95
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_6
Chapter 6
Factors Controlling N 2 Fixation
The growth and activity of N 2 -fixing microorganisms in habitats are constrained by
physical and chemical factors (Stal 2009) (Table 6.1). The major controls on the
distributions and activities of microorganisms in the ocean are light, temperature,
nutrients (including organic matter for heterotrophs), pH, pressure and especially
for N 2 - fixing microorganisms, O 2 (Stal 2009) (Table 6.1). The factors that control
microbial growth and N 2 fixation are not homogenous in the oceans but are variable
in space and time with large fluctuations and gradients (Table 6.1).
6.1 Oxygen
O 2 is a primary regulator of N 2 fixation as the structural proteins are rapidly inactivated by molecular O 2 . As a result, N 2 -fixing microorganisms either avoid O 2 , or
have physiological adaptations for avoiding O 2 inactivation, and avoiding wasting
energy synthesizing the proteins when O 2 is present. N 2 -fixing cyanobacteria have a
physiological challenge because of the O 2 evolved through photosynthesis (Fay
1992). N 2 -fixing cyanobacteria must balance the need to reduce O 2 concentrations
at the intracellular site of N 2 fixation while maintaining sufficient O 2 diffusion to
maintain respiration for energy generation (Stal and Zehr 2008). Marine N 2 -fixing
cyanobacterial taxa exhibit a range of major physiological and morphological adaptations and strategies to avoid O 2 inactivation of nitrogenase.
Some unicellular cyanobacteria avoid inactivation of nitrogenase by photosynthetically evolved O 2 by separating the two activities in time. Crocosphaera (and
Cyanothece) have light-dark cycles of N 2 fixation and photosynthesis, that are
underlain by day-night cycles in metabolism and gene expression (Colon-Lopez
et al. 1997; Shi et al. 2010; Toepel et al. 2008). A circadian rhythm was demonstrated in a N 2 -fixing cyanobacterium (Cyanothece sp. first called Synechococcus
sp. RF-1) in this phylogenetic group (Huang and Grobbelaar 1995). Fixed carbon is
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_6
Chapter 6
Factors Controlling N 2 Fixation
The growth and activity of N 2 -fixing microorganisms in habitats are constrained by
physical and chemical factors (Stal 2009) (Table 6.1). The major controls on the
distributions and activities of microorganisms in the ocean are light, temperature,
nutrients (including organic matter for heterotrophs), pH, pressure and especially
for N 2 - fixing microorganisms, O 2 (Stal 2009) (Table 6.1). The factors that control
microbial growth and N 2 fixation are not homogenous in the oceans but are variable
in space and time with large fluctuations and gradients (Table 6.1).
6.1 Oxygen
O 2 is a primary regulator of N 2 fixation as the structural proteins are rapidly inactivated by molecular O 2 . As a result, N 2 -fixing microorganisms either avoid O 2 , or
have physiological adaptations for avoiding O 2 inactivation, and avoiding wasting
energy synthesizing the proteins when O 2 is present. N 2 -fixing cyanobacteria have a
physiological challenge because of the O 2 evolved through photosynthesis (Fay
1992). N 2 -fixing cyanobacteria must balance the need to reduce O 2 concentrations
at the intracellular site of N 2 fixation while maintaining sufficient O 2 diffusion to
maintain respiration for energy generation (Stal and Zehr 2008). Marine N 2 -fixing
cyanobacterial taxa exhibit a range of major physiological and morphological adaptations and strategies to avoid O 2 inactivation of nitrogenase.
Some unicellular cyanobacteria avoid inactivation of nitrogenase by photosynthetically evolved O 2 by separating the two activities in time. Crocosphaera (and
Cyanothece) have light-dark cycles of N 2 fixation and photosynthesis, that are
underlain by day-night cycles in metabolism and gene expression (Colon-Lopez
et al. 1997; Shi et al. 2010; Toepel et al. 2008). A circadian rhythm was demonstrated in a N 2 -fixing cyanobacterium (Cyanothece sp. first called Synechococcus
sp. RF-1) in this phylogenetic group (Huang and Grobbelaar 1995). Fixed carbon is