Ocean acidification: sea urchin proteome response

Living with ocean acidification

The Earth’s oceans are suffering from an increase in acidification as they absorb the carbon dioxide that we produce from human activity, the key one being the burning of fossil fuels. Ocean absorption accounts for about half of the anthropogenic carbon dioxide. Despite the increase in acidity, the actual pH of the oceans remains alkaline but less so than before, having altered from 8.2 to 8.1 since the industrial revolution. It is expected to fall by another 0.3-0.4 points by the end of the century. These changes might seem small but it is important to remember that the pH scale is logarithmic, not linear.

The natural removal of carbon dioxide from the atmosphere by the oceans was thought to be a good thing at first but now it is clear that there are long-term consequences to marine life as the pH is modified. Coral reefs are at risk since more acidic water corrodes the coral skeletons and slows the growth of new ones. Similarly, shelled creatures such as mussels and clams find it more difficult to grow new shells. Conversely, crustaceans such as crab, lobster and shrimp will benefit because they grow stronger shells under increased ocean acidity due to a different growth mechanism.

In this context, sea urchins have been extensively studied due to their need to calcify as well as their critical role in controlling the local seaweed and seagrass abundances. Their immune cells have been described as “the sentinels of environmental stress” and the effects of elevated carbon dioxide on them have been evaluated in a number of studies conducted in the lab or in artificial ocean conditions. In order to measure the effects under natural conditions, an international group of scientists has now carried out a detailed study of sea urchins living near carbon dioxide vents in the ocean, comparing their immune responses and protein profiles with those living at non-vent sites.

Carbon dioxide oceanic vents

The team comprised researchers from Italy, Australia and New Zealand and the results were published by senior reporter Anna Palumbo from the Anton Dohrn Zoological Station in Naples. They collected Paracentrotus lividus sea urchins living near carbon dioxide vents in the Tyrrhenian Sea off Italy as well as from two control sites of similar habitat and aquatic conditions, apart from the pH. Due to their limited travel habits, the vent urchins were considered to have adapted to life in their local environments.

The immune cells were harvested from the fluid in the coelome cavity and the proteins were characterised by liquid chromatography/mass spectrometry in tandem mode. This procedure identified 588 proteins in total, of which 311 were differentially expressed between the urchins from the three sites. About 20% of the proteome was exclusive to urchins from the individual sites but the key comparisons were between the vent creatures and those from non-vent areas.

The protein functions indicated a strong shift towards antioxidant processes in response to the more acidic conditions and this was accompanied by increased total antioxidant activity. The researchers attributed these findings to “a general cellular protective mechanism involved in the adaptation to stress conditions.” The ammonium metabolic pathway was also stimulated under vent conditions as shown by the raised levels of enzymes involved in amino acid degradation. In addition, proteins associated with phagosome and microsomal activity and carbon metabolism were affected by the reduced pH conditions in the sea.

Acidic adaptation

Several other factors were also measured. The number of immune cells in the vent urchins was the same for one of the control sites. Higher numbers at the second control site might be caused by local stressors that remain to be identified.
Other properties of the sea urchins such as the pH of the coelome fluid and the concentrations of lipid hydroperoxide and nitric oxide were unchanged. The animal respiration system, nitrogen excretion and skeletal mineralogy were also the same for vent and non-vent urchins.

Taken together, these observations strongly suggest that the sea urchins have adapted to life near the vent in the more acidic waters. So, as the oceans become more acidic in general, rather than solely at the vent sites, it is highly likely that the creatures will be able to adapt, at least to the same level of acidity. These findings back up reported research on sea urchin adaptation, but carry more weight because they were conducted on oceans from the vent fields.

Steve Down, 6 June 2019, Spectroscopy Now. Article.

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