Potential for green microalgae to produce hydrogen, pharmaceuticals and other high value products in a combined process

Abstract

Green microalgae for several decades have been produced for commercial exploitation, with applications ranging from health food for human consumption, aquaculture and animal feed, to coloring agents, cosmetics and others. Several products from green algae which are used today consist of secondary metabolites that can be extracted from the algal biomass. The best known examples are the carotenoids astaxanthin and β-carotene, which are used as coloring agents and for health-promoting purposes. Many species of green algae are able to produce valuable metabolites for different uses; examples are antioxidants, several different carotenoids, polyunsaturated fatty acids, vitamins, anticancer and antiviral drugs. In many cases, these substances are secondary metabolites that are produced when the algae are exposed to stress conditions linked to nutrient deprivation, light intensity, temperature, salinity and pH. In other cases, the metabolites have been detected in algae grown under optimal conditions, and little is known about optimization of the production of each product, or the effects of stress conditions on their production. Some green algae have shown the ability to produce significant amounts of hydrogen gas during sulfur deprivation, a process which is currently studied extensively worldwide. At the moment, the majority of research in this field has focused on the model organism, Chlamydomonas reinhardtii, but other species of green algae also have this ability. Currently there is little information available regarding the possibility for producing hydrogen and other valuable metabolites in the same process. This study aims to explore which stress conditions are known to induce the production of different valuable products in comparison to stress reactions leading to hydrogen production. Wild type species of green microalgae with known ability to produce high amounts of certain valuable metabolites are listed and linked to species with ability to produce hydrogen during general anaerobic conditions, and during sulfur deprivation. Species used today for commercial purposes are also described. This information is analyzed in order to form a basis for selection of wild type species for a future multi-step process, where hydrogen production from solar energy is combined with the production of valuable metabolites and other commercial uses of the algal biomass.

Figure 1.

Production of valuable metabolites from algae in commercial use today (A), compared to the proposed processes where stress factors are applied to induce both hydrogen production and production of valuable metabolites simultaneously (B), or in sequence (C).

Figure 2.

Overview of the combined process for production of hydrogen and bioactive metabolites. Green microalgae can be cultured under optimal growth conditions, followed by exposure to stress conditions (high light intensity, nutrient deprivation). The algal biomass can be harvested and used for different purposes, for example direct use as food supplement, aquaculture and animal fodders. Several valuable components can be extracted for the purpose of pharmaceutical industry, cosmetics or other types of industrial purposes.

Figure 3.

Schematic overview of suggested mechanisms for hydrogen production during sulfur deprivation in light, as it has been described for Chlamydomonas reinhardtii. Deprivation from sulfur leads to a degradation of PSII components, which partly inhibits the oxidation of water, and less oxygen is thereby produced in the photosystem. The low level of oxygen that is still produced in PSII is continuously consumed by the respiration, and the culture becomes anaerobic. Sulfur deprivation also leads to degradation of the enzymes in the Calvin cycle, causing this CO₂ fixation pathway and energy sink to come to a halt. When the Calvin cycle is no longer available for reducing CO₂, the whole system of PSII and PSI is reduced, creating a potentially dangerous situation for the algae. To remove the reductive pressure, the algae dispose of the electrons by transferring electrons from ferredoxin to hydrogenase. This enzyme then uses the reductive energy to form hydrogen which can easily be released from the cell. Depending on culturing conditions and other factors, a certain amount of electrons released in the form of hydrogen may originate from degradation of starch. This reducing power enters the electron transport chain from the PQ pool.

Figure 4.

Graphic summary of the most common stress reactions having influence on the synthesis of some important valuable metabolites in green algae.