Page 193 - Algae

176 Algae: Anatomy, Biochemistry, and Biotechnology

can be observed. Although each cell is invisibly small, there can be as many as a thousand billion
21
billion (10 ) of them in a large bloom, and the population as a whole has an enormous impact. E.
huxley blooms are processed through the food web, with viruses, bacteria, and zooplankton all con-
tributing to the demise and decomposition of blooms. Some debris from the bloom survive to sink
to the ocean floor, taking chemicals out of the water column. While they live and when they die, the
phytoplankton cells leak chemicals into the water. A bloom can be thought of as a massive chemical
factory, extracting dissolved carbon dioxide, nitrate, phosphate, etc. from the water, and at the same
time injecting other chemicals such as oxygen, ammonia, DMS, and other dissolved organic com-
pounds into the water. At the same time, the chemical factory pumps large volumes of organic
matter and calcium carbonate into the deep ocean and to the ocean floor. Some of this calcium car-
bonate eventually ends up as chalk or limestone marine sedimentary rocks, perhaps to cycle through
the Earth’s crust and to reappear millions of years later as mountains, hills, and cliffs. Coccolitho-
phorids are primarily found at low abundance in tropical and subtropical seas, and at higher
concentrations at high latitudes in midsummer, following diatom blooms. Hence, export of
inorganic carbon by diatoms in spring at high latitudes can be offset by an efflux of carbon to
the atmosphere with the formation of coccolithophore blooms later in the year.
The contemporary ocean export of organic carbon to the interior is often associated with diatom
blooms. This group has only risen to prominence over the past 40 million years.
Coccolithophorid abundance generally increases through the Mesozoic, and undergoes a
culling at the Kretaceous/Tertiari (K/T) boundary, followed by numerous alterations in the Cen-
ozoic. The changes in the coccolithophorid abundances appear to trace eustatic sea level variations,
suggesting that transgressions lead to higher calcium carbonate fluxes. In contrast, diatom sedimen-
tation increases with regressions and because of the K/T impact, diatoms have generally replaced
coccolithophorids as ecologically important eukaryotic phytoplankton. On much finer time scales,
during the Pleistocene, it would appear that interglacial periods favor coccolithophorids abundance,
whereas glacial periods favor diatoms. The factors that lead to glacial-interglacial variations
between these two functional groups are relevant to elucidating their distributions in the contem-
porary ecological setting of the ocean.
Coccolithophores influence regional and global temperature, because they can affect ocean
albedo and ocean heat retention, and have a greenhouse effect. Coccoliths do not absorb
photons, but they are still optically important because they act like tiny reflecting surfaces, diffusely
reflecting the photons.
A typical coccolith bloom (containing 100 mg m 23 of calcite carbon) can increase the ocean
albedo from 7.5 to 9.7%. If each bloom is assumed to persist for about a month, then an annual
2
6
coverage of 1.4 10 km will increase the global annual average planetary albedo by

1 1:4
(9:7 7:5) ¼ 0:001% (4:9)
12 510

2
6
where 510 10 km is the surface area of the Earth.
This is a lower bound on the total impact, because sub-bloom concentration coccolith light scat-
tering will have an impact, over much larger areas (estimated maximum albedo impact ¼ 0.21%).
A 0.001% albedo change corresponds to a 0.002 W m 22 reduction in incoming solar energy,
whereas an albedo change of 0.21% causes a reduction of 0.35 W m 22 . These two numbers can
be compared to the forcing due to anthropogenic addition of CO 2 since the 1700s, estimated to
be about 2.5 W m 22 . Coccolith light scattering is therefore a factor of only secondary importance
in the radiative budget of the Earth. However, the scattering caused by coccoliths causes more heat
and light than usual to be pushed back into the atmosphere; it causes more of the remaining heat to
be trapped near to the ocean surface, and only allows a much smaller fraction of the total heat to
penetrate deeper in the water. Because it is the near-surface water that exchanges heat with the