Marine Biogeochemistry - Ecological Systems - page 211

The Biogeochemical Cycle of Silicon

Silicon, despite being a nutrient for only onemajor functional group of phytoplankton
(diatoms), is a major factor in regulating other biogeochemical cycles, such as
carbon. This is because diatoms are extremely important primary producers,
generating approximately as much oxygen on an annual basis as do pine trees in
terrestrial systems. In addition, diatoms are among the largest forms of phyto-
plankton, and hence can sink passively to depth. Diatoms also produce transpar-
ent exopolymer particles, which serve as the primary mechanism for aggregating
particles in the ocean's surface layer, thus producing large, rapidly sinking
particles that are the major component of organic carbon and nitrogen flux to
deeper water ( Fig. 12.2 ). Finally, diatoms are also heavily grazed by herbivorous
organisms, and serve as a means to transfer photosynthate to the large organism-
based food web. All of these characteristics contribute to the substantial impor-
tance of diatoms in the ocean.
Silicon is a major component of rocks and terrestrial minerals, and as a result the
inputs to the ocean in riverine waters are substantial. However, silicon is not readily
dissolvable, and dissolved silicon, which occurs as Si(OH) 4 , remains at relatively
low levels. Aeolian and oceanic weathering of seafloor rocks also constitutes
a significant source of dissolved silicon. Silicon also is found in high concentrations
in waters exiting hydrothermal vents, and while quantitative estimates are uncer-
tain, the contribution of this source to total silicon inputs is likely to be significant.
Silicon is incorporated into diatoms and other marine organisms as opal ( Si

ð Þ 4 nH 2 O ), which is slightly more soluble than pure SiO 2 and undersaturated in

all ocean waters. Opal is found in the sediments as siliceous deposits of biogenic
origin; these deposits are largely focused in the Southern Ocean's polar front region
[ 23 ]. Silicon is recycled within the water column, but rates of this cycling are
modest, and silicon regeneration is often markedly uncoupled from that of carbon
and nitrogen in some regions. The reason for this appears to result from the different
controls of each: organic matter regeneration is largely biologically mediated (by
heterotrophic processes), whereas silicon regeneration is regulated by temperature
[ 24 ]. As a result, in polar regions a large fraction of the organic matter that sinks
from the euphotic zone is regenerated in the upper 250 m, whereas a substantial
amount of silicon sinks to a greater depth as biogenic particles. This uncoupling
contributes to the formation of large zones of biogenic silica deposits in polar
regions and are reflective of surface layer diatomaceous productivity. In a more
recent reanalysis of the global silicon budget, it was concluded that the deposition
of silicon in continental margins may have been greatly underestimated [ 25 ]. If this
were true, then the coupling between the silicon and organic matter budgets would
be even stronger than previously thought.
An additional mechanism to couple the biogeochemistry of silicon and organic
carbon is the presence of an organic membrane that covers diatom frustules [ 26 ].
Silica dissolution does not begin until this membrane is degraded by bacteria, which
decreases the time for the dissolution of opal during the transit of a particle through

OH