[Effect of cobalt and vitamin B 12 on growth and survival of rumen ciliates in vitro as a function of bacterial population].

The effect of inorganic cobalt or organic cobalt in the form of vitamin B12 was studied on the growth of rumen ciliates in the presence of normal bacterial population and in the presence of reduced bacterial population. It was stated that concentration from 0.1 to 1 cobalt microgram/ml and 0.12 to 0.24 vitamin B12 microgram/ml stimulated growth of rumen ciliates. At rates 10 to 20 times weaker, cobalt in the form of vitamin B12 was more active than cobalt in inorganic form. With reduced bacterial population (10(3) B/ml) protozoa died within a few days but their survival was prolonged by adding cobalt, this effect being more pronounced with vitamin B12 than with inorganic cobalt. The growth of ciliates treated with antibiotics was stimulated by addition of bacteria, and this effect was more pronounced when the bacteria had been preincubated with cobalt. Incorporation of cobalt into rumen ciliates was studied using electron microprobe analysis. Cobalt was present in all the ciliate but essentially accumulated as granules in the endoplasm inside or outside the vacuoles. With reduced bacterial population, cobalt was also incorporated into the ciliates whatever the Co concentration. Indeed, ciliates seemed to need bacteria in order to use the organic cobalt for growth.