Freshwater organisms are especially vulnerable to environmental changes as they are exposed to both atmospheric changes as well as run-off from nearby cities; in particular, rising temperatures and increasing carbon dioxide levels from both natural factors (rainfall, geology, etc) as well as human influence (deforestatin, agriculture, urbanization). For example, studies of rivers around the world have found that carbon dioxide levels vary from 647 – 38,000 µatm. Higher levels are also typically observed in warmer, drier seasons.
Freshwater mussel populations are decreasing. A new study published in American Journal of Physiology – Regulatory, Integrative and Comparative Physiology wanted to determine whether increasing water temperatures and carbon dioxide could be at least partly to blame. Typically researchers focus on only one stressor, which means they may miss how multiple stressors interact and impact a population. Since both temperature and carbon dioxide levels may threaten mussel populations, the research team decided to study both. To do that, they exposed juvenile freshwater mussels (Lampsilis siliquoidea) to temperatures ranging from 22-34°C (current water temperatures in the Southern US can be up to 35-40°C) and carbon dioxide concentrations of ~230 – 58,000 µatm – (levels of carbon dioxide predicted near planned fish deterrent barriers).
They found that animals exposed only to higher temperatures had elevated metabolic rates and increased levels of an enzyme that helps buffer the blood, carbonic anhydrase. Under increased temperature conditions, this enzyme is thought to help the animals grow larger by promoting mineralization. These mussels also had increases in heat shock protein 70, which helps the body deal with damage to proteins, although mortality was still higher with increasing temperatures.
Responses to increased environmental carbon dioxide also elevated levels of the enzyme carbonic anhydrase as a result of the animals becoming more acidic. The enzyme works by converting carbon dioxide and water to biocarbonate and hydrogen ions, the latter of which can be disposed to raise pH back to normal.
When animals were exposed to both stressors, they lost the ability to respond to the heat stress, resulting in higher mortality rates. Interestingly, larger mussels were better able to adapt to both heat and carbon dioxide stress. These findings show that juvenile mussels are more sensitive to environmental changes. Not a good sign for sustainability of the population.
Jeffrey JD, Hannan KD, Hasler CT, Suski CD. Hot and bothered: effects of elevated Pco2 and temperature on juvenile freshwater mussels. American Journal of Physiology -Regulatory, Integrative and Comparative Physiology. 315(1): R115-R127, 2018.https://doi.org/10.1152/ajpregu.00238.2017