Will ocean acidification affect marine microbes?

Since the beginning of the industrial revolution, as a result of anthropogenic CO₂ emissions in the atmosphere, ocean pH has already decreased by about 0.1 units. If the emissions will continue at the present rate, by the end of this century it is projected to fall a further 0.3-0.4 pH units, a rate of change that has not been experienced by our planet, for at least 400,000 years.

This fundamental alteration in basic ocean chemistry is likely to have wide and severe implications for all ocean life. Several detailed studies have already demonstrated various adverse effects of ocean acidification in a large number of marine organisms. However, to date, it is still not clear how marine microbes will respond to changes in pCO₂-pH in the ocean and the results of the experiments carried out so far are often inconsistent and at times conflicting.

For example, altough numerous studies suggest a negative effect of acidification on coccolithophores, at least two studies indicate enhanced calcification under elevated pCO₂.

The same conflicting results were obtained for cyanobacteria (increasing photosynthesis with elevated CO₂ is generally observed in Synechococcus but not in Prochlorococcus) and for many others eukaryotic phytoplankton species (e.g. Diatoms) which have carbon concentrating mechanisms that considerably diminish the sensitivity of photosynthesis to CO₂ variations.

Microbes are involved in the most important biogeochemical processes of our planet and it's clear that assessing if and how they will respond or adapt to ocean acification is of crucial importance for predicting the global consequences of climate changes.

Will variations in external pH affect the cell surface chemical and protonic equilibria? Will microbial communities continue to function at a lower pH? Do they have the metabolic and genetic plasticity to acclimatize in a rapidly changing sea? How microbes will react to the synergistic or antagonistic effects of global warming? We urgently need answers to these questions, because to date unfortunately, we don't have any reliable estimates on how microbial physiology will be affected by ocean acidification and even less information is available about how primary productivity, microbial communities composition and biogeochemical cycles will change in a more acidic sea.

More ecological, multifactorial, physiological and genomic studies are needed as well as microcosm/mesocosm comparative perturbation experiments and large-scale, long-term surveys are strongly required before we can try to predict how marine microorganism will react to ocean acidification.

The autors points out that, in reality, pH and CO₂ in the ocean are not constant at all, and in many acquatic habitats such as freshwater lakes (diel variations of 2-3 pH units), deep oceans (water below 350m has a pH less than 7.8), estuaries and coastal regions, microbes already experience remarkable short-term, local and seasonal changes in pH that are often many times greater than those projected for the end of this century and yet, they are still there.

In these environments infact, most of the pH variability is a consequence of microbial activity in it-self, such as respiration or photosynthesis and often phytoplankton blooms can locally and rapidly reduce pCO2, with a concomitant increase in pH. Thus the authors conclude that marine microorganisms somehow, must be already capable of accomodating rapid and sometimes large changes in pH and with some exception, the microbial community and the biogeochemical cycles should not be assumed to be at risk until more detailed studies will prove the contrary.

Reference:

Joint, I., Doney, S. C., & Karl, D. M. (2011). "Will ocean acidification affect marine microbes?" The ISME Journal. 5 (1): 1-7.