Philosophiae Doctor - PhD / 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.

Ces travaux de thèse CIFRE, réalisés au sein du Laboratoire de Biotechnologie et Chimie Marines de l’Université de Bretagne-Sud et du Groupe Olmix, s’inscrivent dans le cadre d’un projet national de valorisation de biomasses d’algues vertes, le projet ULVANS. Ce projet est le fruit d’une collaboration entre cinq industriels bretons et deux laboratoires universitaires. Les objectifs de ces travaux de thèse concernent i) la caractérisation approfondie de la matière première, en particulier des polysaccharides matriciels solubles (ulvanes) et des protéines, ii) la mise au point d’un procédé d’extraction assistée par enzymes des métabolites algaux, iii) la caractérisation biochimique et moléculaire des hydrolysats enzymatiques, avec pour objectif la compréhension de l’effet des enzymes commerciales sur les algues, iv) l’évaluation des activités antioxydantes et antivirales des hydrolysats, v) l’étude de l’influence des paramètres d’hydrolyse en vue de déterminer les conditions optimales pour l’extraction des métabolites d’intérêt et enfin vi) de conclure, à partir des éléments fournis par l’étude, sur la viabilité d’un tel procédé à l’échelle industrielle. En conclusion de ces travaux, l’hydrolyse enzymatique apparait comme un procédé intéressant pour le bioraffinage des algues vertes. Bien que les préparations enzymatiques commerciales utilisées ne soient pas spécifiques des composés algaux, les protéases ont conduit à une augmentation significative des rendements d’extraction, alors que l’effet des carbohydrases reste modéré. L’étude de l’influence des paramètres d’hydrolyses a confirmé les résultats préliminaires et montré que la température, la concentration d’enzyme et la méthode de broyage présentaient peu d’effets sur les activités des protéases testées. Un résultat majeur de cette étude aura été la mise en évidence d’activités anti-herpétiques dans les hydrolysats. La caractérisation des fractions actives a montré que les activités étaient liées à la présence de protéines ou glycoprotéines dans les extraits. Ce résultat présente un intérêt majeur car à l’heure actuelle, a priori, aucune activité de ce type n’a été identifiée chez Ulva sp. / This PhD Thesis works, conducted in the Laboratoire de Biotechnologie et Chimie Marines of the Université de Bretagne-Sud and in the Olmix Group, was included in a national project for the upgrading of green seaweed biomasses, the Ulvans project. This project results in the collaboration of five industrial companies and two academic research laboratories. The objectives of this thesis works had been i) the characterization of the raw material, particularly the soluble matrix polysaccharides (ulvans) and proteins, ii) the development of an enzyme-assisted extraction process of algal metabolites, iii) the biochemical and molecular characterization of the enzymatic hydrolyzates, with the aim of understanding the effect of enzymes on algae thallus, iv) the screening of antioxidant and antiviral activities of hydrolyzates, v) the study of the influence of hydrolysis parameters to determine the optimum conditions for the extraction of metabolites of interest and finally vi) to conclude, according to the results provided by this study, on the viability of the industrial upscaling of the process. In conclusion of this work, enzymatic hydrolysis appeared to be an effective process for biorefinery of green seaweeds. Although commercial enzymatic preparations were not specific of algal compounds, protéases led to a significant increase in the extraction yields, whereas the effect of carbohydrases were moderate. The study of hydrolysis parameters confirmed the preliminary results and showed that the temperature, the concentration of enzyme and the grinding method had no effect on the protease used. A major result of this study has been the highlighting of anti-oxidant and anti-herpetic compounds in hydrolyzates. The antiviral activity of ulvans had several times been demonstrated but the biochemical characterization of actives fractions showed that the activity could be associated to proteins or glycoproteins. This result is very interesting because, a priori, any antiviral activity has also been related to this type of compounds in Ulva sp.