99
planktonic invertebrates such as copepods (Braun et al. 1999; Proctor 1997; Scavotto
et al. 2015) have associated diazotrophs, which may benefit from low O 2 environments in the gut.
6.2 Light
Since light is a source of energy and reductant, it is important for N 2 fixation in
those microorganisms which convert light energy to photosynthate, ATP and reductant. Light is directly important for cyanobacteria, but also for phototrophic bacteria
including purple and green sulfur bacteria and purple nonsulfur bacteria. The purple
and green sulfur N 2 -fixing bacteria are found in microbial mats in shallow lagoons
and assume their vertical position in mats based on light intensity and quality
(Roberts and Ludden 1992; Wynn-Williams and Rhodes 1974). Microbial rhodopsins which couple light absorption to energy generation have been found widely
distributed amongst heterotrophic Bacteria and Archaea (Gómez-Consarnau et al.
2019; Beja et al. 2000) although they have not yet been linked to powering N 2 fixation (Zehr and Capone 2020).
In the ocean, particularly the oligotrophic open ocean where diazotrophic cyanobacteria reside, there is sufficient energy for photosynthesis to depths of greater than
100–150 m. This energy is available directly (cyanobacteria) or indirectly (photosynthate to feed heterotrophs) to support N 2 fixation. The quality of light (color)
varies from a broad spectrum including red and shifts progressively to predominantly blue with depth (Kirk 2000), which is of significance to phycobiliprotein
containing cyanobacteria, that can absorb energy from intermediate wavelengths
(Vincent 2000). Except for the macroscopically visible Trichodesmium (Carpenter
et al. 1993, 2004), which can also form surface accumulations (blooms, “slicks”),
the depth profile of N 2 fixation is not well-characterized for most marine planktonic
cyanobacteria (Church et al. 2009, 2005b). Light intensity directly affects photosynthesis in Trichodesmium, which can adjust its PSI: PSII ratio (Cai et al. 2015). This
is undoubtedly due to balancing the needs for carbon and nitrogen, while avoiding
inactivation of nitrogenase by O 2 (Cai et al. 2015).
Light is indirectly important for heterotrophic diazotrophs that are solely dependent on organic compounds synthesized by photoautotrophic organisms for their
needs for cell C and energy generation (Bonnet et al. 2013; Halm et al. 2012). It is
also currently unknown whether N 2 -fixing bacteria in the euphotic zone may directly
use light to provide some of the energy required for N 2 fixation, since so many bacteria, including heterotrophs, have been shown to use light energy with rhodopsinlike proteins (Gómez-Consarnau et al. 2019). In oligotrophic oceanic systems, the
photoautotrophs (or primary producers) are predominantly unicellular, except in
shallow benthic environments where seagrasses and macroalgae may be prevalent
(Carpenter et al. 1978). This is in contrast to terrestrial environments where multicellular plants are the dominant sources of organic matter.
6.2 Light
planktonic invertebrates such as copepods (Braun et al. 1999; Proctor 1997; Scavotto
et al. 2015) have associated diazotrophs, which may benefit from low O 2 environments in the gut.
6.2 Light
Since light is a source of energy and reductant, it is important for N 2 fixation in
those microorganisms which convert light energy to photosynthate, ATP and reductant. Light is directly important for cyanobacteria, but also for phototrophic bacteria
including purple and green sulfur bacteria and purple nonsulfur bacteria. The purple
and green sulfur N 2 -fixing bacteria are found in microbial mats in shallow lagoons
and assume their vertical position in mats based on light intensity and quality
(Roberts and Ludden 1992; Wynn-Williams and Rhodes 1974). Microbial rhodopsins which couple light absorption to energy generation have been found widely
distributed amongst heterotrophic Bacteria and Archaea (Gómez-Consarnau et al.
2019; Beja et al. 2000) although they have not yet been linked to powering N 2 fixation (Zehr and Capone 2020).
In the ocean, particularly the oligotrophic open ocean where diazotrophic cyanobacteria reside, there is sufficient energy for photosynthesis to depths of greater than
100–150 m. This energy is available directly (cyanobacteria) or indirectly (photosynthate to feed heterotrophs) to support N 2 fixation. The quality of light (color)
varies from a broad spectrum including red and shifts progressively to predominantly blue with depth (Kirk 2000), which is of significance to phycobiliprotein
containing cyanobacteria, that can absorb energy from intermediate wavelengths
(Vincent 2000). Except for the macroscopically visible Trichodesmium (Carpenter
et al. 1993, 2004), which can also form surface accumulations (blooms, “slicks”),
the depth profile of N 2 fixation is not well-characterized for most marine planktonic
cyanobacteria (Church et al. 2009, 2005b). Light intensity directly affects photosynthesis in Trichodesmium, which can adjust its PSI: PSII ratio (Cai et al. 2015). This
is undoubtedly due to balancing the needs for carbon and nitrogen, while avoiding
inactivation of nitrogenase by O 2 (Cai et al. 2015).
Light is indirectly important for heterotrophic diazotrophs that are solely dependent on organic compounds synthesized by photoautotrophic organisms for their
needs for cell C and energy generation (Bonnet et al. 2013; Halm et al. 2012). It is
also currently unknown whether N 2 -fixing bacteria in the euphotic zone may directly
use light to provide some of the energy required for N 2 fixation, since so many bacteria, including heterotrophs, have been shown to use light energy with rhodopsinlike proteins (Gómez-Consarnau et al. 2019). In oligotrophic oceanic systems, the
photoautotrophs (or primary producers) are predominantly unicellular, except in
shallow benthic environments where seagrasses and macroalgae may be prevalent
(Carpenter et al. 1978). This is in contrast to terrestrial environments where multicellular plants are the dominant sources of organic matter.
6.2 Light