The Bergen Mesocosm – a case study

The Bergen mesocosm experiment was one of several major research efforts in recent years to try to untangle the complex consequences of ocean acidification. Project leader Dr Ian Joint explains the research, and gives a taste of what it found.

One of the least-understood consequences of increasing carbon dioxide concentrations in the atmosphere is that the oceans are becoming more acidic.

This is because CO2 in the air dissolves in seawater to form carbonic acid – a weak acid that makes the oceans slightly more acidic. The rate of change is extremely rapid and it is expected that by the end of this century, the oceans will be more acidic that they have been for more than 20 million years.

Over geological time scales, the pH of the seas has changed significantly in response to variations in atmospheric CO2. Indeed, the ocean has in the past been more acidic than we expect it to become over the coming decades.

But what is different this time is the speed of change. Ecosystems have proved their ability to accommodate change when it is gradual, usually over hundreds of thousands of years. We do not know how well the marine ecosystem will adapt to changes that will occur over decades.

Microbes are the most important organisms in the sea. In contrast to the land, where plants are large and long-lived, in the seas most of the primary production comes from microscopic algae, or phytoplankton. These have tiny biomass and their generations last only days.

The productivity of this phytoplankton depends in turn on bacteria and archaea to regenerate nutrients. So it was a priority to determine how marine microbes would respond to a high-CO2 world.

Microbes control biogeochemical cycles and keep the planet habitable. This means we need to know how microbial populations will respond to rapid climate change.

As a first step to address this question, a NERC-funded experiment took place in May 2006 to test the effect of an instantaneous change of CO2 concentration to the level that is projected for the year 2100. The aim was to identify the components of the microbial ecosystem that are most sensitive to pH change.

These experiments are complex because pH varies naturally over the course of a year. When phytoplankton populations begin to grow in the spring they consume CO2 that is dissolved in seawater. The result is that pH increases for a period of a few weeks.

The dissolved CO2 gradually increases as a result of bacterial activity and the pH declines to a more typical values. In a high CO2 world, this seasonal variability will still exist but the microbial populations will be exposed to a different range of pH values than in the present day. So we need to understand how marine microbes will adapt to the new conditions.

We used the latest approaches to study the problem, including metagenomics – the analysis of the genetic information of all of the microbes present in the system. Recent advances in the technology to sequence DNA mean that it is now possible to analyse the genetic information from organisms living in their natural environment.

We have used these approaches to investigate which parts of the microbial population might be most vulnerable to the changes in pH that will occur by the end of the century. In addition, we have used novel approaches to investigate changes in metatranscription – that is, to find out which genes are switched on or off as a result of pH change.

The approaches used in this mesocosm experiment have told us how the existing population responds to pH change. But these short-term experiments do not indicate how populations might adapt over time; some bacteria might find the new conditions preferable and so they might become more abundant.

Answers to these questions will require a different type of experiment, which will last for many months or even years.

PlanetEarth online, 1 January 2008. Article.

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