Environmental Engineering Reference
In-Depth Information
2. State of the Science
2.1. Microorganisms
Several types of microorganisms, undergoing different, yet related metabolisms, are
involved in H2 production. To aid in clarifying and distinguishing these mechanisms, let us
first consider the microorganisms involved.
2.1.1. Oxygenic phototrophs
. Oxygenic phototrophs include both aerobic eukaryotes
(plants and green algae) and aerobic prokaryotes (cyanobacteria) that are able to use photons
to energize electrons from water, thus yielding O
2
, for the ultimate purpose of generating the
reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) for CO
2
fixation into
organic carbon (Figure 16). The electrons are energized by passage through two successive
photosystems, photosystems II (PS II) and I (PS I), each of which harvests photons with
networks of chlorophyll and other accessory pigments and uses the photon energy to impart
greater reducing power to the electrons. After nicotinamide adenine dinucleotide phosphate
(NADP) is reduced to NADPH, and CO
2
is fixed into organic carbon, phototrophs typically
respire this substrate aerobically to generate adenosine triphosphate (ATP).
Dashed arrows represent light reactions that extract electrons from H
2
O and deliver them to ferredoxin
(Fd), from which they can be used to fix CO
2
via NADPH or to produce H
2
via hydrogenases.
Abbreviations represent common electron carriers, arranged to show their relative redox potentials,
Em.
Figure 16. The Z-scheme for electron transport in the oxygenic photosynthesis of cyanobacteria and
green algae.
Alternatively, electrons may also be returned by photosystem I to the plastoquinone pool
in a process called cyclic photophosphorylation (Figure 16). This process allows additional
