Page 194 - Algae

Biogeochemical Role of Algae 177

atmosphere, all three of the effects just described conspire to mean that coccolithophore
blooms may tend to make the overall water column dramatically cooler over an extended
period, even though this may initially be masked by a warming of the surface skin of the ocean
(the top few meters).
All phytoplankton growth removes CO 2 into organic matter and reduces atmospheric CO 2
(by means of photosynthesis). However, coccolithophores are unique in that they also take up
bicarbonate, with which to form the calcium carbonate of their coccoliths (calcification process).
The coccolithophorid blooms are responsible for up to 80% of surface ocean calcification. In the
equilibrium of calcification process, an increase in CO 2 concentration leads to calcium carbonate
dissolution, whereas a decrease in CO 2 levels achieves the reverse. While photosynthetic carbon
fixation decreases the partial pressure of CO 2 as dissolved inorganic carbon is being utilized,
conditions favoring surface calcification by coccolithophorid blooms contribute to the increase
of dissolved CO 2 .
2
The relative abundance of the components of the carbonate system (CO 2 ,H 2 CO 3 , HCO 3 ,
22
and CO 3 ) depends on pH, dissolved inorganic carbon, and the total alkalinity, and the equi-
librium between the components can shift very easily from being in one of these dissolved
forms to being in another. How much of the total carbon in each form is determined
mainly by the alkalinity and by the water temperature? When the seawater carbon system
2
is perturbed by coccolithophore cells removing HCO 3 to form coccoliths, this causes a
rearrangement of how much carbon is in each dissolved form, and this rearrangement takes
2
place more or less instantaneously. The removal of two molecules of HCO 3 and the addition
of one molecule of CO 2 change the alkalinity and this indirectly causes more of the dissolved
carbon to be pushed into the CO 2 form. Although the total dissolved carbon is obviously
reduced by the removal of dissolved carbon (bicarbonate ions) into solid calcium carbonate,
yet the total effect, paradoxically, is to produce more dissolved CO 2 in the water. In this
way, coccolithophore blooms tend to exacerbate global warming by causing increased atmos-
pheric CO 2 (greenhouse effect), rather than to ameliorate it, as is the case when dissolved CO 2
goes into new organic biomass. However, recent work is showing that additional properties of
coccoliths may make the situation yet more complicated. Coccolith calcite is rather dense
21
(2.7 kg l 21 compared to seawater density of 1.024 kg l ), and the presence of coccoliths in
zooplankton faecal pellets and marine snow (the two main forms in which biogenic matter
sinks to the deep ocean) causes them to sink more rapidly. Slow-sinking organic matter
may also adhere to the surfaces of coccoliths, hitching a fast ride out of the surface waters.
If organic matter sinks faster then there is less time for it to be attacked by bacteria and
so more of the locked-in carbon will be able to escape from the surface waters, depleting
the surface CO 2 . Probably this co-transport of organic matter with coccoliths offsets the atmos-
pheric CO 2 increase that would otherwise be caused, and makes coccolithophore blooms act to
oppose global warming, rather than to intensify it.

SUGGESTED READING
Ærtebjerg, G., Carstensen, J., Dahl, K., Hansen, J., Nygaard, K., Rygg, B., Sørensen, K., Severinsen, G.,
Casartelli, S., Schrimpf, W., Schiller, C., and Druon, J. N., Eutrophication in Europe’s coastal waters.
Topic report 7/2001, European Environment Agency, Copenhagen, 2001.
Anbar, A. D. and Knoll, A. H., Proteozoic ocean chemistry and evolution: a bioorganic bridge? Science, 297,
1137–1142, 2002.
Berman-Frank, I., Lundgren, P., Chen, Y., Kupper, H., Kolber, Z., Bergman, B., and Falkowski, P.,
Segregation of nitrogen fixation and oxygenic photosynthesis in the marine cyanobacterium
Thricodesmium, Science, 294, 1534–1537, 2001.