Methane is the second most important greenhouse gas emitted from anthropogenic activities with a global warming potential 23 to 25 times higher than that of carbon dioxide. The agricultural sector is a major contributor to methane emissions with ruminant livestock production as the main source within the sector. Ruminant emissions account for 7 to 18% of total global greenhouse gas emissions. Due to the impact that methane has on the climate, several approaches have been developed to mitigate the emission of methane from ruminants. Among these approaches, nutritional strategies are the most developed and likely to be implemented at farm scale because they have the potential to reduce enteric methanogenesis and increase animal performance and overall production. This thesis investigated the potential of tropical macroalgae as a natural alternative for the mitigation of greenhouse gas emissions, particularly CH₄, for the beef industry. In a first experiment the antimethanogenic activity and nutritional value of twenty species of tropical macroalgae were evaluated in vitro (Chapter 2), at an inclusion rate of 16.7% of the total organic matter using rumen fluid from Bos indicus steers fed a low quality roughage diet characteristic of northern Australia. All species of macroalgae resulted in lower total gas and methane production than decorticated cottonseed meal (CSM). The freshwater macroalga Oedogonium, demonstrated a rich nutritional profile which resulted in a decrease in the production of methane of 30.3%, however, increased the production of total volatile fatty acids (VFA) by 20% compared to CSM control. In contrast, Asparagopsis had the strongest effect inhibiting methanogenesis by 98.9% compared to CSM. However, this species also had the lowest concentration of VFA, indicating that anaerobic fermentation was also affected. In a second experiment, the potential to maximize the mitigation of the production of methane while minimizing the effects on in vitro fermentation was investigated (Chapter 3). This experiment demonstrated that Asparagopsis was highly effective in inhibiting methanogenesis with a reduction of 99% at doses as low as 2% organic matter (OM) basis. At this dose, the negative effects of Asparagopsis were minimized, with no significant effects on degradability of organic matter and pH. In addition, combining Asparagopsis (2% OM) and Oedogonium (25 and 50% OM), demonstrated that the antimethanogenic activity of Asparagopsis was not affected by the nutritional value of the basal substrate. The effectiveness of Asparagopsis demonstrates its potential for the mitigation of methane emissions from ruminants at inclusion rates of ≤2% OM. In a third experiment, the secondary metabolites responsible for the antimethanogenic activity of Asparagopsis were elucidated (Chapter 4). This experiment identified bromoform as the most abundant secondary metabolite within Asparagopsis biomass. Bromoform was the only metabolite present in sufficient quantities in the biomass (≥1 μM) to drive the antimethanogenic activity of Asparagopsis. Notably, at this concentration the fermentation parameters of degradability of organic matter and total volatile fatty acids were not adversely affected. In a fourth and final experiment, the mode of action of Asparagopsis and bromoform was elucidated at the microbial level (Chapter 5). Quantitative PCR demonstrated that the decrease in the production of methane induced by Asparagopsis and bromoform was directly correlated with a decrease in the relative number of methanogens. High throughput amplicon sequencing confirmed that both treatments decreased the overall number of OTU sequence reads of the three dominant orders of methanogens Methanobacteriales, Methanomassiliicoccales and Methanomicrobiales. Asparagopsis and bromoform also led to an increase in the accumulation of hydrogen within the gas phase, which was reflected in a decrease of the number of sequence reads of hydrogenproducers and an increase in the number of sequence reads of hydrogen-consumers and species that are less sensitive to the increase of the partial pressure of hydrogen. Nevertheless, the minimal effects on microbial fermentation shown in previous chapters supports that the rumen microbial ecosystem is robust, with the decrease in abundance of some species being compensated functionally by the proliferation of others. Therefore, the outcomes of this thesis consistently demonstrate that Asparagopsis is a promising and potent natural alternative to other antimethanogenic agents for mitigation of enteric CH₄ emission through the direct inhibition of populations of enteric methanogenesis.