Page 188 - Algae Anatomy, Biochemistry, and Biotechnology

Biogeochemical Role of Algae 171

silica frustule has recently been shown to play a role in CO 2 acquisition, which indicates that Si
limitation can induce CO 2 limitation in diatoms. Specifically, the silica frustule facilitates the enzy-
matic conversion of bicarbonate to CO 2 at the cell surface by serving as a pH buffer thus enabling
more efficient photosynthesis. This control by diatoms and the reduction in silicate input from rivers
due to cooling and drying of the climate offers a feedback mechanism between climate variability,
diatom productivity, and CO 2 exchange. These features of diatom physiology almost certainly con-
tribute to the in situ observation that diatoms have greater maximum growth rates relative to com-
parable algae. Further, so long as silicic acid is abundant (and other nutrients non-limiting), diatoms
are found to dominate algal communities. Diatoms are estimated to contribute up to 45% of total
oceanic primary production, making them major players in the cycling of all biological elements.
They globally uptake and process 240 Tmol Si yr 21 . Currently, risk to diatoms comes from both
climate forced impacts and anthropogenic sources. Reduced input of silicate due to damming of
rivers and changes in water use patterns, and increased input of inhibitory levels of ammonium
to estuaries and adjacent coastal waters affect diatom success. The high ammonium concentrations
prevalent in some estuaries, a result of anthropogenic inputs from sewage treatment plants and
agricultural runoff inhibit the uptake of nitrate by diatoms which draw primarily on nitrate for
high growth rates.
Aside from their role in the silicon cycle, the diatoms have also attracted attention because of
their importance to the export of primary production to the ocean’s interior. Aggregation and
sinking is an important aspect in the life history of many diatom species, and high sinking vel-
ocities, whether as individuals, aggregates, or mats, allow diatoms to rapidly transport material
out of the surface mixed layer. Additionally, mesozooplankton grazers that consume diatoms
produce large, fast-sinking faecal pellets. These processes remove nutrients and carbon from the
productive surface waters before they can be remineralized, making the diatoms crucial to
“new” (or export) production. So long as silicic acid is available, diatoms act as a conduit for
nutrients and carbon to deep waters, contrasting with the production of other algae, which
“traps” nutrients in a regeneration loop at the surface.
Though diatoms are by far the most important organisms that take up silicic acid to form their
encasing structures, silicoflagellates also deserve mentioning. These unicellular heterokont algae
belong to a small group of siliceous marine phytoplankton. Silicoflagellates live in the upper
part of the water column, and are adapted for life in tropical, temperate, and frigid waters. Silico-
flagellates have a multi-stage life-cycle, not all stages of which are known. The best-known stage
consists of a naked cell body with a single anterior flagellum and numerous plastids contained
within an external lateral skeleton. This skeleton is composed of hollow beams of amorphous
silica, forming a network of bars and spikes arranged to form an internal basket. The siliceous skel-
eton of silicoflagellates is very susceptible to dissolution, and therefore their preservation is often
hindered by diagenetic processes; moreover, their abundance is relatively low compared with that
of other siliceous microfossils, because they form a small component of marine sediments; both
these reasons make their presence rare in the sedimentary record.

ALGAE AND THE SULFUR CYCLE
Sulfur is an essential element for autotrophs and heterotrophs. In its reduced oxidation state, the
nutrient sulfur plays an important part in the structure and function of proteins. Three amino
acids found in almost all proteins (cysteine, cystine, and methionine) contain carbon-bounded
sulfur. Sulfur is also found in sulfolipids, some vitamins, sulfate esters, and a variety of other
compounds.
In its fully oxidized state, sulfur exists as sulfate and is the major cause of acidity in both natural
and polluted rainwater. This link to acidity makes sulfur important to geochemical, atmospheric,
and biological processes such as the natural weathering of rocks, acid precipitation, and rates of
denitrification. Sulfur cycle is also one of the main elemental cycles most heavily perturbed by