Effective role of medium supplementation in microalgal lipid accumulation.

The present study investigated the interaction between starch and lipid accumulation in a green microalgae enrichment culture. The objective was to optimize the lipid content by manipulation of the medium in regular batch culture. Two medium designs were evaluated: First a high ortho-P concentration with vitamin supplementary (Pi-vitamins supplemented medium), second normal growth medium (control). Both media contained a low amount of nitrogen which was consumed during batch growth in three days. The batch experiments continued for another 4 days with the absence of soluble nitrogen in the medium. When the mixed microalgal culture was incubated in the Pi-vitamin supplemented medium, the lipid, and starch content of the culture increased within the first 3 days to 102.0 ± 5.2 mg/L (12.7 ± 0.6% of DW) and 31.7 ± 1.6 mg/L (4.0 ± 0.2% of DW), respectively. On the last day of the experiment, the lipid, and starch content in Pi-vitamin medium increased to 663.1 ± 32.5 mg/L (33.4 ± 1.6% of DW) and 127.5 ± 5.2 mg/L (6.4 ± 0.3% of DW). However, the lipid and starch content in the control process, reached to 334.7 ± 16.4 mg/L (20.1 ± 1.0% of DW) and 94.3 ± 4.6 mg/L (5.7 ± 0.3% of DW), respectively. The high Pi-vitamin medium induced storing lipid formation clearly while the starch formation was not affected. The lipid contents reported here are among the high reported in the literature, note that already under full growth conditions significant lipid levels occurred in the algal enrichment culture. The high lipid productivity of the reported mixed microalgae culture provides an efficient route for efficient algal biodiesel production.

Influence of silicate on enrichment of highly productive microalgae from a mixed culture.

Microalgae have the potential to supply a biobased society with essential feedstocks like sugar and lipids. Besides being productive, strains used for this purpose should grow fast, be resistant to predators, and have good harvestability properties. Diatoms, a class of siliceous algae, have these and other preferred characteristics. In this paper, we describe the enrichment of microalgae in sequencing batch reactors with and without supply of silicate. Both reactors were operated with a light-dark cycle. To maximize storage compound production, carbon fixation and nitrogen uptake were uncoupled by limiting the availability of nitrate to the dark phase. After ten cycles, a stable culture was established in both reactors. The diatom Nitzschia sp. dominated the silicate-rich reactor, and the green algae Chlamydomonas sp. dominated the silicate-depleted reactor. Over the remaining 27 cycles of the experiment, the microalgal community structure did not change, indicating a highly stable system. Although the dominant microalga was highly dependent on the presence of silicate, the performance of both microalgal enrichments was similar. Polymers of glucose were stored during the nitrogen-limited light period. On organic matter dry weight basis, the sugar content of the biomass increased during the light period from 17 ± 4 to 53 ± 4 % for the silicate-limited culture, and from 14 ± 4 to 43 ± 4 % (w w-1) for the silicate excess culture. These results show that storage compound production can be achieved under various conditions, as long as a selective environment is maintained.

Ecology-based selective environments as solution to contamination in microalgal cultivation.

Large-scale production of energetic storage compounds by microalgae is hampered by competition and evolution. Both phenomena result in contamination and arise due to a mismatch between the desired productive microalgal strain and the constructed environment. The prevailing approach to solve this issue involves increasing the survival potential of the desired strain, for example by working in closed systems or at extreme conditions. We advocate adjusting the environment in such a way that lipid production, or any other desired characteristic, gives a competitive advantage. Competition and evolution become a value rather than a threat to processes in which the desired characteristic is ensured by a selective environment. Research and cultivation efforts will benefit from this approach as it harnesses the microalgal diversity in nature.