Cultivation of Chlorella vulgaris in nutrient solution from greenhouse tomato production : a possibility to reduce nutrient levels and produce commercially interesting metabolites

Abstract: The idea of using algae in different areas and exploring its possibilities is nothing new. Already in the early 1950s microalgae was explored as a possible food resource because of its high protein content. Lately there has been an increased interest in its potential for industrial use. Wastewater treatment is a possible use of algae since the algae grow well on the nutrients present in the water. This offers the possibility to grow algae for nutrient reduction and use the biomass for energy or animal feed since many alga species has a good nutritional value. Approximately 800-1000 m3 water is applied per 1000 m2 of greenhouse area during one season in a tomato production. Out of this is about 20-25% drained off and become waste, if there is no recirculation system in the greenhouse. The nutrient composition of the drainage water varies over the season, but is generally high in nitrogen. Considering a sustainable development of the Swedish greenhouse industry, it is therefore important to reduce the nutrient levels in the water before it is drained away. Chlorella vulgaris is a robust and fast growing microalgae species commonly cultivated and interesting regarding the production of secondary metabolites with health beneficial properties such as omega-3 fatty acids and carotenoids. A successful alga production requires a high growth rate and a high biomass with a desirable composition e.g. distribution between fatty acids, proteins and pigments, which is influenced by several abiotic and biotic factors. For a grower all those parameters might be hard to control. However the primary purpose of the algae is to reduce the nutrients and less complicated deposition is therefore desirable. This might be as raw material for biogas or animal feed or possibly as biofertilizers. Two experiments on Chlorella vulgaris capability, to grow and reduce nutrient levels in excessive water from greenhouse production, were performed. In experiment 1, Z8 (a standard medium for alga) was compared with an artificial nutrient solution, high in nitrogen. In experiment 2, Z8 was compared with a nutrient solution from a greenhouse tomato production. The growth of C. vulgaris was measured by optical density (OD) at 405 nm during two weeks. At the start and at the end of the experiments, nutrient analyses were performed to calculate the reduction. Also a fatty acid analyse was performed with gas chromatography (GC) in order to look for possible interesting fatty acids in C. vulgaris. The results gave a significant reduction of nutrients, which indicate that Chlorella vulgaris can grow and reduce nutrient levels in a nutrient solution with a distribution that can be expected from a tomato production. On the other hand it is uncertain if C. vulgaris can compete strong enough in a nutrient solution that contains several other microorganisms. The iron reduction in both experiments was close to 100% and it might be possible that the iron deficiency limited the growth. The result from the fatty acid analysis was consistent with earlier reported results of fatty acid composition in Chlorella vulgaris and no longer fatty acids then 18:3 were found. In a larger system outside the lab, the amount of biomass and the purity as well as the obtained nutrient reduction will all give a variable result from time to time and between growers due to all culture parameters that are hard to control and environmental conditions which will fluctuate.