H2 production pathways in nutrient-replete mixotrophic Chlamydomonas cultures under low light. Response to the commentary article “On the pathways feeding the H2 production process in nutrient-replete, hypoxic conditions,” by Alberto Scoma and Szilvia Z. Tóth

Abstract

BackgroundA recent Commentary article entitled “On the pathways feeding the H2 production process in nutrient-replete, hypoxic conditions” by Dr. Scoma and Dr. Tóth, Biotechnology for Biofuels (2017), opened a very interesting debate about the H2 production photosynthetic-linked pathways occurring in Chlamydomonas cultures grown in acetate-containing media and incubated under hypoxia/anoxia conditions. This Commentary article mainly focused on the results of our previous article “Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures,” by Jurado-Oller et al., Biotechnology for Biofuels (7, 2015; 8:149).Main bodyHere, we review some previous knowledge about the H2 production pathways linked to photosynthesis in Chlamydomonas, especially focusing on the role of the PSII-dependent and -independent pathways in acetate-containing nutrient-replete cultures. The potential contributions of these pathways to H2 production under anoxia/hypoxia are discussed.ConclusionDespite the fact that the PSII inhibitor DCMU is broadly used to discern between the two different photosynthetic pathways operating under H2 production conditions, its use may lead to distinctive conclusions depending on the growth conditions. The different potential sources of reductive power needed for the PSII-independent H2 production in mixotrophic nutrient-replete cultures are a matter of debate and conclusive evidences are still missing.

Algae-Bacteria Consortia as a Strategy to Enhance H2 Production

“…In addition, the presence of acetic acid in the culture media has been reported as a key parameter for photo-H 2 production in Chlamydomonas monocultures [ 14 ] and co-cultures [ 56 ], whose role is partially independent of its capacity to promote hypoxia [ 14 ]. It has been suggested that, under light, nutrient-repleted conditions and hypoxia, the assimilation (or photoassimilation) of acetic acid, and not starch mobilization, can provide, directly or indirectly, electrons for the PSII-independent H 2 production pathway [ 13 , 14 ]. Physiologically, the photoassimilation of acetate under hypoxia could be equivalent to the H 2 photo-fermentation described in photosynthetic bacteria.…”

“…In the PSII-independent pathway, starch degradation has been identified as the most common source of reductants under sulfur (S)-depleted conditions [ 12 ]. However, under hypoxia and nutrient replete conditions, acetic acid assimilation has been suggested to play an important role as source of reductants for H 2 production [ 13 , 14 , 15 , 16 ]. The third pathway is known as the fermentative or dark pathway.…”