Liquid fuels have excellent properties in terms of storage, logistics and energy density compared to gaseous fuels or electricity. A major disadvantage of liquid fuels is that a vast majority of them is derived from fossil resources. Currently, the consumption rate of fossil fuels by far outcompetes the natural production rate, resulting in elevated atmospheric CO 2 concentrations. Photosynthetic organisms (plants and algae) xate atmospheric CO 2 using solar energy. CO 2 consumption and emission would be balanced if liquid fuels would be derived from plants or algae. However, growing terrestrial plants for biofuel production means less agricultural land and fresh water remains available for food production. Microalgae can grow under marine conditions and outcompete terrestrial plants in terms of areal productivity. On the other hand, cultivation of microalgae introduces new challenges. Species control is, compared to terrestrial plants, much more difficult. Any cultivation system is prone to contamination by undesired algal species threatening stable production. In this study we show that we can overcome this hurdle by creating a selective environment. Our approach allows for large scale, stable production of biofuel precursors and is therefore a substantial step forward in the production of renewable fuels.

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Large-scale production of microalgal storage compounds will likely involve marine microalgae. Previously, we described a method to enrich microalgae with a high storage compound productivity from a natural inoculum. Here, this strategy was implemented under marine conditions in a sequencing batch reactor. The influence of the volume exchange ratio and the moment of ammonium addition in the day-night cycle on the storage compound productivity are described. Storage compound productivity was maximal if ammonium was supplied at the start of the dark period rather than the light period, irrespective of the volume exchange ratio. Increasing the volume exchange ratio from 33 to 50 % per cycle induced a decrease in storage compound production if ammonium was supplied in the light whereas the storage compound productivity was comparable when ammonium was supplied in the dark. The latter indicates a shift of cell division processes to the light period at increasing volume exchange ratio, although ammonium uptake completely occurred in the dark period.

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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.