New paper in PLOS Biology: Symbiotic gut bacteria may drive carbonate production in marine fish

A new Discovery Report from our lab, led by Anthony Bonacolta and published in PLOS Biology, reveals that symbiotic bacteria living in the guts of marine fish may play a critical — and largely overlooked — role in the oceanic carbon cycle. 🐟🦠🪨

We are excited to share our latest work in PLOS Biology!

Symbiotic bacteria may support calcium carbonate precipitation in the Gulf toadfish
Bonacolta AM*, Kravitz T*, Mozo R, Baker LJ, Heuer RM, Grosell M†, del Campo J† (2026). Symbiotic bacteria may support calcium carbonate precipitation in the Gulf toadfish. PLOS Biology 24(5): e3003764. (*equal contribution, †senior authors)

Marine fish are already known to produce calcium carbonate (CaCO₃) precipitates — called ichthyocarbonates — in their guts as part of their osmoregulation strategy. Collectively, marine fish may produce as much as 9 Pg of CaCO₃ per year, a figure that rivals or exceeds the contributions of coccolithophores and foraminifera. Yet the biological mechanisms behind this mineralisation have remained poorly understood.

One intriguing observation has long been hiding in plain sight: bacteria are consistently found in close association with ichthyocarbonate precipitates. Could they be more than bystanders?

To find out, we subjected the Gulf toadfish (Opsanus beta) to different salinity treatments known to modulate CaCO₃ precipitation, and combined microbiome characterisation across the gut with meta-transcriptomic analysis to ask what the host — and its microbiota — were actually doing.

Key findings:

• Across the toadfish gut, we identify a high abundance of Vibrio bacteria directly associated with ichthyocarbonate precipitates and possessing the metabolic potential for CaCO₃ precipitation.
• We observe expression of the transcriptional activator of urease (ureR) by Photobacterium damselae subsp. damselae, a pathway that drives bicarbonate production and can directly induce CaCO₃ precipitation.
• This suggests that carbonate formation in marine fish guts may not be a purely host-driven process, but rather the outcome of a functional symbiosis with gut-associated bacteria.

The parallel with corals is striking: just as photosymbionts enable corals to build reefs, fish and their microbial partners may be synergistically contributing to oceanic carbonate production. If confirmed at scale, this discovery expands our understanding of how symbiosis shapes marine biomineralization and global biogeochemical cycles.

Huge congratulations to Anthony, Tristan, Rocío, and all co-authors — and to the teams at the Rosenstiel School and IBE who made this work possible! 🎉