Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




Concomitant with the addition of large amounts of potato starch wastes to the Grafton, North Dakota, municipal sewage lagoon was the development of a new major microbial population, the purple sulfur bacteria. This study attempts to define ecologic relationships of the lagoon, with special emphasis placed on the action of purple sulfur bacteria in the lagoon.

Chemical changes in the lagoon were investigated by monitoring Biochemical Oxygen Demand (BOD), sulfide, sulfate, phosphate, pyruvate, total carbohydrates, volatile acids, alkalinity, and pH. Lagoon water temperatures were observed daily. Microbial ecologic relationships were deduced by enumerating coliforms, fecal conforms, enterococci, total bacteria (TGE agar), methane formers, sulfate reducers, purple sulfur bacteria, and algae. Finally, two strains of purple sulfur bacteria were characterized metabolically. Optimum pH, temperature, and sulfide levels were determined and the utilization of certain organic substrates was investigated and correlated with organic substrate changes in the lagoon.

The following observations summarize this ecologic study:

1. Two populations, purple sulfur bacteria and total bacteria, reached maximal concentrations in the warmest part of the 1967 summer. During 1968 no correlation of temperature values and microbial growth was observed.

2. Purple sulfur b acteria reached maximal numbers as concentrations of sulfide and volatile acids were depleted. Formic acid, which was not utilized by the isolated strains of Thiocapsa floridana and Chro- matium vinosum, remained as the major volatile acid constituent in August of 1968, while levels of acetic, butyric, and propionic acids were depleted. Decreases in carbohydrate and alkalinity values in 1968 may also be related to purple sulfur populations. Low sulfate levels observed during the purple phase may be attributable to storage of sulfur within purple sulfur bacteria.

3. Populations of methane bacteria were low during the early portion of the summer when optimal volatile acid levels were present. Removal of sulfide, which inhibits methane bacteria, by purple sulfur bacteria, probably aided the development of these organisms.

4. No biological, chemical, or physical agent was linked to the removal of coliforms, fecal conforms, and enterococci.

5. Increases of algal populations in the latter parts of summers 1966 and 1967 may have been related to the low organic content of the lagoon during these periods.

6. Populations of sulfate reducing bacteria, the contributors of sulfide to the lagoon, were not limited by depletion of sulfate (Shreve, 1967).

7. Sulfide concentrations of 45 - 60 mg/l, pH values of 7.5 - 8.0, and incubation temperatures of 25 - 30°C were optimal for maximal growth of Thiocapsa floridana and Chromatium vinosum. Although lagoon pH (7. 7-8.2) was favorable for purple sulfur growth, temperatures and sulfides were always minimal in the lagoon for these organisms. However, these organisms can grow at temperatures as low as 18®C and in sulfide concentrations of 1 - 5 mg/l.

8. Chromatium vinosum utilized succinate, pyruvate, fumarate, malate, glycolic acid, hexanoic acid, histidine, glucose, fructose, lactose, and sucrose, while Thiocapsa floridana, in addition to the substrates metabolized by Chromatium vinosum, used methionine, benzoic acid, maltose, valerate, propionate, and acetate.

9. Purple sulfur bacteria materially lowered BOD levels, as demonstrated by the growth of Thiocapsa floridana in sterilized sewage.

10. An absence of a direct correlation between BOD removal and a specific physical or chemical parameter in the lagoon was evident in this study. In 1967, maximal BOD reduction occured when populations of purple sulfur bacteria and total bacteria (TGE agar) were high.