High-Temperature Ecosystems to nullify the effects of the factor, or (c) simply adapt in more minor ways to live with the factor The second and third strategies differ quantitatively but not qualitatively For example, enzymes can be changed to different temperature optima by relatively small evolutionary steps, but to cope with temperatures that will destroy (denature) the enzyme, a major new structure must be developed Geothermally heated lakes and pools characteristically are well mixed thermally, with little or no directional current This produces, typically, a body of water with a relatively homogeneous temperature regime The consequences for organisms attempting to colonize such ecosystems is that they cannot escape the thermal load, thus any organisms with limited motility must adapt to survival at the temperature regime of the system Algal/bacterial mats in such thermal ecosystems, at temperatures where photosynthesis is possible (discussed later), form on the bottom of the lake or pool There is little or no thermal gradient in these systems, so any eucaryotic consumers must adapt to the prevailing temperature regime or fail to colonize Thus in Yellowstone National Park in the United States and in other geothermally active areas of the world, the thermal lakes and pools can be ranked from low to higher diversity as an inverse function of the mean maximum annual temperature of the system Because of the lack of strong currents and replacement of surface water, cooling of these systems is relatively slow and the annual variation in temperature is low, even in the cold temperate latitude and high elevation of Yellowstone Park Thermal streams share with thermal lakes and pools the characteristic that the layer of productive organic photo- or chemoautotrophs is on the bottom and organisms attempting to colonize must adapt to tolerate the temperature regime But the stream has a significant unidirectional flow and hot water emerging from the substrate onto the surface immediately begins to cool The result is the establishment of a strong thermal gradient from the source downstream, within which different specific conditions for community development can be found In theory, the cooling curve will be exponential, but other factors intervene, among which the most important are the current turbulence and the temperature, humidity, and the wind speed immediately above the surface of the stream For example, in a small Yellowstone thermal stream (Gentian Stream, Firehole Lake Drive), I have found that the temperature in summer can fluctuate rapidly as the sun goes behind clouds and wind velocity changes This effect takes place in minutes, whereas longer lasting weather changes (cloudy days, for example) cause changes in hours to days In Yellowstone, 250 m from the source, the annual fluctuation in temperature is approximately 20 1C, significant, but also a testimony to the thermal capacity of water since the flow of this small stream is measured in liters per minute Thus in contrast to the organisms in thermal lakes and pools, the inhabitants of geothermally heated streams are far from living in a constant temperature natural chemostat, unless the stream habitat is very close to the source Outflows from thermal springs differ from lakes, pools, and streams in the volume of water discharged, its depth and pattern of flow after discharge, and in the types of communities and organisms that colonize the outflows In contrast to lakes and pools, spring discharges have strong flows and 113 turbulent mixing, with a relatively steep thermal gradient established from the source to the point where temperature of the water approximates that of ambient air Outflows not generally follow a well-defined channel (except near some of the sources), and in any case the depth is shallow Once the temperature reaches the point that is tolerated by the filamentous cyanobacteria (blue green algae), a mat of microorganisms forms that can thicken and directly affect the flow of the water This creates a community with distinct cool patches; some are cool on top with hot water flowing underneath, others are devoid of flow This temperature heterogeneity provides an additional manner in which motile organisms can colonize thermal communities, by avoiding lethal temperatures as a consequence of adapting physiology, life history, or behavior to take advantage of the temporary cooling of the algal mat Spring outflows exhibit considerable variety The sources are of different temperatures, primarily a consequence of the water traveling for variable distances in the soil after emergence from the underlying rock and before emergence onto the surface The chemical characteristics of the emerging waters may differ (although I am eliminating from this discussion those waters with abnormally low pH, caused by excessive sulfur content) Finally, spring outflows are often intermittent, creating yet another form of temperature heterogeneity These intermittent flow communities vary greatly in their biotic composition, depending not only on the temperature gradient but also on the period and volume of flow, the chemical composition of the source water, and so on The biodiversity found at temperatures less than 40 1C is, however, low compared to the biodiversity of similar temperature zones in a relatively constant thermal gradient Thermal vent communities are formed by the emission of superheated water from fissures in the ocean floor that form along the line marking the meeting of tectonic plates Because of the pressure of deep water, temperatures are possible that greatly exceed the 100 1C temperatures of water boiling at sea level Unfortunately, the great depths at which these vent communities form makes their study both difficult and extremely expensive In general outline, the water is colonized by strains of high temperature heterotrophic bacteria utilizing the organic compounds dissolved in the superheated water A strong thermal gradient is set up where the heated water meets the cold seawater and several groups of filter-feeding marine invertebrates have evolved to utilize these resources Whether this evolution involves tolerance to high temperatures, however, is problematic, since samples and temperatures are hard to match up under such difficult sampling conditions Furthermore, the thermal gradient is so steep that small distances may see a radical change in mean temperature Numbers of individuals and biomass of the invertebrate consumers are large, because of the richness of the production by heterotrophic microorganisms, but number of species is low, although perhaps similar or even higher than that of the cold, dark, relatively sterile community away from the thermal vents at these depths Although the microorganisms are clearly living at temperatures far higher than any found at the surface of the earth, the question of whether multicelled eucaryotes have evolved to survive at temperatures above 40 to 50 1C is still an open question