Marine Biogeochemistry - Ecological Systems - page 212

the water column (ca. 3,000 m). Sinking rates of large aggregates are ca. 200 m
day 1 , so that a reduction in the already low rate of dissolution by the necessity for
organic degradation can decrease dissolution of silica markedly. Similar effects of
grazing can occur, as fecal pellets are usually composed of an organic pellicle that
must be degraded prior to chemical silica dissolution.
As with other nutrients, silicic acid has substantial interactions with other
elements, such as nitrate and iron. Under iron-limiting conditions, diatoms continue
to assimilate silicon, but because iron is needed in the enzymes used for nitrate
assimilation, nitrate uptake decreases [ 3 ]. As a result, Si:N ratios increase by nearly
an order of magnitude in diatoms under iron limitation and elevated ratios observed
in natural systems have been used to infer iron limitation.
The Biogeochemical Cycle of Sulfur
In marine systems sulfur is largely present in its most stable form, which is sulfate
(SO 4 2 ). Sulfate is present in high concentrations in most marine systems, and
relatively low concentrations are required by organisms to survive [ 27 ]. As a result,
sulfur does not normally become growth limiting. Sulfate concentrations in marine
systems are primarily controlled by physical rather than chemical processes.
Variations in concentration only have a significant biological impact in anoxic
zones where sulfate reduction occurs [ 2 ]. Sulfur is also present as other inorganic
(H 2 S) and organic (dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS),
carbonyl sulfide (COS), and methanethiol (MeSH)) forms. Sulfate is transformed
into these compounds via the sulfur cycle, which operates primarily in the
photic zone of the upper water column, in the sediments, and around hydrothermal
vents ( Fig. 12.4 ).
In aerobic environments sulfur is converted between inorganic compounds
(sulfate and hydrogen sulfide) and organic sulfur compounds including DMSP,
DMS, COS, and amino acids. Most algae and bacteria use sulfur assimilation to
form amino acids, such as cysteine and methionine [ 27 ]. Some phytoplankton
species, particularly prymnesiophytes and dinoflagellates, use methionine to pro-
duce DMSP, a compound with antioxidant properties [ 28 , 29 ]. DMSP can be
released into the water and subsequently used to produce amino acids through
assimilation by bacteria or phytoplankton, including some species of diatoms and
cyanobacteria, demethylated by bacteria to produce MeSH, or oxidized into DMS
and acrylic acid [ 30 ]. DMS is either broken down in the water into sulfate through
bacterial uptake or photooxidation, or is volatilized into the atmosphere, where it
can act as an important aerosol [ 27 ].
The sulfur cycle in ocean sediments can be divided into reactions that occur in
the upper oxic layer and those that occur in the lower, oxygen-depleted (anoxic)
region. In the anoxic sediments, sulfur-reducing bacteria carry out anaerobic
respiration using sulfate or sulfur-containing organic compounds to oxidize organic
matter, resulting in the production of sulfide, typically as H 2 S, a form of sulfur that