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.
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