Nitrogen Ecophysiology of Asparagopsis Armata Tetrasporophytes

Agricultural methane (CH₄) emissions primarily from livestock farming account for 37.5% of the total greenhouse gas emissions in New Zealand. One promising strategy to reduce these emissions involves adding the red seaweed Asparagopsis spp. as a feed supplement for livestock. Asparagopsis spp. contains bromoform, the key compound behind the suppression of methane production by bacteria in ruminants’ guts. When Asparagopsis spp. is included at only 0.2 – 5% of the feed volume, it can reduce methane emissions from ruminants by up to 98%. Efficient aquaculture systems must be developed to cultivate Asparagopsis spp. at volumes required to have a meaningful impact on livestock methane emissions. To achieve this, a detailed understanding of Asparagopsis spp. ecophysiology is required to optimise culture productivity. This thesis aimed to understand the nitrogen ecophysiology of Asparagopsis armata tetrasporophytes and its potential for a co-culture system using kingfish (Seriola lalandi) waste as an inexpensive nutrient source that could replace artificial fertilisers and drive high levels of production. Time course nutrient depletion laboratory experiments quantified the uptake rates of ammonium (NH₄⁺), nitrate (NO₃^-) and urea (CH₄N₂O)  as individual nitrogen sources as well as in a combined treatment of all three nitrogen forms in A. armata. Phases of surge uptake and steady-state uptake were observed in all treatments. In the combined treatment, surge uptake rates were significantly more variable in all nitrogen forms, compared to the individual treatments. Steady-state nitrate uptake rates were reduced by 92% when nitrate was added in combination with ammonium and urea compared to a treatment where nitrate was the sole nutrient added. A three-week co-culture experiment examined A. armata growth, bromoform tissue concentrations, carbon:nitrogen status, and nutrient uptake in response to six kingfish (Seriola lalandi) effluent concentrations. Optimal growth was achieved at 10 – 30% effluent concentrations; however, bromoform concentration was strongly correlated with nitrogen status (% nitrogen and C:N). Nitrogen-limited cultures had bromoform tissue concentrations 42 times higher than nitrogen-replete cultures. This study provides valuable information about the nitrogen ecophysiology and cultivation potential of A. armata. It highlights the trade-off between high growth rates in nitrogen-replete cultures and high bromoform tissue concentrations in nitrogen-limited cultures. This study also offers insights into sustainable seaweed aquaculture, in addition to fish cultures, and supports the integration of A. armata for methane mitigation strategies to manage agricultural emissions.