The role of the endolithic alga Ostreobium spp. during coral bleaching recovery

Abstract

In this study, we explore how the Caribbean coral Orbicella faveolata recovers after bleaching, using fragments from 13 coral colonies exposed to heat stress (32 °C) for ten days. Biological parameters and coral optical properties were monitored during and after the stress. Increases in both, the excitation pressure over photosystem II (Qm) and pigment specific absorption (a*Chla) were observed in the stressed corals, associated with reductions in light absorption at the chlorophyll a red peak (De675) and symbiont population density. All coral fragments exposed to heat stress bleached but a fraction of the stressed corals recovered after removing the stress, as indicated by the reductions in Qm and increases in De675 and the symbiont population observed. This subsample of the experimentally bleached corals also showed blooms of the endolithic algae Ostreobium spp. underneath the tissue. Using a numerical model, we quantified the amount of incident light reflected by the coral, and absorbed by the different pigmented components: symbionts, host-tissue and Ostreobium spp. Our study supports the key contribution of Ostreobium spp. blooms near the skeletal surface, to coral recovery after bleaching by reducing skeleton reflectance. Endolithic blooms can thus significantly alleviate the high light stress that affects the remaining symbionts during the stress or when the coral has achieved the bleached phenotype.

Greater functional diversity and redundancy of coral endolithic microbiomes align with lower coral bleaching susceptibility

“…Under normal conditions, these photosynthetic algae receive very low light levels [24] and are well adapted to photosynthesize in near-darkness [25]. However, when corals bleach following environmental stress, the expulsion of the intracellular Symbiodiniaceae cells from coral tissues results in an increase in the amount of light reaching Ostreobium, enabling these endolithic algae to rapidly photo-acclimate and bloom [26][27][28]. Endoliths can directly interact with the coral tissue through the transfer of photosynthates and constitute a source of energy or nutrients for the host during environmental stress [29][30][31].…”

Genomic view of the diversity and functional role of archaea and bacteria in the skeleton of the reef-building corals Porites lutea and Isopora palifera

“…form conspicuous green bands in the skeletons of several coral species. Ostreobium can play an active role in both providing carbon substrates to coral hosts during thermal stress-induced bleaching and facilitating coral recovery [26][27][28]. Endolithic microbes have also been reported to actively participate in nutrient recycling and primary productivity [13,29].…”

54 years of microboring community history explored by machine learning in a massive coral from Mayotte (Indian Ocean)

“…Within the past few years, several studies have thus investigated the genetic diversity of the endolithic microbiome, and especially that of the dominant euendolith, the chlorophyte Ostreobium sp. (Marcelino and Verbruggen, 2016;Sauvage et al, 2016;Yang et al, 2016;Del Campo et al, 2017;Masséet al, 2020) and its possible implications in coral growth, physiology and photoprotection (Sangsawang et al, 2017;Masséet al, 2018;Galindo-Martıńez et al, 2022). Conversely, the species composition, distribution and abundance of microboring communities in living corals remain poorly known and most studies focused only on communities located within the first few centimeters below coral tissues of adult colonies (Odum and Odum, 1955;Lukas, 1973;Kühl and Polerecky, 2008;Fordyce et al, 2021;Galindo-Martıńez et al, 2022).…”