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.
Radiographic images of a live European flounder accumulating carbonate precipitates in its gut. The fish on top was living in freshwater and lacks "gut rocks". The same fish is shown below after only three hours in seawater. Note the opacities in the gut resulting from the accretion of carbonates (white arrows).
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
Interesting.
There are many good reasons to consider curtailing fish consumption, including that many of the most highly sought after species are close to extinction because people are buying them in the supermarket.
There seems to be misunderstanding going on concerning the implications of Wilson et al results. According to Wilson et al data, fish actually exacerbate (rather than mitigate) the problem of acidification of the surface waters. Fish essentially strip the protons off the bicarbonate and excrete them into the water through gills, while the generated calcium carbonate sinks and either dissolves in the deeper water or gets buried in the sediment if the fish are in the shallows. Wilson et al say that this process may lead to the previously unexplained alkalinization of waters at 500-1000 m – which implies that the surface waters must get correspondingly acidified, just like being titrated with HCl. Moreover, Wilson et al predict that this process will be intensified as global CO2 rises. Bad news for coral reefs.
Misha – thanks for pointing out some of the complexities of this paper. The authors do not clearly state how the production of carbonates by fishes would affect ocean pH other than hypothesizing that these carbonates lead to the higher than expected alkalinity found in the upper layers of the ocean (shallower than 1000 m). The patchy nature of fish biomass and their presence over continental shelves (where as you point out carbonates may become buried in sediment before dissolving) and the effect of upwelling zones on carbonate distribution seem to make generalizing about the effects of these carbonates difficult. So in that sense, I probably did put too much stress on the conclusion that fish reduce the acidity of the ocean.
What is unclear to me is whether reduced fish biomass would lead to a decrease in overall ocean acidity. You point out that fish have a negative effect (increased acidity) in epipelagic waters. I suggested that the production of these carbonates would have an overall net effect of decreasing acidity in the ocean as a whole. These don’t seem mutually exclusive.
Of course any heterotroph is going to raise acidity by producing CO2 during its metabolism and releasing it into the environment. The activity of carbonic anhydrase will also promote the production of H+, also increasing acidity. Fish would produce bicarbonate ions even if they did not make carbonates in their gut. Does the increased carbonic anhydrase activity needed to make these gut carbonates so outweigh the buffering effects of the carbonates that the fish have a net acidifying effect? If it is true that all heterotrophs have a net acidifying effect, does gut carbonate production in fish mitigate this in surface waters, or make it worse? What about in the entire ocean?