Fig. 23.7 Drawings of currents (left) and sea ice (right) in

the Southern Ocean surrounding the Antarctic continent.

bottom-dwelling algae on the bottom of the ice, and algae growing in the ice itself.

1The bottom-dwelling algae are mostly diatoms such as Nitzschia stellata and Amphiprora sp. These bottom-dwelling diatoms occur principally on the bottom of land-fast ice. Pack ice, though, does not have a well-developed community of bottom-dwelling diatoms. This is probably because the bottom of pack ice is extensively grazed by the krill (Garrison et al., 1986). Observers on ice-breaking ships have seen swarms of krill on upturned ice floes or

in holes of decaying ice floes.

2Algae growing in the ice. The algae growing in the ice depend on the conditions of the ice. Initially, salts are excluded from the ice as the salt water freezes during the austral fall and winter. This results in brine inclusions in the ice with salinities as high as 10%. The brine inclusions contain cryoand halotolerant algae that are able to tolerate the cold and high salinity. Dinoflagellates, chrysophytes, and the green Mantoniella are common in these brine inclusions (Stoecker et al., 1998). These brine algae peak and form resting spores before the melting of the surface ice dilutes the brine and produces hyposaline conditions and a new

population of algae. There is an abundance of nutrients available in the meltwater and diatoms such as Nitzschia corta proliferate. As pack ice melts, the algae in the ice are released to the water column where the algae form a seed population for the phytoplankton bloom that occurs during the austral summer (Smith and Nelson, 1986).

The grand experiment

Man’s activities have resulted in an increase in CO2 in the atmosphere and potential warming of the atmosphere of the earth due to the “greenhouse effect.” This increase in CO2 in the atmosphere can be addressed in one of two ways, either by a reduction in the burning of fossil fuels or by removing the CO2 from the atmosphere.

John Martin of the Moss Landing Marine Laboratory in California put forth the hypothesis that iron availability limits phytoplankton production in nutrient-rich seas. He further suggested that it might be possible to fertilize the Southern Ocean (which has an abundance of unused nutrients) with iron, increase photosynthesis by plankton, and increase the flux of CO2 from the atmosphere to the deep ocean, which contains 60 times more CO2 than the atmosphere.

I first said this more or less facetiously at a Journal Club lecture at Woods Hole Oceanographic Institute in July 1988. I estimated that with 300 000 tons of Fe, the Southern Ocean phytoplankton could bloom and remove two billion tons of carbon dioxide. Putting on my best Dr Strangelove accent, I suggested that with half a ship load of Fe, I could give you an ice age. Chisholm and Morel (1991)

Historically, there has been variation in the CO2 concentration in the atmosphere. Man’s activities have resulted in an increase in CO2 concentration today to almost 350 parts per million (ppm). This is an increase from 200 ppm during the last ice age (glacial maximum, 18 000 years ago). The glacial minimum that preceded this, however, had an atmospheric concentration of 280 ppm, approximately the same as in 1900. The decrease in CO2 concentration during the last glacial maximum is explained as follows. There was a fivefold increase in the arid areas of the earth, along with a 1.5 times increase in the winds. These two factors resulted in a 50-fold increase in the airborne dust particles. Since iron is the fourth most common element on Earth, the airborne dust contained a significant amount of iron, much of which was deposited in the oceans. This resulted in a threefold increase in photosynthesis and a decrease in CO2 to around 200 ppm. This produced a decrease in the greenhouse effect, a cooling of the Earth, and an ice age (Martin, 1990).

The suggestion of adding iron to the Southern Ocean to reduce atmospheric CO2 triggered a debate about whether we should engage in intentional large-scale intervention with the Earth’s natural biogeochemical cycles. Martin put forward the following:

One could argue that this really is not such a new or weighty issue. After all, we have already changed dramatically the landscape of terrestrial ecosystems, we have converted forests to croplands, croplands to deserts, rivers to lakes and deserts to greenbelts. So why is there such a big fuss about the prospect of spreading some iron around the ocean? If we are inadvertently, but knowly, changing the chemistry of the atmosphere through fossil fuel burning, why should we not change it purposely through iron fertilization or some other scheme? Chisholm and Morel (1991)

Ultimately it was decided to try a test of the hypothesis. John Martin orchestrated the scientific and logistical planning for a large-scale iron enrichment experiment in the open ocean, although his untimely death from cancer in 1993 prevented him from seeing the outcome. In midNovember 1993, the RV Columbus Iselin arrived 500 miles south of the Galapagos Islands with 480 kg of iron. The iron was pumped into the propellar wash as the vessel steamed to and fro across an 88 km field over 24 hours, raising the iron concentration from about 0.05 mM to about 4 mM. Water samples were taken and monitored for phytoplankton and nutrients, while a P-3 Orion airplane optically scanned the water for changes in phytoplankton pigments. The results showed that there was an increase in phytoplankton, although the increase was not as great as was predicted from laboratory cultures. This was probably due to increased grazing by zooplankton, which showed a 50% increase over the period (Wells, 1994). The experiment was repeated in 1995 (Coale et al., 1996) and again enrichment of photoplankton was obtained.

The experiment showed it is possible to remove 1000 to 100 000 tons of carbon from the atmosphere for each one ton of iron added to the Southern Ocean (Boyd, 2004; Dalton, 2002). The political realities of performing Fe enrichment in quantities high enough to significantly effect the atmospheric CO2 concentration is on hold as the debate over the increase in CO2 in the atmosphere, and global warming, continues.

