Results so far – observations at the end of the experiment

-- Post by Ian Joint --

We have now reached the end of the experiment and I will try to summarise what we have found so far. Again it is important to stress that these are still very preliminary observations and we will need many months to complete all of the analysis. Molecular biology is quite a slow process – fictional TV forensic scientists may be able to get an instantaneous DNA profile – the rest of us have to work harder at it.

As with the first summary of our observations that we posted a week ago, the graphs show the changes that we have observed in one bag from a high CO₂ and one from a normal CO₂ treatment. The graphs show the changes that we have seen since the experiment began on 6 May, until we took the final samples on 23 May.

The experiment should be thought of as happening in two phases. In the first phase, which lasted for 10 days, we concentrated on phytoplankton – we wanted to see how they might respond in a high CO₂ world. But as the phytoplankton grew, they used up the CO2₂that we had added – so we were no longer in a high CO₂ world. In the second phase, we added more CO₂ to make the water more acidic again, so that we could study how bacteria might respond to pH change.

pH

The graph shows how pH changed during the experiment. At the start of the experiment, there was a clear difference between our treatments – bubbling with CO₂ had made the water more acidic – just as we expected. But over the next 10 days, the added CO₂ was used up by the phytoplankton as they grew and the difference in pH between the 2 treatments was gradually reduced. By bubbling with CO₂ enriched air for just 12 hours, we were able to reduce the pH again. This time, it stayed very constant because the phytoplankton cells were no longer growing actively. So, the pH difference was maintained during the second phase of the experiment.

Nutrients

In phase 1, nutrients declined rapidly as the phytoplankton grew. However, there was a large difference between the treatments. At high CO₂ nitrate was always present; but at normal CO₂, the concentration was reduced to low levels – about the same concentration that was the lower limit of our analysis methods. Phosphate appears to behave differently and was depleted in both treatments. After we bubbled with CO₂ for a second time, we saw only very slight changes in phosphate and nitrate. The phytoplankton cells were probably not able to grow as rapidly as before because there were not enough nutrients left in the water.

Chlorophyll

Chlorophyll – the green pigments contained in all plants – increased quickly in the first phase of the experiment. This pigment is a good indicator of how well the phytoplankton was growing. In the second phase, chlorophyll concentrations remained low. However, in the last few days, we began to detect that some phytoplankton species were beginning to grow again and the chlorophyll concentrations started to creep up again. At the same time, the CO₂ concentrations were falling slightly and the water was slightly less acidic.

Specific organisms

The flow cytometer (see earlier blog) has been very valuable in giving us an indication of which phytoplankton species were growing in the two treatments. We have seen large differences in the way in which different microorganisms grew in the high CO₂ and normal CO₂ treatments

The changes in the major plant groups in the different bags were as follows:

Coccolithophores – These were the dominant plants in the first phase of the experiment. As expected, in the high CO₂ world, these plants did not grow well because they would not be able to produce their calcium carbonate shells. They also did not recover their numbers in the second phase of the experiment in either the high or normal CO₂ treatments.

Very small phytoplankton cells - These very small plants (called picoeukaryotes) showed different responses to pH change. They appeared to like the conditions and grew rapidly in the first phase of the experiment. When we bubbled with CO₂ for a second time, the numbers declined. Under normal pH conditions, growth was slower. But the numbers continued to increase during the second half of the experiment.





Photosynthetic bacteria (cyanobacteria) - In the high CO₂ world of our bags, cyanobacteria did not grow very well and there were always fewer than in the bags at present-day conditions. This pattern continued during the second phase of the experiment.

Bacteria - The second phase of the experiment was designed to study the marine bacterial communities and to see how they might respond in a high CO₂ world. When we bubbled with CO₂ for the second time, we expected the bacteria to grow, because the phytoplankton blooms were over and there was a lot of organic material that the bacteria could use. But we were all surprised at how rapidly the bacteria grew and how many bacteria were in the water in the bags after just a few days.





There were big differences between the treatments. The bacteria grew very rapidly in the more acidic condition. They then disappeared very quickly too – much faster than we expected or had ever seen before in any ocean. In contrast, at the normal pH, bacteria grew more slowly and were still present at high numbers at the end of the experiment.

Now we need to discover what bacteria grew so well in the high CO₂ conditions. This will be done back in our laboratories using molecular biology techniques. These have been described in other blogs in this series. It will be very interesting to discover which bacteria have been affected by the pH change. Will this have an affect on the turnover of organic matter and the regeneration of nutrients in the future oceans? We hope to discover this in the coming months as more results become available.

--