*Review* **Recent Achievements in Microalgal Photobiological Hydrogen Production**

**Eleftherios Touloupakis 1, Cecilia Faraloni 2, Ana Margarita Silva Benavides 3,4 and Giuseppe Torzillo 2,3,\***


**Abstract:** It is well known that over the last 60 years the trend of long-lived greenhouse gas emissions have shown a strong acceleration. There is an increasing concern and a mounting opposition by public opinion to continue with the use of fossil energy. Western countries are presently involved in a socalled energy transition with the objective of abandoning fossil energy for renewable sources. In this connection, hydrogen can play a central role. One of the sustainable ways to produce hydrogen is the use of microalgae which possess two important natural catalysts: photosystem II and hydrogenase, used to split water and to combine protons and electrons to generate gaseous hydrogen, respectively. For about 20 years of study on photobiological hydrogen production, our scientific hopes were based on the application of the sulfur protocol, which indisputably represented a very important advancement in the field of hydrogen production biotechnology. However, as reported in this review, there is increasing evidence that this strategy is not economically viable. Therefore, a change of paradigm for the photobiological production of hydrogen based on microalgae seems mandatory. This review points out that an increasing number of microalgal strains other than *Chlamydomonas reinhardtii* are being tested and are able to produce sustainable amount of hydrogen without nutrient starvation and to fulfill this goal including the application of co-cultures.

**Keywords:** biohydrogen; microalgae; *Chlamydomonas reinhardtii*; *Chlorella* sp.; photobioreactors; light conversion efficiency
