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Biological oceanography

Who’s benefiting from increasing CO2?

Bach, L. T., Hernández-Hernández, N., Taucher, J., Spisla, C., Sforna, C., Riebesell, U., & Arístegui, J. (2019). Effects of elevated CO2 on a natural diatom community in the subtropical NE Atlantic. Frontiers in Marine Science, 6, 75. https://doi.org/10.3389/fmars.2019.00075

As the concentration of carbon dioxide (CO2) in the atmosphere steadily increases, oceans are taking up more and more CO2. Many marine organisms get their food by photosynthesizing, which requires sunlight, water, nutrients, and CO2. So, if CO2 in the ocean is increasing, will the photosynthetic organisms grow bigger and more numerous?

Beyond more food being available, there’s an explanation for this theory that is specific to photosynthetic organisms. Photosynthesis involves an enzyme called RuBisCo. RuBisCO’s primary job is to take up CO2 and pass it on to a series of reactions to produce high-energy molecules. But RuBisCO is bad at differentiating between oxygen and CO2 molecules, and more often than not ends up taking oxygen, if the number of CO2 molecules is less or comparable to oxygen molecules. The organism stocks up on and surrounds its RuBisCO enzymes with CO2 to avoid taking up oxygen. So, if CO2 concentrations increase, organisms wouldn’t have to waste energy in stocking up CO2, and instead use this energy for getting other resources, potentially enhancing their growth.

Figure 1: Assorted diatoms under a microscope. (Prof. Gordon T. Taylor, Stony Brook University)

 

To investigate the fate of marine photosynthetic organisms in a higher-CO2 world, Dr. Lennart Bach of GEOMAR Helmholtz Centre of Ocean Research Kiel and his colleagues ran an experiment on phytoplankton called diatoms on the coast of the Canary Islands in Spain. Diatoms (Figure 1) constitute a major phytoplankton group with a characteristic silica shell. Collectively, diatoms contribute to 1/4th of the global ocean productivity. Diatoms are heavier than other phytoplankton because of their silica shell; thus, when diatoms die, they sink faster, leading to transport of carbon to the bottom of the ocean.

 

The scientists suspended large transparent plastic bags in the ocean (Figure 2), letting them fill up with seawater and all the organisms (including diatoms) floating within. The scientists then filled these bags with seawater aerated with different amounts of CO2. There were seven of these bags with different CO2 concentrations and one mesocosm which received non-aerated water. Initially, apart from the CO2 addition, the diatom community was allowed to grow without any perturbations. Halfway through the experiment, nutrients were added to the mesocosm to emulate the natural process in this region that injects nutrients into the water column.

Figure 2: Experimental Mesocosm of GEOMAR, Germany. Installed at Tvarminne in 2012. (Source: Maike Nicolai)

 

The authors indeed saw an overall increase in the total mass of diatoms under increased CO2 conditions. The scientists came up with two main explanations for this result. First, owing to the higher-CO2 conditions, the diatoms may not be spending the energy on stocking up CO2 molecules around the RuBisCo enzyme instead utilize the energy to grow. However, the scientists also observed that the high-CO2 conditions were negatively impacting copepods, predators of the diatom, which could have led to an increase in the diatom biomass.

The authors, apart from the increase in the biomass of the diatoms, also noticed a change in the types of diatoms that were present, with some species of diatoms out-competing other species under the increased CO2 conditions. Mostly larger diatom species benefited the most from the increased CO2 after the addition of nutrients, perhaps explained by previous laboratory studies showing that larger diatom species are more efficient in acquiring carbon under the elevated CO2 conditions. Larger diatoms sink faster and result in higher transport of carbon from the atmosphere to the ocean bottom.

The restructuring of the diatom community under the high CO2 conditions has the potential to impact the food web interactions. Although several studies point towards enhanced growth under high CO2 conditions, there are still some uncertainties to be resolved to determine the winners and the losers within the diatom community.

 

 

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