Untangling the molecular basis of coral response to sedimentation

Abstract

Urbanized coral reefs are often chronically affected by sedimentation and reduced light levels, yet many species of corals appear to be able to thrive under these highly disturbed conditions. Recently, these marginal ecosystems have gained attention as potential climate change refugia due to the shading effect of suspended sediment, as well as potential reservoirs for stress‐tolerant species. However, little research exists on the impact of sedimentation on coral physiology, particularly at the molecular level. Here, we investigated the transcriptomic response to sediment stress in corals of the family Merulinidae from a chronically turbid reef (one genet each of Goniastrea pectinata and Mycedium elephantotus from Singapore) and a clear‐water reef (multiple genets of G. pectinata from the Gulf of Aqaba/Eilat). In two ex‐situ experiments, we exposed corals to either natural sediment or artificial sediment enriched with organic matter and used whole‐transcriptome sequencing (RNA sequencing) to quantify gene expression. Analysis revealed a shared basis for the coral transcriptomic response to sediment stress, which involves the expression of genes broadly related to energy metabolism and immune response. In particular, sediment exposure induced upregulation of anaerobic glycolysis and glyoxylate bypass enzymes, as well as genes involved in hydrogen sulphide metabolism and in pathogen pattern recognition. Our results point towards hypoxia as a probable driver of this transcriptomic response, providing a molecular basis to previous work that identified hypoxia as a primary cause of tissue necrosis in sediment‐stressed corals. Potential metabolic and immunity trade‐offs of corals living under chronic sedimentation should be considered in future studies on the ecology and conservation of turbid reefs.

Transcriptomic signatures across a critical sedimentation threshold in a major reef-building coral

“…Our analyses identified significant changes in gene expression profiles related to energy metabolism, immune responses, and apoptosis across a previously identified critical threshold of sedimentation across which coral mortality increases and community changes significantly in Fouha Bay, southern Guam (Minton, Burdick and Brown, 2022). Our results obtained from corals growing under natural field conditions are in agreement with prior experimental work conducted under laboratory conditions (Bollati et al ., 2021), suggesting that the previously proposed acclimation mechanisms to turbid, sedimentation-impacted reef ecosystems are acting on an ecologically relevant scale.…”

“…GO terms associated with aerobic energy generation for Porites lobata were enriched in the outer zone compared to the inner zone (Fig. 4a), further indicating that corals in the inner zone experienced hypoxic stress, as can be expected under conditions of severe sedimentation (Weber et al ., 2012; Bollati et al ., 2021). In ex situ sedimentation experiments, Bollati et al (2021) found genes associated with an anaerobic glycolytic pathway upregulated in two coral species.…”

Characterizing transcriptomic responses to sediment stress across location and morphology in reef-building corals

“…Stressful conditions have made these kinds of enzymes more active in plants and corals (Bouchard and Yamasaki 2008;Zdunek-Zastocka and Sobczak 2013). Upregulation of metabolism related genes was also observed in a study that examined transcriptomic responses of corals in response to two different sediment experiments (Bollati et al 2021). Downregulation of Mo-molybdopterin cofactor synthesis may indicate that the coral does not have enough energy to synthesize molybdopterin which in turn makes it so the activity of these specific enzymes is decreased or stopped altogether, suggesting a decrease in metabolism for A. cervicornis.…”

“…While there were some methodological differences between their experiments, shared mechanisms were identified across different species and populations, demonstrating a conserved response to sediment stress across species and sediment types. However, the corals evaluated in that study, G. pectinata and M. elephantotus, both have similar morphological characteristics (massive and encrusting), indicating that this response may only be relevant to massive and encrusting corals (Bollati et al 2021…”

“…uPAR is associated with proteolysis, wound healing and inflammation, and is hypothesized to contribute to coral tissue remodeling in response to elevated levels of sedimentation (Morgan, Edge, and Snell 2005). Genes related to immunity, as well as energy metabolism, were also implicated in the transcriptomic response to sediment stress in corals from two locations, Singapore ( Goniastrea pectinata and Mycedium elephantotus ) and Eilat, Israel ( G. pectinata only) using RNASeq (Bollati et al 2021). While there were some methodological differences between their experiments, shared mechanisms were identified across different species and populations, demonstrating a conserved response to sediment stress across species and sediment types.…”