23 Terrestrial Algal Ecology

The previous two chapters present the ecology of planktonic and attached algae in freshwater and marine ecosystems. Algae are not, however, restricted to aquatic systems. Cyanobacteria and eukaryotic algae may be found in essentially every type of terrestrial environment on Earth. They occur in every type of soil, from the tropics to the poles, and at every level of precipitation, from deserts through grasslands to forests and agricultural land, including soils polluted by various industrial and mining activities. Cyanobacteria and eukaryotic algae may grow in the surface layers of snowfields at high elevations and in the Arctic and Antarctic in icefields and the upper active layers of permafrost. Cyanobacteria and eukaryotic algae may also grow on and under the surfaces of rocks and beneath translucent rocks, where they form lithic ecosystems. Algae that grow on surfaces that lie above the surface level of soil, water, snow, and ice, where they are exposed to the air, are considered to be subaerial. Thus algae growing on the surfaces of rocks are subaerial and epilithic (on stones), while algae growing beneath the surfaces of rocks or under translucent rocks are endolithic (inside stones) or hypolithic (under stones). Subaerial algae form epilithic communities on the surfaces of natural stone and stone-like substrates, such as brick and concrete, used in human constructions. Subaerial algae also grow as epiphytic communities on plant surfaces, including bark, wood, and leaves. As symbionts with various species of fungi, cyanobacteria and green algae may form lichens, which are found in practically every terrestrial environment, including soils, rocks, the surfaces of other organisms, such as the bark of trees and shrubs, and on the surfaces of human-made structures.

In aquatic ecosystems, most algae are continuously bathed in water and therefore capable of being continuously active metabolically. The only exceptions occur in the intertidal zones of the oceans, where attached macroalgae and periphytic algae undergo periodic exposure, and the shallow littoral zones of large lakes, where persistent winds from a single direction may cause the lake water to pile up on the downwind shore and expose the periphytic algae along the upwind shore for long periods. In contrast, terrestrial algae are not continuously immersed in liquid water. Algae in terrestrial ecosystems can be metabolically active only when there is sufficient water as liquid or vapor to support that activity. The algae in soils are active when liquid water is present in the surface layers as a flow from rain or a film around the soil particles. As the soils dry, some soil algae can move to lower layers and follow the water, while other algae become dehydrated and inactive until the next precipitation falls. In snow and ice fields, algae are active when air temperatures are above freezing and liquid water is present around crystals of snow or ice. If temperatures are continuously below freezing, algae will be very sparse because there will not be sufficient liquid water to permit photosynthesis and growth.

Terrestrial algae must be adapted to survive long periods of metabolic inactivity either in a desiccated state at temperatures above 0ºC or frozen at temperatures below 0ºC. Moreover, terrestrial algae must be able to survive the transition from a desiccated or frozen state to an active hydrated condition without loss of cellular integrity and viability. These transitions often occur repeatedly over short time spans. Terrestrial algae possess a number of genes that code for specific substances that maintain cellular integrity, structure, and viability through these extreme transitions. These genes are the subject of current research interest because of their potential use in developing agricultural crops that are more resistant to drought or frost. Some subaerial algae and lichens can remain active without liquid water, provided that the amount of water vapor in the air (the relative humidity) is above 70%. These algae are truly terrestrial in the sense that they are not dependent on liquid water. In Antarctica viable algal cells have been recovered from 18 m-deep layers of permafrost, where they have lain for more than 3 million years. These ancient microbial communities may be the best analog to any potential life on the planet Mars, where permafrost also exists adjacent to the polar zones and may contain remnants of early martian microbial life, if it ever existed.

Terrestrial algae play important roles in every ecosystem. They contribute to the fertility and stability of soils everywhere, through fixation of carbon and nitrogen, release of organic compounds, and binding together of soil particles to reduce soil erosion. In mesic (moist) ecosystems, the contributions of terrestrial algae tend to be overshadowed by the dominant bryophytes in cooler ecosystems and vascular plants in temperate to tropical ecosystems. In more climactically extreme ecosystems, where bryophytes and vascular plants are reduced or excluded entirely, terrestrial algae may be dominant and the only significant source of primary production. In permanent snow and ice fields at high altitudes and latitudes, where temperatures rise above 0ºC for some appreciable period of the year, snow algae carry out photosynthesis and grow, generating the only primary productivity. Similarly, in cold deserts, such as the Arctic polar deserts and extremely cold and dry deserts, such as the Antarctic dry valleys, the algae of soils and lithic ecosystems are the only source of primary productivity. Less extreme ecosystems, such as arid and semiarid lands, contain bryophytes and vascular plants, but the vascular plants are sparse, with wide spaces of open land between plants. In these arid systems, soil fertility, water retention, and stability from erosion depend on development of biological soil crusts in the spaces between vascular plants. Biological soil crusts are communities made up of bacteria, fungi, algae, and, in the most developed forms, various lichens and bryophytes. Disturbances such as excessive livestock trampling and human commercial and recreational activities degrade biological soil crusts and result in increased erosion and invasion by exotic species. Conservation of these biological soil crusts is an important part of rangeland management.

In the following sections, the terrestrial algae of soil ecosystems are discussed first, followed by the cryophilic algae of terrestrial snow and ice fields. The third section considers subaerial algae, which is a broad category containing lithic algae, algae on living and dead plant surfaces, and algae found on human-made constructions such as buildings. Lichens are discussed in each of the categories in which they occur.