The ability of green algae to produce hydrogen was discovered over 70 years ago [438, 439]. Hydrogen is recognized in the US and EU as a promising future fuel [440]. While the technology of generating hydrogen from algae is far from being at the industrial scale, several laboratories are working on improving the hydrogen production efficiency by using transcriptomics, proteomics, and metabolomic data [441-443].

The ADP can be used to process surplus algal biomass into methane or hydro­gen. A combined biorefinery concept has recently been proposed [157]. During the first step, C. reinhardtii produces hydrogen via a sulfur deprivation method. During the second step, it is digested anaerobically for methane production. The authors reported approximately 123% more biogas production from algae after the hydro­gen production cycle compared to fresh algae due to the accumulation of lipids and carbohydrates during the sulfur deprivation step [443, 444].

Phototrophic Microbial Fuel Cell

Light energy can be converted into electrical energy via a PMFC [445-448]. The combination of an ADP and a PMFC in a closed loop system can produce methane and electricity [237]. In this system, algal biomass serves as a substrate for AD and provides oxygen (final electron acceptor) for the fuel cell. The liquid phase of digested biomass contains compounds and nutrients to supply electrons in the fuel cell and for subsequent use as media for algal growth. While the system failed to operate in continuous-flow mode, in batch experiments it successfully produced methane (0.32 L/gVS) and power (1.33 mW/m2 illuminated footprint area). Possible reasons for the failure of a continuous system are low biomass concentration, high nitrate concentration, or high water circulation flow rate [237] .