Mud made of fish poop

Seafloor sediment composed of ground up shells and coral also contains a significant amount of fish poop, according to new linkurl:research;https://www.pnas.org/content/early/2011/02/15/1015895108.abstract published on February 21 in Proceedings of the National Academy of Sciences.This research shows that mineralization in fishes' intestines composes around 14 percent of carbonate mud in the shallow seas of the Bahamas, sediment that is subsequently transformed into the limestone geologists use to reconstruct earth's climate.

Crystallized carbonate from fish intestine under an electron microscope
Image: Chris Perry, Manchester Metropolitan University

"Compositions of sediments are used very widely for trying to deduce past climatic and environmental conditions," said linkurl:Chris Perry,;https://www.sste.mmu.ac.uk/users/cperry/Tropicalcoastal/chrisperry.htm a sedimentologist at Manchester Metropolitan University and coauthor of the study. "This could completely change peoples' thinking perhaps about the kind of pathways by which mud is produced."Scientists have known that fish crystallize carbonates in their intestines and release them into the water, said linkurl:Robert Riding,;https://robertriding.com/ a geologist who studies algal and bacterial carbonates at the University of Tennessee and was not involved in the research, but this is the first time fish poop has been shown to exist in the seafloor. "What's new, of course, is that they're looking at the Bahamas and being more specific [about what happens to those carbonates] -- putting a bit more flesh on the bone." Most carbonate mud is composed of structures once belonging to marine organisms, such as corals' calcium carbonate skeletons and mollusks' carbonate shells. Even some single-celled phytoplankton form plates around their body using calcium carbonate, which eventually contribute to the sediment. Over geologic time, the mud is compacted and cemented by pressure into the limestone that characterizes coastal habitats.Because the ability of these organisms to produce the carbonate-based structures is affected by climatic variables such as light, temperature, and nutrients, scientists can analyze various qualities of the limestone to estimate an area's climate history. But identifying the source of the particles in carbonate mud is critical to these analyses, and the new research suggests that scientists have long been missing a main contributor -- fish.Fish are constantly drinking and filtering seawater, converting minerals such as calcium and magnesium into carbonates and excreting them as mucus-bound fecal pellets. In open water, these crystals can provide food for other organisms, or dissolve into the water column, affecting ocean chemistry, but because of the great depth, they rarely make it to the seafloor in their crystalline state. In shallower waters, however, this may not be the case. To tackle this question, Perry teamed up with linkurl:Rod Wilson,;https://biosciences.exeter.ac.uk/staff/index.php?web_id=rod_wilson a fish physiologist at the University of Exeter, and together they collected fish fecal pellets in the shallow seas of the Bahamas to sample the carbonate crystals and compare them to the crystals found in the sediment. Sure enough, the same crystals found in fish poop were also seen in sediment across multiple habitats, including seagrass beds, reefs, and fringe mangrove areas. (See the slideshow below for examples of these crystals.)"I really wasn't very optimistic, but we found them everywhere," said Perry. Using the rates of production of different fish species and their distributions, they calculated the contribution of fish carbonates to mud at 14 percent across these habitats, varying from under 1 percent in low cover areas to 70 percent in fringe mangrove areas.Because these crystals look the same as those produced by other marine organisms, Ridings was skeptical about their precise calculation of 14 percent, and noted that geologists have a great deal of work ahead of them to fully understand the carbonate record. Still, he added, this new knowledge that fish have been contributing to sediments over the past 100 million years shows that fish are an important and unrecognized source of carbonates. Examples of fish crystal precipitates

free slideshow

Photos provided by Chris PerryPerry, C.T., et al, "Fish as major carbonate mud producers and missing components of the tropical carbonate factory," Proceedings of the National Academy of Sciences, DOI: linkurl:10.1073/pnas.1015895108;https://www.pnas.org/content/early/2011/02/15/1015895108.abstract
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[22nd February 2011]