Antarctic lakes as a model for life on the planet Mars or Jupiter’s moon Europa

The McMurdo Dry Valleys in Antarctica (Fig. 23.8) have one of the driest and coldest deserts on Earth and house the only permanently icecovered lakes on our planet (Pocock et al., 2004). The headquarters of Captain Robert F. Scott (who perished with his party returning from the South Pole) was across McMurdo Sound from the dry valleys. In December 1903, Scott wrote, “It is worthy to record, too, that we have seen no living thing, not even a moss or lichen; all that we did

Fig. 23.8 Profiles of water conditions from Lake Bonney, Taylor Dry Valley, Antarctica. The dominant alga,

Chlamydomonas raudensis, occurs primarily between 10 and 17 meters depth. (Modified from Spigel and Priscu, 1996.)

find, far inland among the moraine heaps, was the skeleton of a Weddell seal, and how that came there is beyond guessing. It is certainly a valley of the dead; even the great glacier which once pushed through it has withered away” (Priscu, 1999). Average annual precipitation in these dry valleys is less than 10 cm and average air temperature is about 20° C. Lake Bonney is in the Taylor Valley and has a permanent ice cover that prevents wind mixing of the column, resulting in vertical mixing only on the molecular scale; vertical mixing time is approximately 50 000 years. Stratification in Lake Bonney is the result of strong salinity gradients. The salinity of the lake varies from freshwater at the surface (from melting glaciers in the austral (Southern Hemisphere) summer) to hypersaline brine more

than five times seawater at the bottom. There are four months of darkness during the austral winter with algal cells generating energy from heterotrophy. During the austral summer the ice cover attenuates about 98% of light. Therefore, photosynthetic inhabitants of the water column below the ice are never exposed to saturating light levels and have adapted to a shade environment. Chlamydomonas raudensis is the dominant alga in this environment. This green alga is an obligate psychrophile because it does not grow above 16° C. Chlamydomonas raudensis lives in a discrete layer in the lake at depths of between 10 and 17 m, which is a transition zone between an oxygen-rich layer where O2 production exceeds respiratory O2 uptake and the deeper oxygendeficient region (Fig. 23.8) (Pocock et al., 2004). The shade adaptation is reflected in its low chlorophyll a/b ratio (Morgan et al., 1998). Light above 680 nm is primarily absorbed by chlorophyll a while chlorophyll b absorbs shorter wavelengths (Fig. 1.13). Similarly there are low levels

of photosystem I to photosystem II; photosystem II utilizes light of wavelengths shorter than 680 nm while photosystem I utilizes light up to 700 nm wavelength. The permanent ice cover in these Antarctic lakes absorbs all light wavelengths above 600 nm (Pocock et al., 2004). The perennial ice overlying liquid water in these Antarctic lakes is similar to the situation on the moon Europa of Jupiter, and on Mars where permanently ice-covered lakes are thought to have existed between 3.1 and 3.8 billion years ago (Priscu et al., 1999).

algae (especially those inhabiting shallow waters) is one mechanism the cells have evolved in dealing with damaging UV light. The most common of these UV-absorbing compounds are the mycosporine-like amino acids (MAAs) that absorb UV radiation between 310 and 360 nm (Karsten et al., 1999) (Fig. 23.9). Mycosporine is a generic term used to describe small, water-soluble, nitrogenous metabolites. The MAAs have amino groups conjugated to a cyclohexane chromophore (Fig. 23.9). The MAAs occur in many algal groups where they attenuate UV-B and reduce the harmful effects of the radiation on cells.

Ultraviolet radiation, the ozone hole, and sunscreens produced by algae

Ultraviolet radiation in customarily divided into three spectral regions: UV-C (200–280 nm), UV-B (280–320 nm), and UV-A (320–400 nm) (Banaszak and Trench, 1999). UV-C, with the shortest wavelengths, is the most potentially damaging to cells but it is almost entirely absorbed by ozone and other atmospheric gases. Biological weighing functions, which estimate the effect of each wavelength on a biological process, indicate the affect of UV-B is most severe. This is because aromatic amino acids (e.g., tyrosine, phenylalanine, tryptophan) strongly absorb UV-B at 280 nm. Therefore, proteins containing these amino acids are highly susceptible to photodestruction, particularly to splitting of disulfide bridges between cysteine residues (which control the tertiary structure of proteins) (Bischof et al., 2000).

Recently there has been a depletion in the ozone layer, particularly in the polar regions, which has been at least partially attributed to chlorofluorocarbons. Springtime ozone reductions up to 60% compared with values 30 years ago have been recorded (Karsten et al., 1999). This has resulted in increased amounts of UV-B reaching the surface of the Earth.

The production of ultraviolet sunscreens by

Hydrogen fuel cells and hydrogen gas production by algae

Hydrogen fuel cells are an attractive and clean energy source for motor cars. Electrolysis of water is the most commonly mentioned method for generating hydrogen gas. Algae can also be induced to produce hydrogen gas. Cyanobacteria produce hydrogen as a byproduct of nitrogen fixation (Fig. 23.10). The nitrogenase enzyme, however, has a low catalytic turnover and has a high energy requirement, leading most investigators to reject it as a source of hydrogen gas (Ghiradi et al., 2000). The production of hydrogen gas by hydrogenases in green algae has been more extensively investigated (Melis et al., 2000; Ghiradi et al., 2000). In the green algae, photons of light are captured in the chlorophyll a in a reaction center (P700) of photosystem I, driving the potential by 1 V. Excitation of P700 results in the transfer of an electron to a membranebound form of ferredoxin, a 11.6 kdal iron-sulfur protein of the Fe4S4 type. Hydrogenases in the green algae combine the electrons from ferredoxin with protons to produce hydrogen gas (Fig. 23.10). This is essentially an anaerobic reaction since the hydrogenase enzymes are inhibited by oxygen.