| Summary: | The lactic acid oxidising, sulphate-reducing bacteria were numerically predominant over the acetic acid oxidising sulphate- reducing bacteria in the sediments at station C in the River Don estuary. The numbers of acetic acid oxidising, sulphate-reducing bacteria were higher than the acetic acid oxidising, sulphur-reducing group. The anaerobic cellulolytic bacteria and Escherichia coli. were, also present in high numbers at station C. Demographic fluctuations in the numbers of these bacterial groups demonstrated periodicity influenced seasonally. High numbers of bacteria were associated with low winter temperatures, high sulphide concentrations, more negative redox potentials and high suspended solids in the river water. The numbers of anaerobic cellulolytic bacteria were highly correlated with the numbers of lactic acid oxidising, sulphate-reducing bacteria. Fluctuations in bacterial, populations demonstrated similar trends to those observed by Parkes for 1975-1976 substantiating the hypothesis of seasonal periodicity. The proportionality of the sulphur-reducing and sulphate-reducing groups of bacteria did not exhibit a fixed ratio throughout the year. Particle size analysis revealed that the sediments were homogeneous, particle size distributions were uniform with depth and for a 1.5 m square at the sampling site. Analysis of the sediment pore water showed low levels of sulphate, sodium and chloride ions and a low pool size of short-chain fatty acids. One of the proposed hypotheses influencing seasonal periodicity in the numbers of sulphur-reducing and sulphate-reducing bacteria is the direct provision of substrates for sulphater-reduction by the anaerobic cellulolytic bacteria. No evidence was obtained in this study which would refute the "temperature moderated sulphide toxicity" hypothesis forwarded by Parkes (1978) as one of the possible influences delineating bacterial periodicity. The vertical distribution of the following groups of bacteria were examined with sediment depth: the lactic, acetic, butyric and propionic acid oxidising sulphate-reducing bacteria and the anaerobic cellulolytic bacteria. The highest numbers of these groups of bacteria were in the 1 centimetre to 5 centimetre region of the core. There was a sharp decline in the numbers of bacteria from 6 centimetres to 10 centimetres (the lowest depth sampled). Isolation of the groups of sulphate-reducing and sulphur-reducing bacteria from the sediment was precluded by persistent contamination of enrichments by the facultative bacterium Escherichia coli. Defined mixed culture studies, involving the cellulolytic bacterium Clostridium papyrosolvens grown in the presence of DesuIfovibrio salexigens or Desulfotomaculum acetoxidans showed that cellulose degradation was enhanced in the presence of these sulphate-reducing bacteria. Clostridium papyrosolvens produced lactic acid, acetic acid, ethanol, carbon dioxide and hydrogen from the degradation of cellulose. Cellulose degradation by Clostridium papyrosolvens in co-culture with Desulfovibrio salexigens was enhanced by the removal of hydrogen by the sulphate-reducer. This interrelationship demonstrated interspecies hydrogen transfer, a form of protoco-operation of benefit to both partners. Cellulose degradation by Clostridium papyrosolvens in co-culture with Desulfotomaoulum acetoxidans was enhanced by the removal of acetic acid by the sulphate-reducer. This interrelationship demonstrated a nutritional link between these bacteria of benefit to both partners. The formation of composite colonies by sulphate-reducing bacteria and Escherichia coli in minimal media; the banding of sulphate-reducers in the region of the interface between agar layers containing each of the bacteria and the removal of hydrogen produced by Escherichia coli under anoxic conditions by Desulfovibrio salexigens suggests an interrelationship between these organisms. Protoco-operative interrelationships were demonstrated between Clostridium papyrosolvens and Desulfovibrio satexigens and the cellulolytic bacterium and Desulfotomaculum acetoxidans. These interrelationships are advantageous to each of the organisms and could be of significance in the environment.
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