Conditions for Vigorous Growth on Sulfide and Reactor-Scale Cultivation Protocols for the Thermophilic Green Sulfur Bacterium Chlorobium tepidum

ABSTRACT We describe a reactor-scale cultivation protocol for the fastest-growing and only known thermophilic member of the family Chlorobiaceae, Chlorobium tepidum. We discovered that C. tepidum would grow with sulfide as the sole electron source at rates and with final cell yields comparable to those found with thiosulfate only if the sulfide concentration was maintained below 0.1 mM and the culture redox potential was at −300 ± 20 mV. Such was also the requirement for growth in a photobioreactor when thiosulfate (optimum level, 12 mM) was used as the preferred electron source. For cultivation of C. tepidum on a 5- to 500-ml scale, we used the system of Balch and Wolfe (Appl. Environ. Microbiol. 32:781–791, 1976) using stopper-sealed serum tubes and bottles as an alternative to the methods commonly used for the cultivation of phototrophic anaerobes and obtained consistent results.

Redox potential as a controlling factor in enhancing carbon tetrachloride biodegradation

Biodegradation of hazardous waste is often the most cost-effective technique suitable for purifying large quantities of polluted groundwater and industrial effluents. In an effort of optimizing environmental conditions for microorganisms to degrade carbon tetrachloride, culture redox potential (Eh) was demonstrated as having a critical role. The microorganisms tested were isolated from contaminated field sites and included Pseudomonas cepacia and Providencia stuartii. Ti(III) citrate was used as a reducing agent to poise Eh at designed values. Over 99% degradation of carbon tetrachloride was effected in 3 days at −250 mV ≤ Eh ≤ −200 mV. Lesser rates were observed at Eh ≥ 0 mV. Kinetic analysis indicated that the overall degradation rate constant increased from 2.75×10−3 h−1 to 4.75×10−2 h−1 by controlling Eh at about −200 mV compared with Eh at ≥ 0 mV. Results indicated that the implementation of critical redox potential may be effective in optimizing CT biodegradation activity.