range [16]. The possibility of APB to use alternative wavelength ranges can be an
important feature when competing for light energy with the visible light
absorbing microalgae and cyanobacteria.
Carotenoids are a group of pigments commonly present in microalgae/
cyanobacteria and APB which have the important function of harvesting light
(transference of resonant energy to Chls/Bchls) [18] and protecting the photosynthetic apparatus from light and oxygen damage. In general, most carotenoids
have an absorption range around 400–550 nm, in the middle of the visible light
spectrum [12].
• Ecological distribution and applications
The metabolic richness, heterogeneity, and ability of phototrophic organisms to
tolerate extreme conditions allow them to adapt to a great diversity of environments. The type of phototrophic organisms that can be found in a specific area is
greatly influenced by the environmental conditions like temperature, light
intensity, salinity, pH, presence of oxygen, CO 2 , sulfide, organic compounds, and
nutrients [19, 20]. In particular, microgradients of light, sulfide, and oxygen
strongly affect the organism’s distribution along the photic zones. Phototrophs
that harbor oxygen-sensitive enzymes, like the strict anaerobic Chlorobi, can be
found in sulfidic environments, deeply in the water columns or mats. To overcome the lower light availability, they use specialized light-harvesting structures,
the chlorosomes [10, 21]. As lithotrophs that can use H 2 S as electron donor, most
studies have reported the utilization of Chlorobi organisms (e.g., Chlorobium
limicola) for sulfide removal and sulfur recovery from biogas and wastewater
streams [22, 23].
In the case of aerobic APB and oxygen-tolerant PPB, these can be found at
shallow depths of water bodies, sediments, moist soils, and paddy fields, in oxic
—anoxic interfaces [24]. With their highly diverse and flexible metabolism, PPB
are capable of growing under phototrophic conditions in the light and under
chemotrophic conditions in the dark, either autotrophically (fixating CO 2 ) or
heterotrophically, using oxygen or inorganic molecules as terminal electron
acceptors in respiratory processes [24]. Though they have so many possible
metabolic combinations, they still have a preferred mode of growth. Consequently, these organisms can be divided into two groups: the purple sulfur bacteria (c-proteobacteria) and the purple non-sulfur bacteria, which are mostly
a-proteobacteria, with the exception of one genus that is b-proteobacteria [12].
Purple sulfur bacteria (PSB) typically grow phototrophically under anoxic
environments using hydrogen, sulfide, and sulfur as electron donors, producing
sulfur granules that can be deposited internally or externally, depending on the
organism family [25]. As for the purple non-sulfur bacteria (PNSB), these
organisms also typically grow under light anoxic conditions, but they present a
much more highly diverse and flexible metabolism in relation to PSB. In fact,
their preferred mode of growth is photoorganoheterotrophy, meaning they use
organic molecules both as electron donors and carbon source [24]. Nevertheless,
many species can also use hydrogen and reduced sulfur compounds as photosynthetic electron donors. Besides phototrophy, some PNSB can grow equally
418
J. C. Fradinho et al.
important feature when competing for light energy with the visible light
absorbing microalgae and cyanobacteria.
Carotenoids are a group of pigments commonly present in microalgae/
cyanobacteria and APB which have the important function of harvesting light
(transference of resonant energy to Chls/Bchls) [18] and protecting the photosynthetic apparatus from light and oxygen damage. In general, most carotenoids
have an absorption range around 400–550 nm, in the middle of the visible light
spectrum [12].
• Ecological distribution and applications
The metabolic richness, heterogeneity, and ability of phototrophic organisms to
tolerate extreme conditions allow them to adapt to a great diversity of environments. The type of phototrophic organisms that can be found in a specific area is
greatly influenced by the environmental conditions like temperature, light
intensity, salinity, pH, presence of oxygen, CO 2 , sulfide, organic compounds, and
nutrients [19, 20]. In particular, microgradients of light, sulfide, and oxygen
strongly affect the organism’s distribution along the photic zones. Phototrophs
that harbor oxygen-sensitive enzymes, like the strict anaerobic Chlorobi, can be
found in sulfidic environments, deeply in the water columns or mats. To overcome the lower light availability, they use specialized light-harvesting structures,
the chlorosomes [10, 21]. As lithotrophs that can use H 2 S as electron donor, most
studies have reported the utilization of Chlorobi organisms (e.g., Chlorobium
limicola) for sulfide removal and sulfur recovery from biogas and wastewater
streams [22, 23].
In the case of aerobic APB and oxygen-tolerant PPB, these can be found at
shallow depths of water bodies, sediments, moist soils, and paddy fields, in oxic
—anoxic interfaces [24]. With their highly diverse and flexible metabolism, PPB
are capable of growing under phototrophic conditions in the light and under
chemotrophic conditions in the dark, either autotrophically (fixating CO 2 ) or
heterotrophically, using oxygen or inorganic molecules as terminal electron
acceptors in respiratory processes [24]. Though they have so many possible
metabolic combinations, they still have a preferred mode of growth. Consequently, these organisms can be divided into two groups: the purple sulfur bacteria (c-proteobacteria) and the purple non-sulfur bacteria, which are mostly
a-proteobacteria, with the exception of one genus that is b-proteobacteria [12].
Purple sulfur bacteria (PSB) typically grow phototrophically under anoxic
environments using hydrogen, sulfide, and sulfur as electron donors, producing
sulfur granules that can be deposited internally or externally, depending on the
organism family [25]. As for the purple non-sulfur bacteria (PNSB), these
organisms also typically grow under light anoxic conditions, but they present a
much more highly diverse and flexible metabolism in relation to PSB. In fact,
their preferred mode of growth is photoorganoheterotrophy, meaning they use
organic molecules both as electron donors and carbon source [24]. Nevertheless,
many species can also use hydrogen and reduced sulfur compounds as photosynthetic electron donors. Besides phototrophy, some PNSB can grow equally
418
J. C. Fradinho et al.