Desiccation resistance and contamination as mechanisms of gaia

The gaia hypothesis, formulated by J.E. Lovelock, asserts that the composition of the reactive gases, the oxidation-reduction state and the temperature of the lower atmosphere of the planet Earth are actively regulated by the biota. Lovelock and Watson, using highly simplified mathematical models, have shown that the modulation of atmospheric temperature can be achieved by exponentially growing populations of differently colored organisms ("dark and light daisies"). It is more likely that the modulation of atmospheric gas composition is based on the colligative properties of exponentially growing mixed populations of microorganisms rather than on "daisies". Exponential growth of one population of microorganisms leads to gaseous and other metabolic products released to the environment, which favor the exponential growth of different populations, each with their own unique emissions. Extremely high densities of mixed populations of microorganisms ensue. These populations form structured microbial communities composed of members in varying states of activity. Growth potential of metabolically diverse populations most likely provides the basis for the responsiveness of the biota to changing environments. We have attempted to measure an aspect of the growth potential and diversity of one microbial community, that from a flat laminated microbial mat dominated by the cyanobacterium, Microcoleus. Microbial mat samples collected at yearly intervals between 1977 and 1982 were allowed to dry. Subsamples were revived under laboratory conditions by rewetting, and the resulting complex microbial populations were analyzed. Greater than 10⁴ viable organisms per ml were estimated to be present in the desiccated samples. Only a portion of the diverse community could be characterized. There were at least 115 different types of desiccation resistant microorganisms present in these samples, primarily bacteria. However, more than a dozen types of rather uncommon fungi and protoctists were removed from naturally desiccated material. Several did not fit descriptions of previously known species or strains. Neither animals nor plants were recovered from these tiny samples. We present minimal diversity estimates for microorganisms in both the desiccated samples and the corresponding fresh laminated microbial mat community from which they were taken. We found that many organisms in laboratory samples capable of immediate growth from a desiccated state were not common components of growing mats in the field. We reaffirm and refine the role of microbial communities, their metabolic diversity, differential exponential growth and resiliency as a fundamental mechanism of environmental regulation by the biota.