Opening Lecture
January 18,Sunday
The Deep-Sea's Contribution to the Extremophiles
Holger W.JANNASCH*
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
The outer edge of our planet's biosphere is determined by organisms that have adapted to life at extreme environmental conditions. The term "extreme" is based on an average range of condition for all known forms of life, especially of the eucarial domain with a slight anthropocentric tinge. Most of the known extremophiles are microbes of the bacterial and archaeal domains. Their definition comprises adaptations to physiologically highly disparate environmental parameters, such as temperature, pressure, salinity, pH, humidity, toxicity, and low substrate levels. Microbes adapted to extreme conditions will often not grow at conditions that are "normal" as defined for non-extremophiles.
The larger part of the Earth's biosphere, namely about 80% by volume, is represented by the oceans' deep and pelagic part. This uniformly well-buffered and moderately saline seawater becomes an extreme environment mainly through its hydrostatic pressure, low temperature (2 - 4℃), and low concentrations of energy- yielding dissolved organic substrates for heterotrophic microbial metabolism. While barophilic and psychrophilic microbes have been studied extensively, research on their uptake and metabolic efficiencies at low levels of organic carbon (oligocarbophily) is more recent. In view of the fact that the amount of dissolved organic carbon in seawater is in the same order of magnitude as inorganic carbon in the atmosphere, the turnover of the former is important for the release of CO2 from the ocean. This climate-affecting process may well be controled by microbial oligotrophic characteristics. The biochemical capabilities of barophilic microbes may differ characteristically from those of non-barophiles. The biotechnological importance in searching for such new barophiles is indicated by our hosts' (JAMSTEC) peerless research efforts in deep-sea microbiology.
The discovery of deep-sea hydrothermal vents provided us with another group of extremophiles: hyperthermophilic archaea as well as bacteria. The presence of liquid water above 100℃ at deep-sea pressures might be the key for the existence of organisms at the upper, yet unknown, temperature limit of life in general. The fact that many of the isolated extreme thermophiles have been obtained from both deep and shallow marine environments might relate to a genetic stability and the microbe's typical dispersibility. In pure culture studies, their maximum growth temperature was recently extended to 113℃. This limit may well reach higher values with the use of
