Amphimedon queenslandica microbiome changes with host health

Marine sponges provide an excellent model to study microbial community changes in relation to host health in the face of climate change. Here, I contribute a study using a tropical coral reef sponge Amphimedon queenslandica (Demospongiae, Haplosclerida, Niphatidae), with a microbiome already partially characterized, making it a particularly valuable focal species. Using electron microscopy, I characterized the four dominant bacterial morphotypes (M1, M3, M4 and M5) present within healthy and compromised adult sponge tissue. SEM revealed that the microbiome of A. queenslandica shifts from healthy grey samples to compromised brown and bleached ones. I identified four health states (state 1, state 2, state 3a and state 3b) each corresponding to a distinct tissue quality and bacterial community composition. The Chromatiale bacteria M1 dominates the microbiota in healthy (state 1) samples and is completely absent from unhealthy brown (state 2) samples. I propose that this bacterial morphotype regulates the bacterial community composition of A. queenslandica. In the absence of M1, smaller M4 bacteria become the dominant morphotype within compromised samples, along with the putative spirillum-type bacteria M5. In bleached samples of state 3a, M1 re-colonises the sponge tissue, possibly leading to resilience. In other bleached samples of state 3b, M4 and M5 are present whilst M1 remains absent, as tissue appears extremely degraded. This study reveals the crucial role that micro-symbionts play in adaptation and survival of the host.

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