When teaching marine biology I warn my students that if they are there to just learn about sharks and dolphins they will be sorely disappointed, because only microscopic plankton have the biomass to really affect the oceans. Being an ichthyologist this always hurt a bit. A recent paper in Science has restored my faith that all that microscopic stuff is just fish food – fish CAN change the world. Better yet, this story involves some animal comparative physiology.
First a little background on how we are killing our oceans. The same CO2 that is accumulating in the atmosphere from the combustion of fossil fuels and other sources, leading to global warming, is diffusing into the oceans and changing their pH. When CO2 reacts chemically with H2O, H+ ions are released making water more acidic. This declining pH is already adversely affecting marine organisms, which are often adapted to a narrow pH range. Calcium carbonate, however, can react with CO2 and limit the drop in pH. The production of calcium carbonate by microscopic organisms like coccolithophores is thought to be the major player in this regulation of ocean pH.
So where do the fish come in? Research on the toadfish, Opsanus beta, showed that this fish produces little calcium carbonate rocks in its digestive tract. Subsequent physiological research showed that the production of these “gut rocks” was involved in the absorption of water in the gut. Marine fish are less salty than the surrounding ocean. Water, therefore, diffuses out of the fish into their environment leaving them very thirsty. But when they drink they fill their guts with salty water, which would pull fluids from their bodies leaving them even thirstier. It is for a similar reason that you should not drink ocean water when stranded in a life raft (that’s when you drink your own urine instead). But the fish apparently have a trick. They accrete some of the salts in their urine as carbonate precipitates, lowering the salinity of the water in their gut and facilitating its absorption. And then the fish defecate the rocks.
But would this calcium carbonate release affect the Ocean’s pH balance considering the relatively low biomass of fishes compared to plankters like the coccolithophores? In their paper Wilson et al. also calculate the total biomass of fishes in the Ocean and the amount of calcium carbonate they produce. While these types of calculations require a good number of assumptions, the authors claim that their conservative estimate is that fishes produce 3-15% of the Ocean’s calcium carbonate.
So next time PETA tries to convince you not to eat fish because they are cute, tell them a better reason is that fish poop could help save the marine ecosystem.
R. W. Wilson, F. J. Millero, J. R. Taylor, P. J. Walsh, V. Christensen, S. Jennings, M. Grosell (2009). Contribution of Fish to the Marine Inorganic Carbon Cycle Science, 323 (5912), 359-362 DOI: 10.1126/science.1157972