Effect of Feeding Red Seaweed Kappaphycus alvarezii on Nutrient Digestibility, Enteric Methane Emissions, and Rumen Metatranscriptome in Growing Sheep

A 126-day study was conducted in growing male sheep to investigate the effect of feeding Kappaphycus alvarezii on nutrient utilisation, enteric methane emissions, and rumen microbiota. Control sheep (C, n = 15) were fed a basal diet consisting of wheat straw and concentrate mixture (35:65, DM basis), whereas the experimental group (Ka, n = 15) received the basal diet, with 5% K. alvarezii replacing an equal amount of the wheat straw on a dry matter (w/w) basis. A 16% reduction was observed in daily enteric methane emission (g/d) (p = 0.0001) in the Ka group compared with the C group. Dry matter digestibility (p = 0.0062) and average daily body weight gain (p = 0.0239) were adversely affected by K. alvarezii supplementation. However, there was no significant (p = 0.2379) difference in the TVFA production (mmol/L) between the C and Ka groups. Total protozoal counts, as well as populations of Entodiniomorphs and Holotrichs, were significantly reduced in the Ka group (p < 0.05). The metatranscriptome profiles of both groups revealed Bacillota and Bacteroidota as the most active bacterial phyla, while the transcript abundance of Thermodesulfobacteriota and Pseudomonadota increased marginally (p = 0.128) following K. alvarezii supplementation. Methanobacteriota was the single most dominant archaeal phylum. At the genus level, Desulfovibrio (p = 0.001) and Succinivibrio (p = 0.014) significantly increased in the Ka-supplemented group. Supplementation of K. alvarezii led to a targeted reduction in the minor methanogen community like Methanobacterium and Methanothermobacter. The fundamental housekeeping and metabolic COG categories, particularly carbohydrate transport and metabolism (FDR = 0.769) and energy production and conversion (FDR = 0.970), exhibited functional stability between the control and Ka groups. Dietary supplementation of K. alvarezii significantly upregulated pathways related to xenobiotic degradation, co-factors, and amino acid catabolism. Conversely, it suppressed lysine biosynthesis and nicotinate and nicotinamide metabolism. Acetate concentration correlated positively with key fibrolytic Firmicutes (Butyrivibrio, Blautia, Roseburia) and negatively with Chloroflexota, whereas propionate and branched-chain VFAs showed distinct associations with taxa such as Xylanibacter, Synergistota, and Treponema, indicating suppressed crude protein digestibility.