Scheme of two-stage cultivation H. pluvialis biorefinery

Scheme of two-stage cultivation H. pluvialis biorefinery producing the high value compound astaxanthin and either edible oil or the biofuel compound biodiesel. Green stage of cultivation can be performed using either photoautotrophic cultivation (deep green section) or hetero/mixotrophic cultivation (pale green section) systems. Red stage cultivation (red section) takes place after green stage of H. pluvialis cultivation and is aimed to maximize astaxanthin content. Recycling of waste is performed through an anaerobic digestion process. (Solid arrows – subsequent steps; dashed arrows – optional steps; double lines – final products; double arrows – inputs; dotted lines – opportunities for recycling resources. Figure source: reproduced from Shah et al. 2016, Front Plant Sci. 7:531. doi: 10.3389/fpls.2016.00531) distributed under the terms of the Creative Commons Attribution License.

Biofuel production is unlikely to be possible without the development of major low-energy and low-cost breakthrough technologies for cultivation, dewatering, and harvesting. Meanwhile, the high cost of microalgae cultivation and processing technologies can be justified by producing high-value nutraceutical compounds such as polyunsaturated fatty acids or astaxanthin from microalgae for commercial applications.

Using a “biorefinery approach,” which is defined as the integration of simultaneous production of numerous compounds within one system, maximizing the benefits and limiting the costs, can be a useful alternative.

The basis of natural astaxanthin, Haematococcus pluvialis, emerges as a very useful microalgae for the development of a dedicated microalgal biorefinery. It fits numerous requirements for the development microalgal biorefineries especially the “high value product first” principle.

Firstly, H. pluvialis can produce a high value (̴$7000/kg) product – astaxanthin – which can easily justify costly cultivation and processing systems required for this microalga. Secondly, H. pluvialis grown under stressed conditions induces both astaxanthin formation and deposition of triglycerides. These two responses are closely related and coincide in both space and time. Triglycerides are prerequisite for deposition of astaxanthin inside lipid bodies to confer its protective mechanism.

Generally in microalgal biofuel production, starvation-induced lipid accumulation is considered a major challenge for commercialization of these systems because the overall lipid productivity can drop significantly due to reduced growth rates under starvation conditions. In astaxanthin production the high value of the main product will compensate for the delay in final product formation. Due to the coexistence of astaxanthin and triglycerides in space and time, simultaneously astaxanthin and a biofuel feedstock (triglycerides) can be obtained from a single algal feedstock.

High fatty acid content (30–60% of DW) in the astaxanthin containing “red” cells makes H. pluvialis a very good choice for a biorefining strain. In addition, its fatty acid profiles have been proven as suitable for biodiesel production by several studies.

Thirdly, H. pluvialis can be grown under mixotrophic conditions, which are highly advantageous for the development of a microalgae biorefinery. This alga can utilize carbon dioxide, carbonates, and carbohydrates as carbon sources, opening the possibility of reducing production costs and/or speeding up the cultivation through using various waste streams like flue gasses or others.

Autotrophic, heterotrophic and mixotrophic cultivation modes require energy and nutrients, which can be recycled from the anaerobic digestion process. Carbon sources may vary depending on the cultivation mode. In photoautotrophic mode, the required CO2 can be recycled from energy production at the anaerobic digestion stage.

Heterotrophic mode requires a reduced carbon source (e.g. carbohydrates or acetate) that needs to be supplied from alternative sources. These compounds can also be originated from waste streams.

For example, carbohydrate-rich food waste can be used in heterotrophic cultivation of H. pluvialis. In case of mixotrophic cultivation both sources of carbon can be used. After simultaneous extraction of astaxanthin and triglycerides, algal biomass can be utilized as a supplementary feedstock for biogas production through anaerobic digestion. This would further help in the extraction of residual energy from this integrated bioprocess.

The above-mentioned three features make H. pluvialis a suitable candidate for algal biorefinery development, which can produce high value product (astaxanthin) and biofuel molecules (biodiesel and/or biogas).

We propose a H. pluvialis biorefinery scheme, as illustrated. We assume that this biorefinery concept can be helpful for the research, development and commercialization of H. pluvialis derived natural astaxanthin production as well as acceleration of the growth of   neutraceuticals, pharmaceuticals and aquaculture industries worldwide in the future.

Dr. Mahfuzur Shah is Associate Research Professor and Microalgal biotechnologist, and Dr. Maurycy Daroch) is Associate Professor and Renewable Bioenergy Researcher, both currently affiliated with School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen, China.