cell density, vary in their physiological optimization of light-harvesting or photoprotective mechanisms and link to host-species distribution and dominance over the reef slope. Symbiont functional diversity appears to have a significant role but does not explain host vertical distribution patterns per se, highlighting the importance of species-specific morphological and physiological properties of the coral host.

Mesophotic coral depth acclimatization is a function of host-specific symbiont physiology

“…Some general trends of light acclimatization could be observed for all central Red Sea corals, and these trends were similar to those reported from studies conducted in other oceans. For instance, the decrease in symbiont cell densities and increase in chl a was comparable to that of other corals over similar depth gradients (Frade et al, 2008a;Cooper et al, 2011), and data from the deep mesophotic Red Sea suggest that the trend continues until corals become azooxanthellate (Fricke et al, 1987;Kaiser et al, 1993).…”

“…In the case of the coral species Madracis pharensis, a study showed that coral colonies switched from a generalist symbiont type (B7) to a depth specialist (B15) in deeper water (Frade et al, 2008b). The latter symbiont contained more photosynthetic pigments per cell and had different pigment ratios, possibly explaining the superior photophysiological efficiency in its host compared to symbiont-generalist hosting coral species (Frade et al, 2008a). This does not seem to be a universal mechanism though (Cooper et al, 2011;Bongaerts et al, 2013), as generalist symbiont types, like e.g., type C1, can have large bathymetric distributions and are found in shallow and mesophotic coral communities alike (Chan et al, 2009;Cooper et al, 2011).…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…For example, Smith et al (2017b) identified the importance of photoconvertible red fluorescent proteins (pcRFPs) to support coral adaptation/acclimation to deeper waters, while Cooper et al (2011) showed that both the host and symbiont needed to have photobiological flexibility to survive across light regimes. Similarly, Lesser et al (2010) found for the coral Montastrea cavernosa in the Bahamas that both physiological and morphological adaptation was required to survive in depths down to 91 m. Changes in the endosymbiont community (e.g., Frade et al, 2008;Lesser et al, 2010;Bongaerts et al, 2015) and density (Frade et al, 2008) have also been reported with depth. Specialization of deeper coral communities suggests plasticity will be required for species to survive across depths and ultimately utilize these systems as potential refugia.…”

Dimethylsulfoniopropionate in corals and its interrelations with bacterial assemblages in coral surface mucus

“…3). Symbiont abundances (per coral surface area), higher in M. meandrites than in the other two species, are known to be a species-specific trait and probably related to threedimensional space availability within the coral tissue [8] and to the genetically constrained cell size of particular symbiont lineages. [52] Although Symbiodinium numbers inside the coral tissue are known to vary over time and space for individual species, [36,53] neither time of the day nor depth explained their variation in the current study.…”

“…Zooxanthellae (Symbiodinium) were visible and gated as a distinct population of events with strong fluorescence signal and high forward scatter (proportional to cell diameter). [8] Symbiont abundances determined by flow cytometry (Accuri C6) were used to standardise DMSP concentrations per cell assuming the zooxanthellae are principally responsible for DMSP production within the coral symbiosis. For standardisation of DMSP relative to coral surface area, the bare skeleton chips resulting from the tissue blasting procedure were used.…”