Environmental Engineering Reference
In-Depth Information
Among experimental organisms, the filamentous cyanobacteria have received the
majority of investigation to date, but non-filamentous cyanobacteria such as Oscillatoria that
protect nitrogenases from O
2
by temporal rather than physical separation of photosynthesis
and N
2
fixation also deserve further examination. In addition, a great diversity of
cyanobacteria adapted to a wide variety of environments exists that should be investigated for
potential contributions to the 112 production effort. The ability of some cyanobacteria to
grow heterotrophically is especially interesting in light of the possibility it presents to convert
organic wastes into 112 [75].
Finally, further work directed toward optimizing cultivation conditions for highly
desirable organisms, particularly outdoor and marine cultivation [80], is essential to the
realization of commercially viable indirect photolysis.
2.5 Photofermentation
Photofermentation is the light-dependent process carried out by anoxygenic phototrophic
bacteria, particularly purple non-sulfur bacteria such as Rhodobacter sphaeroides and
Rhodobacter capsulatus, in which 112 is evolved by nitrogenase under nitrogen-deficient
conditions, using ATP supplied by photoheterotrophic growth [19]. Although photosynthetic,
this process does not split water and does not evolve O2 as in the oxygenic photosynthesis of
green algae and cyanobacteria (Figure 19). It requires an anoxic atmosphere, because
anoxygenic phototrophs do not protect their nitrogenases from O2 intracellularly in the way
that cyanobacterial heterocysts do; however, the absence of O2 production causes the
continuous production of 112 for >100h to be relatively uncomplicated, in contrast to the case
with direct photolysis.
2.5.1. Cultivation conditions
. Rhodobacter and other anoxygenic phototrophs are
versatile organisms capable of a wide variety of growth modes, including aerobic respiration,
anaerobic respiration, fermentation, and photoautotrophy, enabling them to withstand the
varying culture conditions that would necessarily accompany outdoor cultivation. In addition,
the nitrogenase activity of these organisms is more stable under diurnal illumination. At the
same time, significant 112 production only occurs during photoheterotrophic growth, with the
result that an important aspect of cultivation design will be the balance between conditions
that favor nitrogenase stability and those that favor 112 production [19, 87].
Another important challenge in cultivation of all phototrophs is the self-shading effect
that intensifies as cultivation volumes become larger or as cell densities become greater. This
problem is already being addressed through genetic modification of the photosynthetic
apparati, as it is in green algae, and a mutant of R. sphaeroides with less than half of the
pigment of wild- type algae has already shown a 50 percent increase in 112 production [88].
Finally, the scale-up of photobioreactors with retention of optimal culture conditions
presents several challenges of its own. Several studies have shown that 112 production is
increased when cells are immobilized, and immobilization of cells on a large scale will
require discovery of gels or solid supports that can minimize the problems of inhomogeneous
distribution of cells and nutrients and difficulty of control, while maximizing the benefits of
higher 112 production rates and a cell-free effluent. In addition, efficient mixing also poses
special problems:while mixing is necessary to distribute substrates and collect gases,
mechanical mixing is difficult in high surface-area-to-volume reactors, and gas sparging risks
diluting the 112 produced [19].
