A bad acid trip

Ocean acidification is a major threat to the world’s oceans. While global warming is relatively well known, ocean acidification has been dubbed the “evil twin” of global warming due to the fact that it is also caused by anthropogenic emissions of CO₂ in the atmosphere.  

About one quarter of the atmospheric CO₂ is absorbed by the oceans’ surface waters which alters the chemistry of the ocean. The process is making the oceans less alkaline and is termed ocean “acidification”.  

Since 2005 there has been a boom in research assessing the impacts of ocean acidification, which has demonstrated that its main effect is to alter the creation of calcium carbonate skeletons of many marine taxa from phytoplankton, through to molluscs, corals and calcifying seaweed. Coral reefs are of particular risk because coral reefs are mostly made of calcium carbonate. Corals are the major framework of the reefs, while a type of calcifying algae called coralline algae are the glue that holds these reefs together. Both taxa are susceptible to ocean acidification, though until recently we have lacked a greater mechanistic understanding of why particular species are more resistant to ocean acidification, or whether this resistance can be gained by acclimatisation. 

Calcifying fluid 
Utilising a combination of cutting-edge geochemical facilities at the University of Western Australia, we have come closer to answering these questions. In corals and coralline algae, calcium carbonate is precipitated in a specific location inside the organisms from a fluid with chemical conditions different from the external seawater. This is referred to as the “calcifying fluid”.  

Corals are the major framework of the reefs, while a type of calcifying algae called coralline algae are the glue that holds these reefs together

During the precipitation process, trace elements and different isotopes of these elements are incorporated in the newly formed skeleton. Some of these trace elements are proxies of pH or concentrations of dissolved inorganic carbon, which allow us to estimate the chemistry of the calcifying fluid when calcium carbonate is precipitated.  

Understanding this chemistry has allowed the scientific community to better understand why ocean acidification impacts corals and coralline algal species to varying degrees. Both corals and coralline algae that are badly impacted by ocean acidification tend to have much lower pH within their calcifying fluid (pHcf) when seawater declines, which creates conditons less favorable to precipitate calcium carbonate.

Variations 
However, there are many examples of species that do not conform to this. For example, some species calcify at similar rates today and under ocean acidification, but their pHcf is also much lower. Advances in geochemical understanding have also allowed us to determine the dissolved inorganic carbon (DIC) and the saturation state of calcium carbonate in the calcifying fluid. This is related to pH, DIC, calcium concentrations, and temperature. The capacity of different species to maintain constant calcification rates under ocean acidification tends to relate to their ability to maintain the saturation rate in the calcifying fluid in the face of ocean acidification. However, the strategies used to maintain this under ocean acidification vary between species.   

At least three different mechanisms of maintaining the saturation rate in the calcifying fluid are apparent: pHcf homeostasis, elevation of dissolved inorganic carbon, and increasing Ca2+concentration in the cf. However, these physiological abilities are species-specific, and it appears that these cannot be gained by acclimatisation.  

Over 12 months we grew corals and coralline algae under control and various ocean acidification scenarios. We measured the geochemical proxies over different time periods after exposure. Importantly, for susceptible species whose pHcf and saturation state of calcium carbonate in the calcifying fluid decline, the effects of ocean acidification on the proxies occurred rapidly and persisted throughout the experiment. 

Effects 
These results have worrying implications for coral reef ecosystems, which are already under threat from mass bleaching events caused by ocean warming. Coralline algae are relatively tolerant of marine heatwaves that induce these mass bleaching events. However, as a taxon they are extremely susceptible to ocean acidification.  

So while over shorter time scales they might persist or even thrive on reefs heavily influenced by mass bleaching events, over longer time scales it is likely that climate change will also greatly reduce their capacity to assist reef building if they are not able to adapt to ocean acidification. Our results indicate that only a subset of species are capable of tolerating ocean acidification. Together, this indicates that these tolerant species recorded in our study and others will be the likely winners under climate change.  

However, this also means that the complex and biodiverse reefs we know today will likely be transformed into less diverse, low profile reefs dominated by a few hardy species that can resist both marine heatwaves and concomitant ocean acidification.  

Globally, our world leaders are now required to curb emissions before what remains of these iconic ecosystems are forever altered.  

Authors: Dr Christopher Cornwall (left) is Rutherford Discovery Fellow at the School of Biological Sciences, Victoria University of Wellington in New Zealand and Dr Steeve Comeau is research scientist at Laboratoire d'Océanographie de Villefranche CNRS-Sorbonne Université.