Anti-Methanogenic Potential of Seaweeds and Impact on Feed Fermentation and Rumen Microbiome In Vitro

A series of in vitro studies were conducted to explore the anti-methanogenic potential of five seaweeds collected from the Indian sea and to optimize the level(s) of incorporation of the most promising seaweed(s) into a straw and concentrate diet to achieve a significant reduction in methane (CH₄) production without disturbing rumen fermentation characteristics. A chemical composition analysis revealed a notable ash content varying between 55 and 70% in seaweeds. The crude protein content was highly variable and ranged between 3.25 and 15.3% of dry matter. Seaweeds contained appreciable concentrations of tannins and saponins. Among the seaweeds, <i>Spyridia filamentosa</i> exhibited significantly higher CH₄ production, whereas the percentage of CH₄ in total gas was significantly lower in the cases of <i>Kappaphycus alvarezii</i> and <i>Sargassum wightii</i>. The ranking of seaweeds in terms of CH₄ production (mL/g OM) is as follows: <i>Sargassum wightii</i> < <i>Kappaphycus alvarezii</i> < <i>Acanthophora specifera</i> < <i>Padina gymnospora</i> < <i>Spyridia filamentosa</i>. A remarkable decrease of 31–42% in CH₄ production was recorded with the incremental inclusion of <i>Kappaphycus alvarezii</i> at levels of 3–5% of the dry matter in the diet. The addition of <i>Sargassum wightii</i> led to a significant decrease of 36–48% in CH₄ emissions when incorporated at levels of 4–5% of dry matter, respectively. The findings of this study revealed a significant decrease in the numbers of total protozoa and <i>Entodinomorphs</i>, coupled with increasing abundances of sulfate-reducing microbes and minor methanogens. Metagenome data revealed that irrespective of the seaweed and treatment, the predominant microbial phyla included Bacteroidota, Bacillota, Pseudomonadota, Actinomycetota, Fibrobacterota, and Euryarchaeota. The prevalence of <i>Methanobrevibacter</i> was similar across treatments, constituting the majority (~79%) of the archaeal community. The results also demonstrated that the supplementation of <i>Kappaphycus alvarezii</i> and <i>Sargassum wightii</i> did not alter the feed fermentation pattern, and therefore, the reduction in CH₄ production in the present study could not be attributed to it. Animal studies are warranted to validate the extent of reduction in CH₄ production and the key processes involved by supplementation with <i>Kappaphycus alvarezii</i> and <i>Sargassum wightii</i> at the recommended levels.