DISSIMILATORY SULFATE REDUCTION: A PRIMITIVE RESPIRATORY PATHWAY FOR MICROBIAL LIFE FORMS LIVING IN EXTREME ENVIRONMENTS

Dissimilatory sulfate respiration is one of the most primitive pathways for energy production; hence, its study under extreme environmental conditions, such as temperature, salinity, and acidity, will provide a better understanding of how life may have survived and diversified on primitive earth. It may also give us clues as to how life may thrive on other planets. Sulfate reduction rates (SRR) were measured in the hydrothermal springs of Yellowstone National Park to determine the factors controlling sulfate reduction.

Sediment and pore-water samples from a number of geothermal hot springs were tested for temperature, pH, alkalinity, and concentrations of sulfate, sulfide, iron, manganese, ammonium, and phosphate, using spectrophotometric methods. In addition, field incubation experiments were conducted in triplicate at each site to determine SRR using a radiotracer technique. At selected sites activation energy and half saturation constant for sulfate reduction were determined. Effort was made to identify primary electron donors by spiking some of the incubated samples with organic acids (formate, lactate, and acetate) at chosen few sites.

Hydrothermal springs varied considerably in their physico-chemical properties, hence, affecting SRR. Measured SRR varied between 0.7 and 1900 nmol cm3 d-1. Calculated activation energy values varied between 16 and 119 (kJ/mol) at three different sites. Several trends are indicated from preliminary results: a direct relationship between temperature and sulfate reduction for each of the tested sites was apparent, consistently higher rates of sulfate reduction were observed in microbial mats as opposed to sites where only sediments were present, and reduction rates did not show any consistent pattern with the sulfate concentration in the hydrothermal springs.