Algae

Many types of algae are microscopic, occurring in single cells or small colonies. The usual habitat of many of the microscopic algae is open waters, in which case they are known as phytoplankton. Many species, however, live on the surfaces of rocks and larger plants within shallow-water habitats, and these are known as periphyton. Other microscopic algae live on the moist surfaces of soil and rocks in terrestrial environments.

Microscopic algae are at the base of their ecological food web—these are the photosynthetic, primary producers that are fed upon by herbivores. In the open waters of ponds, lakes, and especially the vast oceans, the algal phytoplankton is the only means by which diffuse solar radiation can be fixed into biological compounds. In these open-water (or pelagic) habitats the phytoplankton are consumed by small, grazing animals known as zooplankton, most of which are crustaceans. The zooplankton are in turn fed upon by larger zooplankton or by small fish (these predators are known as planktivores), which may then be eaten by larger fish (or piscivores). At the top of the open-water food web may be fish-eating birds, seals, whales, very large fish such as sharks or bluefin tuna, or humans. Therefore, the possibility of all of the animals occurring higher in the food webs, including the largest of the top predators, are ultimately dependent on the productivity of the microscopic phytoplankton of the pelagic marine ecosystem.

Other algae are macroscopic, meaning they can be readily observed without the aid of magnification. Some of these algae are enormous, with some species of kelps commonly reaching lengths greater than tens of meters long. Because they are primary producers, these macroscopic algae are also at the base of their ecological food webs. In most cases, however, relatively few herbivores can directly consume the biomass of macroscopic algae, and the major trophic interaction of these plants is through the decomposer, or detritivore part of the food web. In addition, because of their large size, macroscopic algae are critically important components of the physical structure of their ecosystems, providing habitat for a wide range of other organisms. The largest kelps develop a type of ecosystem that is appropriately referred to as a marine "forest" because of the scale and complexity of its habitat structure.

Some species of green algae occur as mutualistic symbionts with fungi, in an association of two organisms known as lichens. Lichens are common in many types of habitats. Other green algae occur in a mutualism with certain animals. In general, the host animal benefits from access to the photosynthetic products of the green alga, while the alga benefits from protection and access to inorganic nutrients. For example, species of unicellular Chlorella live inside of vacuoles within host cells of various species of freshwater protozoans, sponges, and hydra. Another species of green alga, Platymonas convolutae, occurs in cells of a marine flatworm, Convoluta roscoffensis. Various other green algae occur inside of marine mollusks known as nudibranchs. Similarly, various species of dinoflagellates occur as symbionts with marine corals.

Each species within an algal community has its particular ecological requirements and tolerances. Consequently, algal species tend to segregate along gradients in time and space, according to varying patterns of environmental resources, and of biological interactions, such as competition and predation. For example, during the growing season there is a time-series of varying abundances of phytoplankton species in open-water habitat. At certain times, particular species or closely related groups of species are abundant, but then these decline and other species of phytoplankton become dominant. This temporal dynamic is not totally predictable; it may vary significantly from year to year. The reasons for these patterns in the abundances and productivity of algal species are not understood, but they are likely associated with differences in their requirements for nutrients and other environmental factors, and perhaps with differing competitive abilities under resource-constrained conditions.

In a similar way, species of seaweeds tend to sort themselves out along stress-related environmental gradients associated with varying distances above and below the high-tide mark on rocky marine shores. The most important environmental stress for intertidal organisms is desiccation (drying), caused by exposure to the atmosphere at low tide, with the intensity of drying being related to the amount of time that is spent out of the water, and therefore to the distance above the high-tide line. For sub-tidal seaweeds the most important stress is the physical forces associated with waves, especially during storms. The various species of brown and red algae are arranged in rather predictable zonations along transects perpendicular to rocky shores. The largest kelps only occur in the sub-tidal habitats, because they are intolerant of desiccation. Within this near-shore habitat the species of algae are arranged in zones on the basis of their tolerance to the mechanical forces of wave action, as well as their competitive abilities in the least stressful, deeper-water habitats somewhat farther out to sea, where the tallest species grow and develop a kelp forest. In the intertidal, the various species of wracks and rockweeds dominate particular zones at various distances from the low-tide mark, with the most desiccation-tolerant species occurring closest to the high-tide mark. Competition, however, also plays an important role in the distributions of the intertidal seaweeds.