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The high-energy input for harvesting biomass makes current commercial microalgal biodiesel production economically unfeasible. A novel harvesting method is presented as a cost and energy efficient alternative: the bio-flocculation by using one flocculating microalga to concentrate the non-flocculating microalga of interest. Three flocculating microalgae, tested for harvesting of microalgae from different habitats, improved the sedimentation rate of the accompanying microalga and increased the recovery of biomass. The advantages of this method are that no addition of chemical flocculants is required and that similar cultivation conditions can be used for the flocculating microalgae as for the microalgae of interest that accumulate lipids. This method is as easy and effective as chemical flocculation which is applied at industrial scale, however in contrast it is sustainable and cost-effective as no costs are involved for pre-treatment of the biomass for oil extraction and for pre-treatment of the medium before it can be re-used.

The toxicity of homologous series of organic solvents has been investigated for the gram-positive bacteria, Arthrobacter sp. and Nocardia sp., and the gram-negative bacteria, Acinetobacter sp. and Pseudomonas sp. The hydrophobicity of the solvent, expressed by its logP(octanol), proves to be a good measure for the toxicity of solvents in a two-phase system. The transition from toxic to nontoxic solvents occurs between logP(octanol) 3 and 5 and depends on the homologous series. No correlation has been found between the hydrophobicity of the substituent on the alkyl backbone of the solvent and the location of the transition point in toxicity. The logP(octanol), above which all solvents are nontoxic, is used to express the solvent tolerance of the bacteria. In general, the solvent tolerance of gram-negative bacteria is found to be slightly higher than that of gram-positive bacteria, but this does not hold for all homologous series of organic solvents investigated.Because the toxicity effects of organic solvents in a two-phase system can be ascribed to molecular as well as phase toxicity effects, molecular toxicity effects were investigated separately in a one-phase system with subsaturating amounts of organic solvent. The solvent concentration in the aqueous phase, at which 50% of the metabolic activity of the bacteria is lost, is used to express solvent toxicity. This concentration is found to be similar for the gram-positive Arthrobacter and the gram-negative Acinetobacter. Assuming the critical membrane concentration theory (G. J. Osborne et al. Enzyme Microb. Technol. 1990, 12: 281-291) to be valid, it can be concluded that differences in solvent tolerance between these two bacteria, cannot be ascribed to differences in response to molecular toxicity. Prediction of the toxicity of any solvent, using the critical membrane theory, appears to be possible in the case of alkanols or alkyl acetates. However, prediction of the toxicity of ethers appears to be impossible.

Simultaneous production and selective extraction of beta-carotene from living cells of Dunaliella salina in a two-phase system of aqueous and organic phases has been investigated. Solvents with values of log P(octanol), which denotes hydrophobicity of a compound, ranging from 3 to 9 were used as organic phase. Viability and activity of Dunaliella salina in the presence of organic solvents were checked by microscopic observation and photosynthetic oxygen-production-rate measurements, respectively. Extraction ability of different solvents for both beta-carotene and chlorophyll was determined spectrophotometrically. In addition, beta-carotene contents of the cells growing in the aqueous phase and extracted beta-carotene by the different organic phases were quantified by the same method. Results showed that solvents having log P(octanol) > 6 can be considered biocompatible for this alga. Moreover, pigment extraction ability of a solvent is inversely dependent on its log P(octanol) value. By increasing the degenerative hydrophobicity the extraction ability for both chlorophyll and beta-carotene, decreases. However, this decrease is more profound for chlorophyll. Therefore, selective extraction of beta-carotene becomes feasible. Comparison of the total beta-carotene produced in the presence and in the absence of solvents shows that the presence of a second phase of biocompatible solvents in the culture media may induce the beta-carotene production pathway. The beta-carotene productivity per cell in a two-phase system with dodecane was the highest observed. Extraction ability of the biocompatible solvents dodecane, tetradecan, and hexadecane was similar.