7Instead of releasing oxygen gas while fixing carbon dioxide as in photosynthesis, hydrogen sulfide chemosynthesis produces solid globules of sulfur in the process:
18H2S + 6 CO2+ 3 O2→ C6H12O6+ 12 H2O + 18 S
Figure 4.12: Large concentrations of tubeworm Riftia pachyptila, with anemones and mussels colonizing in close proximity in the Galapagos Rift.
Figure 4.13: An azimuthal projection showing (top) the Arctic Ocean and the North Pole, and (bottom) the South Geographic Pole (1), South Magnetic Pole (2), South Geomagnetic Pole (3; not of our concern) and South Pole of Inaccessibility (4; ditto).
Outermost blue lines are 60°.
8Schofield, O., Ducklow, H. W., Martinson, D. G., Meredith, M. P., Moline, M. A., and Fraser, W. R.
(2010). How do polar marine ecosystems respond to rapid climate change? Science, 328(5985):1520–3
fuelled by chemical compounds as energy sources instead of light (chemoautotrophy).7The chemosynthetic bacteria grow into a thick mat which attracts other organisms, such as amphipods and copepods, which graze upon the bacteria directly. Larger organisms, such as snails, shrimp, crabs, tube worms, fish (especially eelpout, cutthroat eel, ophidiiforms and Symphurus thermophilus), and octopuses (notably Vulcanoc-topus hydrothermalis), form a food chain of predator and prey relationships above the primary consumers. The main fami-lies of organisms found around seafloor vents are annelida, tubeworms, gastropods, and crustaceans, with large bivalves and “eyeless” shrimp making up the bulk of non-microbial organisms.
Tube worms (Siboglinidae), which may grow to over 2 m tall in the largest species, often form an important part of the community around a hydrothermal vent (Fig. 4.12). They have no mouth or digestive tract, and like parasitic worms, absorb nutrients produced by the bacteria in their tissues.
About 10 billion bacteria are found per g of tubeworm tissue.
Tubeworms have red plumes which contain hemoglobin.
Hemoglobin combines with hydrogen sulfide and transfers it to the bacteria living inside the worm. In return, the bacteria nourish the worm with carbon compounds.
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Figure 4.14: ROV image of krill grazing under the ice. In this image most krill swim in an upside down position directly under the ice.
usually is much warmer than the air above it, the extent of sea ice is largely controlled by the winds and currents that push it northwards. If it is pushed quickly, the ice can travel much further north before it melts. Most ice is formed along the coast, as the northward-moving ice leaves areas of open water (coastal latent heat polynyas), which rapidly freeze.
4.3.1 Sea Ice
Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean’s surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth’s surface and about 12% of the world’s oceans. Much of the world’s sea ice is enclosed within the polar ice packs in the Earth’s polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean.
In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere. The uppermost layer of the ocean is supercooled to slightly below the freezing point, at which time tiny ice platelets (frazil ice) form. With time, this process leads to a mushy surface layer, known as grease ice. Frazil ice formation may also be started by snow-fall, rather than supercooling. Waves and wind then act to compress these ice particles into larger plates, of several me-ters in diameter, called pancake ice. These float on the ocean surface, and collide with one another, forming upturned edges. In time, the pancake ice plates may themselves be rafted over one another or frozen together into a more solid ice cover, known as consolidated pancake ice. Such ice has a very rough appearance on top and bottom.
When sea water freezes, the ice is riddled with brine-filled channels which sustain “sympagic” (ice-associated) organisms.
In fact, a number of varieties of algae such as diatoms engage in photosynthesis in polar regions of the earth. Other energy sources include Aeolian dust and pollen swept in from other regions. These ecosystems also include bacteria and fungi, as well as animals such as flatworms and crustaceans. A number of sympagic worm species are commonly called “ice worms”.
Additionally, the ocean has abundant plankton, and prolific algal blooms occur in the polar regions each summer as well as in high mountain lakes, bringing nutrients to those parts of the ice in contact with the water. In spring, krill scrape off the green lawn of ice algae from the underside of the pack ice (Fig. 4.14). They in turn provide food for animals such as krill and specialised fish like the bald notothen, fed upon in turn by larger animals such as Emperor penguins and Minke whales.
