Ocean Acidification

Human activities are altering the chemistry of the seas at a fundamental level. Atmospheric concentrations of carbon dioxide are increasing as a result of burning of fossil fuels. As CO2 from the atmosphere dissolves in the surface ocean, seawater becomes more CO2-rich and more acidic (increased hydrogen ion levels and lower pH), resulting in substantial changes in a host of chemical properties. The pH of the oceans has been relatively constant for more than 20 million years but is now changing very quickly and will decrease from on average pH 8.1 to pH 7.8 by the end of the century – probably at least a 10 times more rapid change than anything over that period of time.

A group of expert microbial oceanographers met at the Center for Microbial Oceanography and Education, Hawaii in February 2009 to assess the consequences of higher CO2/lower pH for marine microbes – crucial regulators of the Earth System. It is widely recognized that marine microbes provide great benefit to human society, through essential ecological services, and maintain the health of the oceans. Microbial photosynthesis, and hence oxygen production, in the ocean is equivalent to that of plants on land. Microbes maintain the productivity of the oceans through nutrient cycling and nitrogen fixation. Microbial activity results in the production of biogases that affect the chemistry of the atmosphere, which in turn affects our planet’s climate.

It is not known if microbes can or will adapt or evolve over the likely time-scale of ocean pH change. There are no robust predictions of how marine microbes will be affected by increased CO2 and pH change or what the resulting impacts would be on marine ecology and biogeochemistry. Research investment should be expanded in laboratory and field experiments and models to make predictions and provide probability estimates of how microbial processes may be sensitive or insensitive to higher CO2/lower pH. Better data and numerical models are required to inform policy makers on the issues that will arise as we tackle the consequences of rising atmospheric CO2. Considerable investment is required to design and carry out experiments to assess which ecological services provided by marine microbes might be at risk from ocean acidification.

It was the view of the experts that experimental approaches should rely heavily on existing environments where higher CO2 and lower pH occurs naturally, such as zones of high respiration, particularly where respiration is much higher than primary production, and in cold polar seas, which have lower calcium carbonate saturation state. Freshwater lakes and estuarine waters offer exceptional opportunities since they are less well buffered than the oceans and experience daily to seasonal changes in hydrogen ion concentration that can be orders of magnitude greater than those projected for the oceans in the next century. Coastal and estuarine environments also experience substantial pH variations over short time and space scales. An important question is whether marine microbes have lost the metabolic flexibility of their freshwater counterparts because they have experienced relatively constant pH for 20 million years.

Our understanding of basic marine microbial physiology is inadequate to answer some important questions involving the consequences of ocean acidification. For example, most phytoplankton species regulate internal pH, which is generally maintained below the pH of seawater, but it is not known how well other marine microbes control pH, nor if a change in external pH will affect this process. Elevated CO2 levels increase photosynthesis rates in some but not all microbial species, and laboratory studies suggest that marine nitrogen fixation may also be enhanced. Carbonate ions – the building block for calcium carbonate shells – will decline in a high-CO2 world. The mechanisms involved in the biological formation of carbonate shells are not well understood, and there is conflicting evidence that shell formation rates could either increase or decrease under future elevated CO2.

Ocean acidification will not occur in isolation to other consequences of CO2-induced climate change and more local human perturbations to the marine environment such as nutrient overloading. Potential feedbacks and synergies are poorly understood, as are impacts on low oxygen zones, the oceanic reservoir of dissolved organic matter, and ocean carbon storage and sequestration.

Microbial diversity in the oceans is enormous and complex, with many thousands of bacterial species in every liter of surface seawater. Although it is highly unlikely that increased CO2 and lower pH will result in species loss, alteration of pH may change the dominant species with subtle but potentially important consequences for biogeochemical processes and food webs. Since natural marine microbial communities are very complex, experiments to investigate higher CO2 / lower pH should attempt to capture that complexity. The best approaches used to date have involved the use of large volume mesocosm experiments, but there are real challenges in using mesocosms for long time periods (> 1 month) or in the open ocean. A large investment in infrastructure will be required if the research community decides that such experimental approaches offer the best chance to understand how marine microbial assemblages will respond to higher CO2 and lower pH changes.