MULTISPECIES ALGAL ASSAY TO STUDY THE ROLE OF INTERSPECIFIC COMPETITION ON OIL PRODUCTION IN MASS CULTURES

For the last 30 years microalgae have been of interest as sources of renewable biofuels (biodiesel) and potential CO2 sinks. However, the commercial production of algal biofuels is yet to become reality. For commercial production the algae must be cultured reliably with a high productivity on a large scale in open-pond systems with efficient enrichment of nutrients and with a minimum energy requirement. While algal species with high oil content were selected in the early studies on oil production from microalgae, it seems that the most oil-rich species are just the most slow-growing species, resulting in low rate of oil production. Further, since the interspecific competition in outdoor ponds can represent a major problem in microalgae mass cultures, fast-growing native species can outcompete the inoculated species under field conditions. In this work an experimental framework, based on an induced-competition algal assay between two fast-growing species, was set up in laboratory with the aim to evaluate the role of competition on oil production. Since the average oil content of Chlorophyceae is the highest among the algal classes, species belonging to the group of green algae were selected for the experiment. The two fast-growing species Selenastrum capricornutum and Scenedesmus quadricauda, from the laboratory algal collection, were used mainly because of their difference in both morphological and dimensional features, which makes easier to discriminate the specific cell-growth in mixed cultures. This experimental strategy enabled to monitor the mixed culture by a multi-channel particle counter, able to discriminate the two-size distributions, around 45 and 85 µm3, respectively. An exponentially growing culture of Selenastrum (ca. 100,000 cells/ml) was contaminated with ca. 5,000 cells/ml of Scenedesmus, while an uncontaminated culture was parallely tested as a control. The evolution trend was monitored using the multisizer counter as well as the microscope until 19 days, i.e. when the stationary-growth phase was reached. Selenastrum (9,900,000 cells/ml) was keeping the dominance according to the 70:30 percent ratio. The biomass has been dried and the solvent-extracted oil was gravimetrically measured and expressed as percent of the biomass dry weight. Compared to the control, the oil yield (45.13% dw biomass) in the mixed culture was similar to the oil extracted from Selenastrum single culture (46.35% dw biomass). These results are indicating that the oil production is not affected by the interspecific competition, suggesting that fast-growing green algae species could be successful even if mixed in mass-culture systems. The experimental framework revealed to be effective to study the environmental factors affecting interspecies competition in open-ponds field conditions.