DISCUSSION
Data presented in this paper show that populations of CL butyricum SS6, D. desulfuricans DT101 and Chr. vinosum PT121 co-existed for extented time periods in mixed
continuous culture. Experimental evidence presented in Table 1 show that the presence of
NO3 ' in the growth medium resulted in increased cell populations of Cl. butyricum SS6
relative to NH 4
+ grown cultures. These data are similar to those recorded for Clostridium
10H52 (Caskey and Tiejdje, 1980) and Cl. perfringens (Hasan and Hall, 1975) and are
consistent with the concept that NO 3
- was functioning in a dissimilatory role. Analysis of
fermentation products from cultures of Cl. butyricum SS6 showed a switch from butyrate
to acetate production in the presence of NO3". Similar findings have been reported by
Ishimoto et al. (1974) for Cl. perfringens. Similarily data presented in Table 2 show that
NO 3
-
can function as an alternative e" acceptor to SO 4
2- in D. desulfuricans DT101. Spent
media analyses showed the accumulation of NH 4
+ indicating a dissimilatory role for
NO3". In conclusion these data show that nitrate dissimilation in strict anaerobes allows
these bacteria to more efficiently derive energy from carbon substrates and thus may have
important ecological implications in environments such as estuaries where the supply of
fermentable substrates is limited.
The mixed population study whilst only a simple experimental system does nonetheless
allow the investigation under laboratory conditions some of the environmental parameters governing the growth and interactions of different physiological groups of microorganisms which can then be related back to the field environment. The results presented in
Figures 2 and 3 show that an obligate commensal relationship was established between
Cl. butyricum SS6 and D. desulfuricans DT101 based upon the generation of carbon
substrates. Desulfovibrio desulfuricans DT101 in turn was involved in a mutualistic
interaction with Chr. vinosum PT121, the relationship between the two populations
being mediated by the cyclical reduction and oxidation of SO4 2 " and S 2 ". Under
conditions of N-limitation (Fig. 2 and Fig. 3b) free S 2- levels reached inhibitory levels
such that growth of the phototroph was severely impaired and the relationship between
D. desulfuricans DT101 and Chr. vinosum PT121 became one of ammensalism. These
data show that carbon flow cannot be considered in isolation and must take into account
environmental parameters such as the availability of oxygen, nitrogen and sulphur.
ACKNOWLEDGEMENT
This work was supported by research grant (GR/3/4208) to R.A. Herbert from the Natural Environmental
Research Council.
BAKER K, 1968. Low cost continuous culture apparatus. Laboratory Practice 17, 817-821.
BROWN C.M., Mac Donald-Brown D.S. and Stanley S.O., 1972. Inorganic nitrogen metabolism in marine
bacteria : nitrogen assimilation in some marine pseudomonads. Journal Marine Biological Association, 52 :
793-804.
CASKEY W.H. and TIEDJE J.M., 1980. The reduction of nitrate to ammonia by a Clostridium sp. isolated from
soil. Journal of General Microbiology, 119: 217-223.
CRUICKSHANK R., 1965. Medical Microbiology, E.S. Livingstone, Edinburgh, London. : 868-869.
HASAN S.M. and HALL J.B., 1975. The physiological function of nitrate reduction in Clostridium perfringens.
Journal of General Microbiology, 87 : 120-128.
86
Data presented in this paper show that populations of CL butyricum SS6, D. desulfuricans DT101 and Chr. vinosum PT121 co-existed for extented time periods in mixed
continuous culture. Experimental evidence presented in Table 1 show that the presence of
NO3 ' in the growth medium resulted in increased cell populations of Cl. butyricum SS6
relative to NH 4
+ grown cultures. These data are similar to those recorded for Clostridium
10H52 (Caskey and Tiejdje, 1980) and Cl. perfringens (Hasan and Hall, 1975) and are
consistent with the concept that NO 3
- was functioning in a dissimilatory role. Analysis of
fermentation products from cultures of Cl. butyricum SS6 showed a switch from butyrate
to acetate production in the presence of NO3". Similar findings have been reported by
Ishimoto et al. (1974) for Cl. perfringens. Similarily data presented in Table 2 show that
NO 3
-
can function as an alternative e" acceptor to SO 4
2- in D. desulfuricans DT101. Spent
media analyses showed the accumulation of NH 4
+ indicating a dissimilatory role for
NO3". In conclusion these data show that nitrate dissimilation in strict anaerobes allows
these bacteria to more efficiently derive energy from carbon substrates and thus may have
important ecological implications in environments such as estuaries where the supply of
fermentable substrates is limited.
The mixed population study whilst only a simple experimental system does nonetheless
allow the investigation under laboratory conditions some of the environmental parameters governing the growth and interactions of different physiological groups of microorganisms which can then be related back to the field environment. The results presented in
Figures 2 and 3 show that an obligate commensal relationship was established between
Cl. butyricum SS6 and D. desulfuricans DT101 based upon the generation of carbon
substrates. Desulfovibrio desulfuricans DT101 in turn was involved in a mutualistic
interaction with Chr. vinosum PT121, the relationship between the two populations
being mediated by the cyclical reduction and oxidation of SO4 2 " and S 2 ". Under
conditions of N-limitation (Fig. 2 and Fig. 3b) free S 2- levels reached inhibitory levels
such that growth of the phototroph was severely impaired and the relationship between
D. desulfuricans DT101 and Chr. vinosum PT121 became one of ammensalism. These
data show that carbon flow cannot be considered in isolation and must take into account
environmental parameters such as the availability of oxygen, nitrogen and sulphur.
ACKNOWLEDGEMENT
This work was supported by research grant (GR/3/4208) to R.A. Herbert from the Natural Environmental
Research Council.
BAKER K, 1968. Low cost continuous culture apparatus. Laboratory Practice 17, 817-821.
BROWN C.M., Mac Donald-Brown D.S. and Stanley S.O., 1972. Inorganic nitrogen metabolism in marine
bacteria : nitrogen assimilation in some marine pseudomonads. Journal Marine Biological Association, 52 :
793-804.
CASKEY W.H. and TIEDJE J.M., 1980. The reduction of nitrate to ammonia by a Clostridium sp. isolated from
soil. Journal of General Microbiology, 119: 217-223.
CRUICKSHANK R., 1965. Medical Microbiology, E.S. Livingstone, Edinburgh, London. : 868-869.
HASAN S.M. and HALL J.B., 1975. The physiological function of nitrate reduction in Clostridium perfringens.
Journal of General Microbiology, 87 : 120-128.
86