Figure 4.15: Polar sea ice matrix covered in algae community showing a typical environment for sea ice microbial communities.
4.3.2 Sea Ice Microbial Community
The fluctuation of brine salinity, which is controlled by atmo-spheric temperatures, is the single-most influential factor on the chemistry of the sea-ice matrix. The solubility of carbon dioxide and oxygen, two biologically essential gases, de-creases in higher salinity solutions. This can result in hypoxia within high heterotrophic activity regions of the sea ice matrix.
Regions of high photosynthetic activity often exhibit internal depletion of inorganic carbon compounds and hyperoxia.
These conditions have the potential to elevate brine pH and to further contribute to the creation of an extreme environ-ment. In these conditions, high concentrations of dissolved organic matter (DOM) and ammonia and low concentrations of nutrients often characterize the ice matrix.
The concentration of nutrients such as nitrate, phosphate and silicate inside the sea ice matrix relies largely on the diffusive influx from the sea ice-water interface and to some extent on the atmospheric deposits on the sea ice-air interface.
The chemical properties of the sea-ice matrix are highly complex and depend on the interaction within the internal sea-ice biological assemblage as well as external physical fac-tors. Winters are typically characterized by moderate oxygen levels that are accompanied by nutrient and inorganic carbon concentrations that are not growth limiting to phytoplank-ton (Fig. 4.15). Summers are typically characterized by high oxygen levels that are accompanied by a depletion of nutri-ents and inorganic carbon. Because of its diffusive interaction with seawater, the lower part of the sea ice matrix is typically characterized by higher nutrient concentrations.
Microorganisms present in the surface seawater during fall are integrated in the brine solution during ice forma-tion. Studies have shown that sea-ice microbial retention can be enhanced by the presence of extracellular polymeric substance/polysaccharides (EPS) on the walls of the brine channels. EPS are proteins expressed on the cell walls of mi-croorganism such as algae. They improve the cell adherence to surfaces and when found in sufficient concentration, are thought to play a role in recruiting other organisms such as microbes. Airborne microorganisms make up a significant proportion of the microbial input to the ice matrix. Microor-ganisms located in the sea or in the ice matrix brine can be incorporated in falling snow or in aerosols.
Both the Antarctic and Arctic sea ice environments present strong vertical gradients of salinity, temperature, light, nutri-ents and DOM. These gradinutri-ents were shown to induce strong vertical stratification in bacterial communities throughout the ice layer. Microbial abundance declines significantly with depth in the upper and middle ice, but not in the lowest, sug-gesting that much of the prokaryotic bacterial community is
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Figure 4.16: Coastal upwelling will occur if the wind direction is parallel to the coastline and gen-erate wind-driven currents. These are diverted to the right of the winds in the Northern Hemisphere and to the left in the Southern Hemisphere due to the Coriolis effect. The result is a net movement of surface water at right angles to the direction of the wind. When this “Ekman transport” is occurring away from the coast, surface waters moving away are replaced by deeper, colder, and denser water.
9The term “silicic acid” has traditionally been used as a synonym for silica, SiO2. Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH)4. What is meant here is really silica, SiO2.
resistant to extreme environmental conditions. Heterotrophic bacteria were also shown to be more abundant at the bottom of the ice layer in zones of greater algae concentration, which characterized by higher DOM and nutrient concentrations.
Metagenomic studies of the Ross Sea illustrate the high abundance of aerobic anoxygenic phototrophic bacteria in sea ice environments. The predominance of Gammaproteobacte-ria in sea ice around the globe have been reported by many studies. A large proportion of the identified sea-ice micro-bial community in these studies were shown to belong to phylotypes associated with heterotrophic taxa.
Bacteria in all environments contribute to the microbial loop, but the roles of sea-ice microbial communities in the microbial loop differ due to the rapidly changing environ-mental conditions found in the Arctic and Antarctic. Sea-ice algae contribute 10%–28% of the total primary production in ice-covered regions of the Antarctic. Microalgae provide a vital source of nutrition for juvenile zooplankton such as the Antarctic krill Euphausia superba in the winter. DOM derived from phototrophic microalgae is crucial to the microbial loop, by serving as a growth substrate for heterotrophic bacteria.