The effects of processing on the in vitro antimethanogenic capacity and concentration of secondary metabolites of Asparagopsis taxiformis

The macroalga Asparagopsis taxiformis is an effective antimethanogen in ruminants due to the presence of bromoform. To date, research on the effects of A. taxiformis on methanogenesis has used freeze-dried material without an understanding of alternative post-harvest processing methods. Therefore, A. taxiformis was processed using a factorial design based on rinsing (unrinsed/dip rinsed/submerged), freezing (frozen/not frozen) and drying (freeze-dried/kiln-dried/dehydrated) to determine the effects on methane (CH4) production in in vitro rumen fermentations and the concentration of bromoform in the biomass of A. taxiformis. The most effective treatment group, which completely inhibited the production of CH4, consisted of treatments that, regardless of rinsing, were frozen and then freeze-dried. Of these, the unrinsed treatment had the highest concentration of bromoform (4.39 ± 0.07 mg g−1 dry weight (DW)) and the rinsed treatments had concentrations between 2.0 and 3.2 mg g−1 DW. The next most effective group consisted of treatments that were unrinsed, and kiln-dried or dehydrated without freezing. These also completely inhibited the production of CH4 but had lower concentrations of bromoform (between 1.0 and 2.0 mg g−1 DW). Importantly, all other treatments inhibited CH4 by <100% and had bromoform concentrations <1.0 mg g−1 DW, demonstrating that the threshold for complete inhibition of CH4 production in vitro is 1 mg g−1 DW of bromoform in A. taxiformis when included at 2% of organic matter. In conclusion, biomass that was unrinsed, frozen and subsequently freeze-dried was the most effective processing method to maintain antimethanogenic activity, while kiln-drying or dehydrating unrinsed biomass without freezing is also effective.