Could global warming have a positive impacted on the marine microbial food web?

As global temperatures rise as a result of climate change one of the major effects it will have is an increase in ocean temperature. The part of the ocean that is most sensitive to this change is the euphotic zone which starts at the ocean surface and continues down several meters to a point that sunlight is heavily diminished. This is also the site of the Earth’s largest CO2 store because CO2 is highly soluble in water. About half of the world’s primary production takes place here by photosynthetic microbes. In the same way plants are at the bottom of the terrestrial food web these microbes are at the bottom of the marine food web. In this study the authors combined evidence from a number of different laboratory experiments and environmental models in order to predict what the impact of rising ocean temperatures could have on microbial primary producers and the possible knock on effects to the marine food web and the level of carbon fixation.

The euphotic zone consists of some important groups of bacteria that are fundamental to the oceans food web. Primary production is carried out by many species of cyanobacteria and other microbial plankton which fix CO2 into organic compounds to use as a fuel source. These phytoplankton are a major food source for other grazing microbes which consume and digest smaller microbes. Heterotrophs rely on dissolved organic material (DOM) in the water which they absorb. Most of the DOM available comes from the primary producers and are released either by messy eating of the primary producers by grazers or by lysis of the cells by viruses. Both grazers and heterotrophs are a food source for larger organisms.

In the conclusion to this study it is believed that a rise in ocean temperature will have some major changes to this fundamental part of the ocean food web. For example, temperate regions which are dominated by the cyanobacteria Synechococcus will shift to a physically smaller group of tropical cyanobacterium Prochlorococcus. One of the reasons for this is the preference for warmer temperature that Prochlorococcus has for metabolic function. This could result in change in the type of DOM found in the area and a resulting shift in heterotrophic populations as well as a shift on grazing populations of microbes. The evidence suggests that heterotrophy will become a larger theme in ocean communities as photosynthesis increases. This could have a positive impact on climate change itself as more CO2 becomes fixed by increasing primary producers. Grazing rates will also increase but not to the same extent as primary producers and heterotrophs. As a whole this does indicate a possible growth in the bottom of the marine food web.

The authors have tried to answer a very important question with this paper but do also admit that it is based on many theoretical predictions and so cannot be concluded as fact. Another negative with this study is that it focuses only on temperature increase and not on ocean acidification resulting from increased CO2 uptake. Growth rates are not just limited by energy inputs from the sun and are still limited by the amount of key elements found in the water column, eg iron. This was not discussed in much detail. A lot more research will be needed before changes to marine food webs can be fully understood.

A review of:
Sarmento, H. Montoya, J. Vazquez-Dominguez, E. Vaque, D and Gasol, J. (2010) Warming effects on marine microbial food web processes: how far can we go when it comes to predictions? Phil. Trans. R. Soc. B. 365,2137-2149.