Organic carbon utilisation by the filamentous alga Tribonema

Abstract Filamentous algae have potential application to wastewater treatment, in particular for efficient recovery of nutrients into biomass. However, supplying inorganic carbon is a major limiting factor. The utilisation of organic carbon present in wastewater may reduce the constraints in carbon supply, however there is little knowledge of mixotrophic growth amongst filamentous algae. This study investigated the utilisation of organic carbon sources relevant to wastewater by the filamentous xanthophyte alga Tribonema . Algae growth was compared in the absence of organic carbon (autotrophic) and in presence of 0.2 g-C L -1 glucose, ethanol or acetate under mixotrophic (presence of organic carbon and light) or heterotrophic (presence of organic carbon and absence of light) conditions. To investigate direct utilisation of organic carbon and indirect utilisation via bacterial CO 2 -genesis, cultivation was performed under both axenic and non-axenic conditions. Tribonema was found to directly utilise glucose, which increased mixotrophic productivity and maintained growth under heterotrophic conditions. In contrast, acetate was only indirectly utilised mixotrophically in the presence of bacteria, whereas ethanol was not utilised under any conditions. The underlying mechanisms of glucose utilisation by Tribonema were also investigated by analysing its photosynthetic rate and respiration rate under glucose concentrations ranging from 0 – 100 g L -1 . Based on the results, enhancements to metabolic pathways and reduced CO 2 requirements provided by glucose utilisation were proposed. Despite the positive results with respect to glucose utilisation, out competition for this resource by bacteria suggest that Tribonema is more suitable for treatment of wastewater with low organic carbon concentrations, such as secondary-treated wastewater effluent.