Warming increases decomposition over production
Phytoplankton fixes carbon dioxide in photosynthesis to support its own growth and the growth of the marine food web. Some of the carbon fixed in plankton biomass end up as calcium carbonate sediment on the deep seabed where it remains for thousands of years, but most of the carbon that does not contribute to growth and development is turned back into carbon dioxide by respiration of the entire plankton community. The balance between photosynthesis (primary production) and community respiration therefore determines whether the oceans are a carbon sink or a carbon source.
A team of scientists led by Angel López-Urrutia of the Spanish Institute of Oceanography in Gijón showed that the balance between production and respiration is profoundly affected by temperature, and that while the rates of photosynthesis and respiration both go up with temperature, respiration goes up faster, eventually outstripping photosynthesis . This turns the oceans from a carbon sink to a carbon source.
In fact, vast areas of the North East Atlantic have already become a carbon source, with respiration of the plankton community almost 150 percent of photosynthesis (Oceans carbon source or sink? this series).
The increase in carbon dioxide released to the atmosphere would unleash a positive feedback to aggravate global warming, leading to further deterioration of phytoplankton production, and even more carbon dioxide released in respiration and decomposition.
López-Urrutia and colleagues have based their prediction on the metabolic theory of ecology due to ecologist James Brown and his colleagues at the University of New Mexico Albuquerque in the United States [2, 3].
The metabolic theory of ecology
At the heart of the metabolic theory of ecology is the universal biochemistry of energy metabolism in the living world. All organisms, from bacteria to giant redwoods, from single-celled animals to whales, share the same fundamental molecular complexes for photosynthesis and for metabolism. As body size increases, metabolic rate decreases because there are less and less molecular complexes for photosynthesis or metabolism per unit of biomass. Metabolic/photosynthetic rates decreases with the exponential power of ¾ with body size, due to the constraints of having to distribute nutrients and substrates through branching fractal networks (such as blood vessels) that become more and more elaborate as body size increases  (Biology’s theory of everything, SiS 21). Metabolic rate also varies with temperature according to a well-known relationship between temperature and chemical reaction rates. Combining these relationships gives a general expression that describes the metabolic rates of practically any organism (see Box).
Biology’s theory of everything in a nutshell
A general expression has been derived that describes the metabolic rate of any organism according to body size and temperature (see main text):
B = b0M3/4 e-E/kT
where B is the metabolic rate, b0 is a constant independent of body size and temperature, M is body mass, and the ¾ power scaling exponent reflects the fractal-like distribution network supplying resources to individual cells within the organism’s body that adds a 4th dimension to a 3-dimensional being. The Boltzmann factor, e-E/kT describes the temperature-dependence of metabolic rate, where E is the average activation energy of metabolism or of photosynthesis and k is Boltzmann’s constant, 8.62 x 10-5 eVK-1.
The expression provides a way to sum up the photosynthetic and respiratory activities of entire ecological communities.
The metabolic rates of marine photosynthetic organisms depend on photosynthetic activity, which too, is dependent on body size and temperature, and also on light. The dependence on light, however, saturates beyond a certain light intensity, as other biochemical reactions become limiting.
The rate of net primary production of a plankton community is the sum of all photosynthesis by photosynthetic organisms allocated to growth (minus respiration), which is also referred to as carbon use efficiency. This carbon use efficiency is independent of body size and environmental temperature and also independent of light, as predicted by the metabolic theory of ecology and confirmed by empirical observations.
Community respiration is the sum of the respiration of photosynthetic plankton plus the respiration of non-photosynthetic plankton.
Although both community respiration and photosynthesis rates are predicted to increase with temperature, respiration goes up faster because the average energy of activation for metabolism is 0.65 eV (electron volt, a measure of energy at the molecular level) compared with the activation energy of photosynthesis, predicted to be 0.32 eV.
Theory matches observations
Using the most comprehensive compilation of plankton metabolism data available, López-Urrutia and colleagues evaluated the values of gross photosynthetic activity, net photosynthetic activity, and community respiration at both the organism and population levels. The predicted constants from metabolic theory of ecology are validated against experimental data, and net primary production and community respiration are calculated.
The data on respiration of individual plankton species and on phytoplankton net production rates as a function of body size and temperature show that indeed, they follow the predictions of the metabolic theory.
Also as predicted, metabolism and photosynthesis increase at different rates with temperature. Respiration rates of non-photosynthetic organisms go up steeply with temperature with activation energy close to the predicted value of 0.65 eV. Phytoplankton respiration and net primary production show weaker temperature dependence with activation energy close to the expected for photosynthetic processes of 0.32 eV.
Phytoplankton production is a rapidly saturating function of light. The fact that phytoplankton respiration is also affected by the incident light intensity provides further evidence that the respiration of phytoplankton is ultimately constrained by photosynthesis, and the carbon-use efficiency of marine phytoplankton is essentially independent of body size, temperature and light availability. The carbon use efficiency in phytoplankton is very high at 83 percent, which lies within the range of reported phytoplankton net growth efficiencies.
Next, the estimates of community respiration and production obtained by the theory were compared with concurrent direct measurements of the amount of oxygen consumed and carbon assimilated by natural communities in situ. There was again very good correlation between the measured and estimated values.
Armed with the great match-up between theory and observations, López-Urrutia and colleagues used the model to predict the effects of global warming on the metabolic balance of the oceans. Because phytoplankton production have different activation energies, with the photosynthesis activation smaller than that of respiration, both community production and respiration will increase as sea temperature rises, but respiration will increase relatively more than production. The effect is to decrease net productivity, releasing more carbon dioxide.
The model also predicts an increase in the threshold for metabolic equilibrium between gross primary production and respiration with temperature. So, if sea temperature increases as a result of human activity, the surface waters would capture relatively less carbon dioxide, because it would need to photosynthesize at a much higher rate to balance the increased respiration and decomposition. This is not taken into account in current climate models.
The plankton of the oceans will capture 4 Gt of carbon less per year by the end of this century, representing a reduction of 21 percent. This is equivalent to one-third of current worldwide emissions by industrial activities and would significantly aggravate the anthropogenic effects on climate change.
A previous study applying the metabolic theory of ecology to the earth’s terrestrial ecosystem gave very similar results and predictions: respiration will increase to outstrip photosynthesis as temperature increases . That means primary production will go down as carbon dioxide from respiration and decomposition go up.
The oceans’ plankton is about to give us a final curtain call in the greatest tragedy the human species has ever enacted. All the evidence is converging towards this terrible end, unless we make determined efforts to end burning fossil fuels right now. Numerous options for sustainable and renewable energies already exist  (Which Energy?) that will save our oceans and our planet.
The Institute of Science in Society