The Effects of Injecting CO2 into Deep Bethypelagic Layers of the Ocean

In the face of increasing CO2 levels in the atmosphere one approach to reducing new CO2 emissions is carbon capture and storage.  Yamada et al. (2010) examine the technique of dissolution; injecting CO2 into deep layers of the ocean.  The limited mixing of these deep waters would prevent the CO2 from entering the atmosphere for a long time, but the CO2 could affect the prokaryotic populations at these depths and their associated nutrient cycles.  The research looked at the effects of increased CO2 on these populations by capturing samples of them in water samples from deep in the Pacific Ocean and conducting laboratory experiments on them, increasing CO2 levels and evaluating the effects.—Anna Fiastro
Yamada, N., Tsurushima, N., Suzumura, P., 2010. Effects of Seawater Acidification by Ocean CO2 Sequestration on Bathypelgic Prokaryote Activities. Journal of Oceanography. Vol 66, p 571-580.

The plan for dissolution is to inject CO2 into the benthypelagic zone, which ranges from 1000 to 3000 meters from the surface.  This is an important area for the regeneration of nutrients and organic material.  The layers of the ocean are separated by temperature and salinity gradients that prevent mixing. Due to limited mixing of the layers of the ocean it is thought that the CO2 would not move up and not be introduced into the atmosphere.  The CO2 would dissolve into the surrounding water and remain at depth, causing a decrease in the pH, also known as acidification, but only locally.  It is important to look at the effects of these elevated CO2 levels on the systems that operate in these layers, specifically the prokaryotes who are responsible for these nutrient cycles.
          Yamada et al. took water samples from two different locations in western North Pacific at 2000 meters deep, which were used in experiments within 10 days of sampling.  CO2 injection conditions were simulated by bubbling air containing different concentrations of CO2 though the tanks containing the samples.  The pH, total cell count, and heterotrophic prokaryotic production rates were monitored in each sample.  Although there was variation between the sites, thought to be due to seasonal differences, clear results were obtained.  The bubbling of CO2 increased the acidity of the water (decreased the pH).  The total cell counts remained relatively constant independent of pH, but the heterotrophic prokaryotic production rates decreased with increasing acidity.  Another way to say this is that with more CO2 in the water, productivity of the organisms living in it went down. 
It seems counter-intuitive that total cell count would remain the same while productivity went down.  In order to further examine this, the researchers looked at the direct viable count, or the number of thriving prokaryotic cells capable of growth.  This was shown to decrease with acidification, explaining the decreased productivity rates.
Another trial was run in which acidification was simulated by adding a chemical buffer.  This showed similar results to the CO2 bubbling method.  As pH decreased and acidity increased, prokaryotic growth and production were lowered.
In these experiments, acidification suppressed bacterial activity more than Archaea activity.  The significance of this is not fully understood, and further research is necessary to look at the life histories of different types of Archaea to better understand their reaction to changing pH levels.

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