Method: Effect of light on microbial fermentation in an in vitro system using rumen fluid from lactating dairy cows

Most in vitro assays used to evaluate ruminal fermentation are carried out using clear glass vessels that allow light penetration, unlike the rumen. Some bacterial species are light-sensitive, and any light stimuli may cause chemical reactions within the microbial cell that impact their survival, growth, and proliferation. Thus, this study’s objective was to evaluate light’s effects on ruminal fermentation in a dual-flow continuous culture. Using four fermenters and three experimental periods, the ruminal fluid of three lactating dairy cows was collected, pooled, and incubated for 10 days. Laboratory lights (234 ± 15 lux) were turned on 24 h per day for the entire experiment duration. Two of the fermenters were covered with aluminum foil to avoid light penetration within the jar. All fermenters were fed 106 g/day split into two feedings of the same diet that had 16% CP; 35% neutral detergent fiber; 31% starch, DM basis. After 7 days of adaptation, samples were collected for 3 days of each period for analyses of ruminal fermentation, nutrient degradation, and microbial analysis. Statistical analysis was performed with the MIXED procedure of SAS and the vegan and phyloseq packages of R. Significance was declared at P ≤ 0.05 and tendencies at 0.05 < P ≤ 0.10. There was no effect of treatment on ruminal fermentation or nutrient degradation in vitro . However, light exposure decreased species richness and diversity, decreased abundances of Actinobacteria and Planctomycetes in the liquid fraction and Actinobacteria, Fibrobacteres, Planctomycetes, and Syngergistetes in the solid fraction. Light exposure increased Synergistetes in the liquid fraction and tended to increase Proteobacteria in the solid fraction. In summary, this pilot study demonstrated that light affects ruminal microbiota. Factors such as light intensity, exposure duration, and affected populations should be further investigated. Those results indicate that some ruminal microorganisms may be light-sensitive, and whenever possible, darker conditions should be practiced.