NEWS Our oceans are becoming more acidified, and the bacteria seem to have many ways to cope with the pH stress. In nutrient-poor conditions the effects on bacteria are more pronounced, and the energy costs to the ecosystem can be substantial.
A large part of the CO2 from human activities is dissolved in oceans, lakes an rivers, and therefore the oceans are getting more and more acidified. A decrease in pH from 8.2 in pre-industrial time to 8.1 today maybe doesn’t sound a lot, but it is enough to disturb the ecosystem. Research has shown effects on the forming of shells, on coral reefs, as well as on phytoplankton.
However, the understanding of how acidification affects bacteria is poor. Marine bacteria account for a large part of the surface ocean respiration, and play a very important role as drivers of nutrient cycles and energy fluxes. To understand how projected climate changes will affect them is therefore crucial for assessing how the marine ecosystem will function in the future.
It is not easy to find out what is happening inside such a small organism as a bacterium. In an animal you could measure blood pressure, heart beat, weight or maybe behaviour. To understand how bacteria handle different stress situations, their gene expression is analysed. If the expression of a certain gene is enhanced when the bacteria are exposed to a certain condition this shows that the bacteria are handling the new condition. When the acidification is enhanced the bacteria are compelled to get rid of excessive protons in order to retain pH homeostasis in the cells. They will be forced to activate energy demanding mechanisms to do so, which will result in an increase in certain measurable gene expressions.
To investigate how bacteria respond to ocean acidification, a mesocosm experiment has been carried out. The study was partly financed by Ecochange, and a group of Ecochange researchers participated in the study. The researchers were interested in comparing not only different levels of acidification, but also the effect of different nutrient conditions. There are some earlier studies performed in nutrient-rich conditions, but nearly nothing done with low nutrient levels. Therefore, the experiments were located to the naturally nutrient-poor Mediterranean Sea.
The results indicate large differences in effects of acidification depending on if the environment is rich or poor in nutrients. In rich conditions high photosynthetic activity leads to an increased pH, and bacteria living under these conditions seem to not be exposed to low pH. In poor conditions the effects of acidification are more pronounced. Moreover, there seems to be a whole series of mechanisms in the bacteria by which they can meet the stress that the acidification causes. However, these mechanisms are energy demanding.
In the low nutrient part of the experiments, the most highly expressed genes encoded proton pumps which transport protons across the membranes. The genes related to respiration proton pumping were highly active as well. The results show that this activity mainly is a way for the bacteria to cope with pH stress and not primarily to increase the respiration rates. It seems as if there are multiple functions in the marine bacteria that are used to cope with the pH stress. In acidified conditions, bacteria need to allocate more energy to cell maintenance instead of growth. Inhibited bacterial growth efficiency could influence bacterial carbon cycling and energy fluxes in the food web. Ocean acidification could therefore have long-term effects on the productivity of the ocean ecosystem.
Editor: Kristina Viklund