Aerobic conditions proved to be best for the microbial conversion of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry. The dry soil contained 400 mg of alpha-HCH per kg. This xenobiotic compound was mineralized within about 18 days at an initial rate of 23 mg/kg of soil per day by the mixed native microbial population of the soil. The only intermediate that was detected during breakdown was pentachlorocyclohexene, which was detected at very small concentrations. Alpha-HCH was also bioconverted under methanogenic conditions. However, a rather long acclimation period (about 30 days) was necessary before degradation started, at a rate of 13 mg/kg of soil per day. Mass balance calculations showed that about 85% of the initial alpha-HCH that was present was converted to monochlorobenzene, 3,5-dichlorophenol, and a trichlorophenol isomer, possibly 2,4,5-trichlorophenol. Under both denitrifying and sulfate-reducing conditions, no significant bioconversion of alpha-HCH was observed. The beta isomer of HCH was recalcitrant at all of the four redox conditions studied. We propose that the specific spatial chloride arrangement of the beta isomer is responsible for its stability. The results reported here with complex soil slurry systems showed that alpha-HCH is, in contrast to the existing data in the literature, best degraded biologically in the presence of oxygen.

The non-legume genus Parasponia has evolved the rhizobium symbiosis independent from legumes and has done so only recently. We aim to study the promiscuity of such newly evolved symbiotic engagement and determine the symbiotic effectiveness of infecting rhizobium species. It was found that Parasponia andersonii can be nodulated by a broad range of rhizobia belonging to four different genera, and therefore, we conclude that this non-legume is highly promiscuous for rhizobial engagement. A possible drawback of this high promiscuity is that low-efficient strains can infect nodules as well. The strains identified displayed a range in nitrogen-fixation effectiveness, including a very inefficient rhizobium species, Rhizobium tropici WUR1. Because this species is able to make effective nodules on two different legume species, it suggests that the ineffectiveness of P. andersonii nodules is the result of the incompatibility between both partners. In P. andersonii nodules, rhizobia of this strain become embedded in a dense matrix but remain vital. This suggests that sanctions or genetic control against underperforming microsymbionts may not be effective in Parasponia spp. Therefore, we argue that the Parasponia-rhizobium symbiosis is a delicate balance between mutual benefits and parasitic colonization.

Halorespiring microorganisms are not only able to oxidize organic electron donors such as formate, acetate, pyruvate and lactate, but also H(2). Because these microorganisms have a high affinity for H(2), this may be the most important electron donor for halorespiration in the environment. We have studied the role of H(2)-threshold concentrations in pure halorespiring cultures and compared them with mixed cultures and field data. We have found H(2)-threshold values between 0.05 and 0.08 nM for Sulfurospirillum halorespirans, S. multivorans and Dehalobacter restrictus under PCE-reducing and nitrate-reducing conditions. The reduction of PCE and TCE can proceed at H(2) concentrations of below 1 nM at a polluted site. However, for the reduction of lower chlorinated ethenes a higher H(2) concentration is required. This indicates that the measured H(2) concentration in situ can be an indicator of the extent of anaerobic reductive dechlorination.

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The occurrence and localization of enzymes involved in energy supply and biosynthesis was studied in root nodules of Alnus glutinosa (L.) Vill. Vesicle clusters of the endophyte, Frankia sp., contain NADP‐dependent isocitrate dehydrogenase, succinate dehydrogenase, fumarase and malate dehydrogenase. The data indicate that both the endophyte and the host are capable of metabolizing carbon compounds via the tricarboxylic acid cycle. Both vesicle clusters of the endophyte and root nodule cells contain glutamate‐oxaloacetate transaminase which can function in a malate‐aspartate shuttle. This might enable transport of reducing equivalents from the host cell cytoplasm to the endophyte.