Using Ulva (Chlorophyta) for the production of biomethane and mitigation against coastal acidification

Abstract

In South Africa the green macroalga Ulva armoricana is the main species of macroalgae cultured. The species is currently the largest aquaculture (2884.61 tonnes) product by weight with a corresponding capacity for biogas (CH₄) production. We have shown that biotransformation of U. armoricana to Liquefied Petroleum Gas (LPG) is viable and economically feasible as a clean fuel. pH toxicity tests showed that U. armoricana can be used as a health index, under potentially increased CO₂ concentrations that can occur in IMTA carbon sequestration. We have shown sporulation to be the morphological response to environmental stress, which is indicative of chlorophyll degradation and a reduction in the photosynthetic activity of the alga. With the exception of Cadmium (Cd), the physico-chemical values obtained and the dissolved nutrient/heavy metals uptake by the alga all fell within the FAO/WHO permissible standards. Our Cd values therefore negate the use of these macroalgae for human consumption. We have also shown that U. armoricana can be used in eco-monitoring by playing a significant role in wastewater filtration and bioaccumulation. Nutrient utilization and proximate composition results show that African mud catfish (Clarias gariepinus) grow well on a protein-enriched Ulva diet, suggesting that enriched Ulva has the potential to be a successful fish feed. This thesis suggests among others, that South Africa could take advantage by being the first African country to propose specific standards for edible macroalgae as its successful research innovations and development provides a template for other African countries to further their aquaculture sectors. Additional benefits (bioremediation, ocean de-acidification through the capture of atmospheric and dissolved CO₂ during growth to assist in climate change mitigation) from Ulva farming activities bode well for the aquaculture industry.