Effects of heavy metals and other trace elements on the fermentative activity of the rumen microflora and growth of functionally important rumen bacteria
The inhibitory effects of high concentrations of essential and non-essential trace elements were tested on the rumen microflora using the rate of fermentation in vitro as the assay. The elements (and the concentration causing 50% inhibition) in decreasing order of toxicity were Hg2+ (20 μg/ml), Cu2+ (21 μg/ml), Cr6+ (70 μg/ml), Se4+ (73 μg/ml), Ni2+ (160 μg/ml), Cd2+ (175 μg/ml), As3+ (304 μg/ml), and As5+ (1610 μg/ml). The elements tested that were either weak or non-inhibitory at concentrations greater than 400 μg/ml included Zn2+, Cr2+, Fe2+, Mn2+, Pb2+, and Co2+. Methylmercury was as inhibitory as mercuric chloride to the fermentation. When the inhibitory effect of Cd2+ was tested on separated bacterial and protozoal fractions, it was more inhibitory to the bacteria. The inhibitory effects of trace elements were also determined for a number of axenic cultures of rumen bacteria. The bacteria which most frequently exhibited the greatest sensitivity were Bacteroides succinogenes, Ruminococcus albus, Bacteroides amytophilus, and Eubacterium ruminantium. Those often exhibiting intermediate sensitivities included Butyrivibrio fibrisolvens, Selenomonas niminantium, and Megasphera elsdenii, while Streptococcus bovis was very refractory to all elements tested. Rumen fluid provided a modest protective effect for the bacteria.