Ocean Acidification and Warming Alter Skeletal Mineralization

As far as I know, this is the first study to quantify the effect of ocean acidification and warming on skeleton mineralization of a water-breathing vertebrate. Overall, simulated ocean acidification increased the density of hydroxyapatite in major components of the skeleton of elasmobranchs. This result stands in sharp contrast with most previous studies in which the calcium carbonate exoskeleton of marine invertebrate showed a decrease in mineralization with ocean acidification. Even though some invertebrate species are able to maintain an elevated pH at their site of calcification to maintain constant calcification rates, this comes at a cost. In a review of more than 40 studies on the effect of ocean acidification on mollusks, the emerging pattern is that shell acidification is reduced at low pH levels. When the shell or skeleton of calcifying invertebrates is thinner and therefore is more susceptible to fracture, predation pressure increases for these animals. While in marine invertebrates the reduction in CaCO3 as a consequence of low pH is due to a relatively straightforward chemical reaction (i.e. H+ precipitate Ca2+ in the skeleton), the possible mechanism underlying the increase in HA in the skeleton of vertebrates needs further investigation. Here, I outline a few likely mechanisms responsible for the increase in mineralization in the skeleton of elasmobranch fishes.

First, CO2 is a poison, known to affect the neurotransmitter responsible for regulation of Ca2+ deposition. In particular, otolith growth and calcification has been shown to be regulated neurally. A study of neuronal control of the calcium supply in the inner ear of cichlids showed a significant precipitation of calcium accumulated at the macular tight junctions. It is in fact hypothesized that neuronal activity is responsible for the release of calcium into the endolymph of the otolith by altering the permeability of these junctions. Although cartilage tissue is not deeply innervated, there is evidence that sensory nerve fibers are in contact with the outer layer of cartilage. Neuronal activity, therefore, may influence cartilage matrix mineralization.

Another likely mechanism responsible for increased mineralization of cartilage with ocean acidification may be linked to the physiological response of fishes when challenged with fluid acidosis. Fishes are able to compensate for changes in acid-base status within their extracellular body fluids by reducing acidosis with bicarbonate and non-bicarbonate buffers. In fact, fish gills respond fairly rapidly to acidification by increasing phosphate in body fluids. In skates, the increase in buffer capacity is proportional to pH decrease in the water, and skates can achieve acidity compensation within 24 hours. Perhaps this exceptional buffering capacity may have the side effect of accumulating phosphate in blood and tissues, including the cartilage. Therefore, CO2-induced acidosis may indirectly contribute to increased skeletal HA density, by increasing phosphate concentration in fish blood. Higher HA density in simulated acidified oceans is, at first, an unexpected result when compared to results from studies on calcifying invertebrates, but a plausible consequence of effective buffering capacity of elasmobranchs during acidosis. In fact, denticles on the skin of sharks are not affected by acidification. A similar results was also demonstrated in a previous study on benthic sharks, supporting the hypothesis that acidification plays a significant role in physiological processes related to mineralization, rather than a direct effect on the physical structure of the skeleton per se, as instead seen in shell-forming invertebrates.

Interestingly, warming had an opposite effect on skeleton mineralization. At higher temperature, HA density decreased in the skates’ skeleton structures that relate to locomotion, i.e., the crura, vertebrae, and wings. The regulation of cartilage mineralization in fishes requires a more in-depth analysis to fully understand the underpinning mechanisms involved in the response to warming. However, one plausible explanation is that a decrease in mineralization with temperature in these skeletal structures might be associated with changes at the cellular and molecular level. In fact, temperature can directly affect cartilage growth by altering gene expression. Some of these genes might be involved in regulation of metabolism, vascularization, matrix production, receptors, channels, and enzymes. Such cellular processes can, in turn, affect growth by changing the physical and biomechanical properties of blood vessels thus affecting the transport of nutrients and minerals. Previous studies on the effect of climate-related stressors showed that skates challenged by warming exhibited a lower body condition and stressed skates depressed metabolic rates beyond their optimal conditions of temperature, pH , and oxygen. Therefore, a reduced mineralization of the skeleton may be an indirect effect of temperature, which reduced the aerobic scope and therefore the energy available for growth and physiological processes such as skeletal mineralization. It is indeed interesting that the only measured significant differences in mineralization linked to temperature are in fact seen in structures associated with growth of the animal (wing and vertebra). This is a least suggest that perhaps mineralization is linked to physiological processes related to digestion and growth, which are already known to be profoundly affected by temperature in fishes. Warming could increase growth rates of ectotherms, however, mineralization of the cartilage might not be able to keep up at similar rates. This study shows that there seem to be no obvious functional trade-offs of ocean acidification and warming on skeletal mineralization. In other words, the measured changes in skeletal density appear to be side effects of possible neurological, physiological and ecological stressors on fishes which need to maintain homeostasis during acid-base and thermal challenges.