Algae can be aquatic or subaerial, when they are exposed to the atmosphere rather than being submerged
in water. Aquatic algae are found almost anywhere from freshwater spring to salt lakes,
with tolerance for a broad range of pH, temperature, turbidity, and O2 and CO2 concentration.
They can be planktonic, like most unicellular species, living suspended throughout the lighted
regions of all water bodies including under ice in polar areas. They can be also benthic, attached
to the bottom or living within sediments, limited to shallow areas because of the rapid attenuation
of light with depth. Benthic algae can grow attached on stones (epilithic), on mud or sand (epipelic),
on other algae or plants (epiphytic), or on animals (epizoic). In the case of marine algae, various
terms can be used to describe their growth habits, such as supralittoral, when they grow above
the high-tide level, within the reach of waves and spray; intertidal, when they grow on shores
exposed to tidal cycles: or sublittoral, when they grow in the benthic environment from the
extreme low-water level to around 200 m deep, in the case of very clear water.
Oceans covering about 71% of earth’s surface contain more than 5000 species of planktonic
microscopic algae, the phytoplankton, which forms the base of the marine food chain and produces
roughly 50% of the oxygen we inhale. However, phytoplankton is not only a cause of life but also a
cause of death sometimes. When the population becomes too large in response to pollution with
nutrients such as nitrogen and phosphate, these blooms can reduce the water transparency,
causing the death of other photosynthetic organisms. They are often responsible for massive fish
and bird kills, producing poisons and toxins. The temperate pelagic marine environment is also
the realm of giant algae, the kelp. These algae have thalli up to 60 m long, and the community
can be so crowded that it forms a real submerged forest; they are not limited to temperate
waters, as they also form luxuriant thickets beneath polar ice sheets and can survive at very low
depth. The depth record for algae is held by dark purple red algae collected at a depth of 268 m,
where the faint light is blue-green and its intensity is only 0.0005% of surface light. At this
depth the red part of the sunlight spectrum is filtered out from the water and sufficient energy is
not available for photosynthesis. These algae can survive in the dark blue sea as they possess accessory
pigments that absorb light in spectral regions different from those of the green chlorophylls a
and b and channel this absorbed light energy to chlorophyll a, which is the only molecule that
converts sunlight energy into chemical energy. For this reason the green of their chlorophylls is
masked and they look dark purple. In contrast, algae that live in high irradiance habitat typically
have pigments that protect them against the photodamages caused by singlet oxygen. It is the composition
and amount of accessory and protective pigments that give algae their wide variety of
colors andx for several algal groups, their common names such as brown algae, red algae, and
golden and green algae. Internal freshwater environment displays a wide diversity of microalgae
forms, although not exhibiting the phenomenal size range of their marine relatives. Freshwater phytoplankton
and the benthic algae form the base of the aquatic food chain.
A considerable number of subaerial algae have adapted to life on land. They can occur in surprising
places such as tree trunks, animal fur, snow banks, hot springs, or even embedded within
desert rocks. The activities of land algae are thought to convert rock into soil to minimize soil
erosion and to increase water retention and nutrient availability for plants growing nearby.
Algae also form mutually beneficial partnership with other organisms. They live with fungi to
form lichens or inside the cells of reef-building corals, in both cases providing oxygen and complex
nutrients to their partner and in return receiving protection and simple nutrients. This arrangement
enables both partners to survive in conditions that they could not endure alone.
Table 1.2 summarizes the different types of habitat colonized by the algal divisions.