Ionized calcium requirement of rumen cellulolytic bacteria

Ionized calcium (Ca+2) appears to be required by the 3 predominant species of rumen cellulolytic bacteria, Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus. The present study evaluated the role of ionized calcium in growth and cellulose digestion. Maximum growth or rate and extent of digestion and lag time were the criteria used to evaluate Ca+2 requirements. All cultures except F. succinogenes A3c grew when repeatedly transferred in a medium without added Ca+2. As Ca+2 concentration increased in cellobiose medium, the rate of growth increased and lag time decreased for F. succinogenes A3c, whereas F. succinogenes S85 exhibited increases in both maximum growth and rate of growth. No responses in any of the criteria were observed for the ruminococci in cellobiose medium. Both strains of F. succinogenes had an absolute requirement for Ca+2 with cellulose as the only substrate. For strain A3c the requirement was 0.36 to 0.42 mM and for S85, >0.64 mM. Increases in extent of cellulose degradation occurred with all strains of ruminococci as Ca+2 concentration increased; however, degradation in Ca+2-free medium was similar to that of F. succinogenes with Ca+2. Although the ruminococci presumably have cellulosomes that require Ca+2 in their structure, such was not evident in our studies. The function of Ca+2 in cellulose degradation by F. succinogenes is unknown, but may be related to the secretion or activation of their cellulolytic enzymes. Based on reported concentrations of Ca+2 in the rumen, it seems unlikely that an in vivo deficiency would occur for these bacteria.