Climate & Heat Stress | del Campo Lab - Microbial Ecology and Evolution

A transcriptomic cell-atlas of reef coral bleaching

Sun, 30 Jun 2024 00:00:00 +0000

Coral reefs support hyperdiverse ecosystems and build natural offshore structures that protect our coastlines from storm surge and coastal inundation. The corals that build these structures are able to do so by forming intimate symbioses with dinoflagellates in the family Symbiodiniaceae that provide photosynthetically fixed carbon to the coral host as an energy source. However, when ocean temperatures increase (particularly during marine heatwaves that are becoming more intense due to climate change), this symbiosis breaks down, the algae are expelled, and corals lose their coloration — a phenomenon popularly known as bleaching. As a result of bleaching, corals can quickly suffer mass mortality, destabilising the entire reef ecosystem and threatening its survival.

After decades of research on bleaching, our understanding of the genomic basis for bleaching — for corals, their algal symbionts, and other partners (i.e., the coral holobiont) — remains very limited. What is missing is a granular understanding of the genomics of bleaching linked to the functional changes that occur during the process. To fill this knowledge gap, we need to comprehensively characterise the transcriptomic response of the coral holobiont at the single-cell level in order to capture the functional genomic response of the different members of the coral holobiont during bleaching events.

Our goal is to develop an approach that can be used to scrutinise the transcriptomic response of the coral holobiont at the single-cell level. Our central hypothesis is that not all coral and symbiont cell types have the same transcriptomic profile, and therefore they will not have the same response to bleaching. Having an integrated, granular understanding of the coral holobiont’s transcriptomic response across coral cell types and symbionts during a bleaching event will transform our understanding of coral bleaching and potentially help us identify better strategies for intervention.

Preprint

Bonacolta AM, et al., including Javier del Campo (2024). A Single-Cell Atlas of Coral Bleaching. Research Square.

The rhythm of the reef — Diel cycles of the coral holobiont

Sun, 30 Jun 2024 00:00:00 +0000

Coral physiology is intimately dependent on diel cycles, as fluctuations in light intensity and spectrum, nutrient availability, and temperature directly change the respiration and energy assimilation capabilities of the coral colony. As complex sessile organisms, corals harbour incredibly diverse microbial assemblages of prokaryotes and microeukaryotes that provide functional roles in the coral ecosphere. Corals and Symbiodiniaceae have co-evolved, resulting in a symbiotic life strategy that benefits the coral host through the intracellular production of organic compounds such as carbon and oxygen. During sunlit hours, photosynthetic efficiency has been observed to increase oxygen levels ~250% of air saturation, stimulating hyperoxic environments. Corals and their aerobic microbial associates consume available oxygen through the night, where levels can become hypoxic. Given this extreme variation in abiotic parameters, it is likely that microbial communities reflect measurable changes through diel cycles, while cellular mechanisms help corals adapt to these extreme abiotic shifts.

This project explored three scleractinian corals — Pseudodiploria strigosa, Orbicella faveolata, and Diploria labyrinthiformis — through three diel cycles to characterise their microbial associates (including surrounding seawater) and the host transcriptional response. Coral samples were collected in triplicate and 1L reference seawater samples were acquired off the leeward side of Curaçao at 6-hour intervals over three days, resulting in a total of 3 replicated diel cycles. DNA was isolated, amplified using the V4 region of the 16S and 18S rRNA genes, and sequenced using Illumina MiSeq 2×250bp; RNA was sequenced using poly-A selection/enrichment.

We hypothesise that microbial taxa follow trends in abundance with respect to anoxic and hyperoxic conditions, and that circadian genes such as cry1, cry2, clock, and cycle show diel oscillations in expression. This work was published in Cell Host & Microbe (Weiler et al. 2026).