The evil twin: Ocean acidification affects proteome of growing barnacles

Ocean acidification on the increase

Climate change is being driven by the phenomenal volumes of anthropogenic carbon dioxide emitted into the atmosphere, which is a key factor in the greenhouse effect and rising surface temperatures. However, there is a second climatic consequence of these emissions, dubbed "the evil twin of climate change" at the 2009 UN Climate Change Conference.

The culprit is ocean acidification, caused by the fact that the oceans absorb about half of the global carbon dioxide emissions. Without that, climate change would be far worse but it is having a detrimental effect on the oceans too.

Unlike atmospheric climate change, ocean acidification is easy to track. It is a slow, progressive, predictable change. Recent estimates put the current surface water uptake of carbon dioxide at 100 times quicker than the glacial age 20,000 years ago, which was the last time carbon dioxide levels rose sharply.

The pH value of the ocean has already fallen from 8.2 to 8.1 since the industrial revolution began, and is predicted to drop by another 0.3-0.5 pH units by the year 2100.

Marine organisms are known to be sensitive to minor changes in ocean conditions, as evidenced by the dying coral reefs of Australia. So, these large changes have the potential to be severely damaging.

Barnacles are an interesting species to examine. They are perceived as robust to ocean acidification and have to overcome various stressors in their transition from larvae to adult forms, including finding a suitable base and attaching to it.

A research team at the Swire Institute of Marine Science and School of Biological Sciences in The University of Hong Kong has been studying barnacles for the last few years. In their latest published work, they examined the proteome of the barnacle, reasoning that it might change in a specific way in response to ocean acidification.

Barnacle testing

Vengatesen Thiyagarajan and colleagues studied barnacles of the genus Balanus amphitrite which they collected from a pier in Kowloon. This species is known to exist over the pH range 7.3-8.6 which has been observed in the local surface coastal waters.

The adults were encouraged to release their larvae, which were studied over four days of growth from the swimming stage II to the competent larval stage. They were raised in media at the current ocean pH of 8.1 and the projected pH for the year 2100 of 7.6.

After four days, the proteins were extracted from whole larvae from the acidic and control tanks and separated by two-dimensional SDS-PAGE. They were visualised on the gel with three stains to highlight the total proteins, phosphoproteins and glycoproteins, respectively.

The proteins spots showing significant differences in intensity between the two pH pools were selected for mass spectrometric identification. In addition, the relative phosphorylation and glycosylation levels of the proteins were measured and those with changes were also identified.

Proteomic changes reflect increasing acidification

From a total of 566 proteins, 34% were phosphorylated and 23% were glycosylated but very few had altered abundances due to acidification. In fact, only nine proteins had altered expressions and seven of those were identified by mass spectrometry. The remaining two remained unidentified due to the lack of a sequenced genome for barnacles.

One of the four up-regulated proteins was elongation factor 2, implying that the more acidic environment triggers the production of stress response proteins. The lack of up-regulation of other translation-related proteins suggests that EF2 was selectively regulated.

Two haemoglobin beta chain-like proteins were also up-regulated, which might be in response to the reduced affinity for oxygen under oceanic acidification caused by higher concentrations of carbon dioxide in the water.

The final protein that was increased by lowering the pH was long-chain specific acyl CoA dehydrogenase, involved in energy production by fatty acid oxidation. It might have been up-regulated to aid the enhanced expression of the first three proteins.

The three down-regulated proteins included an EH domain-containing protein, which regulates endocytosis, and cathepsin L-like protein, which is a cysteine protease involved in the degradation of cell matrix proteins.

The third protein in this group was heat shock protein 83, a molecular chaperone which is involved in protein folding. One protein that was neither up- nor down-regulated but experienced an increase in phosphorylation was heat shock protein 90, also suggesting a change in molecular chaperone function.

Similarly, two bioenergetic enzymes displayed increased glycosylation at lower pH, which was explained in terms of a shift in energy towards the enhanced production of EF2 and haemoglobin-like proteins.

Thiyagarajan proposed that the set of seven proteins could be used as a protein expression signature for seawater acidification stress for barnacles. Their involvement in energy metabolism, respiration and molecular chaperone duties illustrates the mechanism by which the barnacles cope with pH stress. In practical terms, it might affect the ability of the larvae to attach to a surface and undergo metamorphosis to the adult form.

This work represents the first published study of changes to the proteome in a non-model larval species in response to oceanic acidification. The procedure shows promise for future studies on the response of other aquatic invertebrate larvae.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.