Shutting Down the Oceans Act III: Global Warming and Plankton;
Snuffing Out the Green Fuse
The oceans’ plankton is about to give us the final curtain call in the greatest
tragedy the human species has ever enacted unless we make determined efforts
to stop burning fossil fuels right now. Numerous options for sustainable and
renewable energies exist (Which
Energy?) that will save our oceans and our planet. Dr.
Mae-Wan Ho
A fully
referenced version of this article is posted on ISIS members’ website. Membership
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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 [1]. 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 ľ 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
[4] (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 ľ 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.
Well-grounded predictions
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 [3]. 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 [5] (Which Energy?)
that will save our oceans and our planet.
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