Will ocean acidification affect marine microbes?

Increasing quantities of carbon dioxide (CO2) in the atmosphere, combined with a decrease in dissolved oxygen (O) in the ocean as a result of temperature increases, is leading to theories of ocean acidification. If this is occurring, it is vital to understand how the functioning of microbes will be influenced due to their importance in marine productivity and planetary habitability. This review collected research surrounding this issue.

Ocean pH has never been constant as a result of natural fluctuations. These fluctuations depend on the location within the ocean in terms of depth and containment of water: deviations of 0.77 pH points have been found at 350m, and steep declines in pH occur within areas that are more contained, such as waters near estuaries. The pH levels in marine environments can fluctuate on a time-basis. Short-term fluctuations occur constantly as a result of gaseous exchanges of CO2 and O, however a majority is linked to microbial activity: high respiration to photosynthesis ratios cause accumulation of CO2, particularly in calm waters and during the night. Biological and physical processes (e.g. temperature), cause fluctuations over intermediate time periods, such as seasons. Although this suggests that natural changes still enable stability the terms of the microbial assemblage, the scale of this variation is almost 13 times lower than the permanent change predicted by 2300. There is evidence that once a pH change is initiated, it will increase expontentially as a result of the microbial communities: a phytoplankton bloom, initiated by an increase in CO2, will increase the concentration of CO2 further.

Of course these are only predictions, and considering numerous factors influencing pH, this may never become reality. One paper states that pH has not varied by more than 0.6 pH units for 350 million years, whilst another states a greater than expected change was observed in the central Pacific.

Research is conflicting in terms of how marine microbes will respond. Some clades are present across varying pH levels, however the species and function changes. For example, calcification rates of phytoplankton have been effected, however the direction of the effect is strain dependant.  Subtle variations in photosynthetic rates have been observed in species that do not have carbon concentrating mechanisms. The quantities of some bacteria have been shown to increase, such as phytoplankton. Steep ecological changes, including algal communities and invertebrates, have occurred alongside pH changes in estuaries.

It is possible for species, such as some representatives of the alphaproteobacteria and the gammaproteobacteria groups, to survive out of and within the core acidified zone: 200m – 4000m deep. I wonder what traits the species exhibit that enables them to survive? Considering organisms that live within the acidic zone live in close proximity to those that do not, and the ability of microbes to transfer genes, it would be interesting to determine at what point the genes for these traits were gained. Perhaps some microbes have a threshold point after reaching acidic conditions in which they must acquire the appropriate genes for survival from other microbes already present. Alternatively, these genes may be present yet not expressed until environmental stimulation is experienced.

In freshwater systems, some microbes persist through pH fluctuations of 2-3pH points daily. However, these microbes are adapted to these conditions, unlike those in marine environments, which are less adapted due to the higher buffering capacity, therefore lower natural variation levels of ~0.3 pH points.  This posed the question: Are the genomes of marine microbes flexible enough to allow them to acclimatize or can they accumulate new genes fast enough to enable them to survive ocean acidification? If so,  how will their function be influenced?

If the genomes of microbes in the water column are not adequate to allow said microbes to adapt, I wonder if this extends to symbiotic bacteria that are inherited directly from the parents, considering the hosts regulation of the internal environment inhabited by the microbes.

Answers to some of the questions posed could be gained through experiments using model marine microbes; comparing marine, coastal and freshwater systems that hold similar initial communities; and comparing stored isolates and fresh isolates.  The responses would be considering to pH changes. Due to problems revolving around culturability of marine microbes and controlling pH of media, a more valid approach is the comparison of the genes and gene expression. Another problematic factor is the number of confounding factors that occur naturally. Therefore laboratory experiments will need to be multi-factorial in order to represent a greater proportion of factors.

Although it is vital that the functional effects of altering the microbial communities as a result of pH changes are understood, considering the natural fluctuations over several time periods and locations it is debatable whether a permanent change will occur. Potential effects of this on microbial communities, and more importantly function, must be understood. Such understanding can be predicted more accurately through comparisons between organisms taken from ‘real’ environments as opposed to laboratory experiementation.

Joint I, Donay SC, Karl DM (2011). Will ocean acidification affect marine microbes? The ISME J.;5:1–7 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3105673/