Chlamydomonas reinhardtii has been comprehensively investigated in terms of potential hydrogen photoproduction; in addition to hydrogen, a variable amount of byproducts are generated as part of the microalgal biomass. Its volatile nature permits the biomass to remain essentially intact (Kruse, Rupprecht et al., 2005).

To obtain further energy, the biomass can itself be processed via extraction and transester­ification of the remaining lipids to produce biodiesel. For example, the total lipid content of the biomass of the C. reinhardtii strain D1 after photobiological hydrogen production is 15 ± 2% on a dry-weight basis; oil may then be extracted, being composed of 3.3% w/woii phytols, 21% w/woii triglycerides, 39% w/woii polar lipids, and 37% w/woii highly polar lipids, to eventually biodiesel composed of 41% saturated fatty esters, 53% mono unsaturated fatty esters, and 7.2% polyunsaturated fatty esters (mainly linoleic acid) (Torri, Samori et al., 2011). This mix of methyl esters adheres to European Union (EU) standard EN 14214 pertaining to biodiesel specifications.

Following biodiesel production from the spent biomass, the lipid-free residue can still be used for animal feed or be anaerobically digested into biogas (Sialve, Bernet et al., 2009) as discussed in further detail in the following paragraph. Moreover, pyrolysis of the residue left after extraction may represent another pathway to produce extra energy (Mohan et al., 2006). Pyrolysis of the microalgal extraction residue may lead to oil with a quality lying between pe­troleum tar and bio-oil from lignocellulosic biomass (Miao et al., 2004); the mass yields of biochar, oil, and gas are 44 ± 1%, 28 ± 2%, and 28 ± 1%, respectively. The ash content of said biochar, obtained via combustion at 700 °C, was 45 ± 5%. On an ash-free basis, the mass yields of biochar, oil, and gas were 24 ± 5%, 38 ± 9%, and 36 ± 1%, respectively (Torri, Samori et al., 2011). Since a major portion of ashes, phosphorus, and nitrogen are retained in biochar, it may be used as fertilizer to improve the productivity of soil, thus contributing to abatement of greenhouse gases while making it possible to convert carbon-neutral energy into carbon­negative bio-energy (Kruse and Hankamer, 2010).

Additionally, production of biogas via fermentation of the microalgal biomass offers the possibility to recycle a large proportion of the original nutrients. Although this option is not economically feasible at low throughput rates, it will become a more interesting possi­bility as medium costs become a larger fraction of the final cost, coupled with consideration of phosphorous limitations (Cordell, Drangert et al., 2009).