100
6.3 Temperature
Temperature is a fundamental physical variable for biology, as it determines the
kinetics of many molecular reactions and interactions. Temperature has been
invoked as a major constraint on N 2 fixation in the open oceans (Breitbarth et al.
2007; Stal 2009), yet N 2 fixation is found in extremophiles at both ends of the temperature spectrum. N 2 -fixation was demonstrated in Archaea isolated from hydrothermal vents at 95 °C (Mehta and Baross 2006) and in Antarctic lakes (Vincent
2000). Thus, it was not completely surprising when N 2 fixation and N 2 -fixing microorganisms were found in cold polar waters (Harding et al. 2018; Shiozaki et al.
2018; Shiozaki et al. 2020), although polar waters had not previously been assumed
to be a site for N 2 fixation.
Trichodesmium is found at relatively warm temperatures and generally is constrained to above 20 °C (Breitbarth et al. 2007). Relationships between temperature
and the distribution of other N 2 -fixing species have been demonstrated in a number
of studies (Church et al. 2008; Moisander et al. 2010), and see review in Stal (2009).
One of the effects of temperature is its influence on the relative solubility of
gases, most importantly O 2 . The observation that heterocyst-forming cyanobacteria
are less common in the marine environment than the nonheterocyst-forming species
Trichodesmium (and now Crocosphaera and UCYN-A), has been the subject of
speculation and multiple hypotheses (Marino and Howarth 2016; Marino et al.
2006). One hypothesis involves the physiological response to O 2 and O 2 solubility
as a function of water temperature (Staal et al. 2003; Stal 2009). As water temperature increases above 25 °C, solubility decreases which reduces the advantage of the
heterocyst cell wall in preventing O 2 diffusion (Staal et al. 2003).
Thus, temperature ranges can directly, or by interacting with O 2 solubility, select
for different species that have different optima or different strategies for avoiding O 2
(i.e heterocyst-forming species). In addition, temperature in the environment correlates with other factors that control N 2 fixation, such as N depletion in the warm
oligotrophic gyres.
6.4 Nutrients
Microbial growth in aquatic environments requires basic major macronutrients and
trace elements. One or more may be in the shortest supply relative to nutritional
requirements, and is called the limiting nutrient (Moore et al. 2013) (see Chaps. 7
and 8). In marine environments, N, P and Fe are often the primary nutrients of greatest relevance and whose relative availability may limit primary productivity
(Falkowski et al. 1998). Other trace elements may be a factor for specific sub-groups
(Morel and Price 2003) such as silicon for Diatom-Diazotroph Associations (DDAs).
Diazotrophs have the same fundamental nutritional requirements as non- diazotrophs,
but possibly in different relative proportions. For example, the additional investment
6 Factors Controlling N 2 Fixation
6.3 Temperature
Temperature is a fundamental physical variable for biology, as it determines the
kinetics of many molecular reactions and interactions. Temperature has been
invoked as a major constraint on N 2 fixation in the open oceans (Breitbarth et al.
2007; Stal 2009), yet N 2 fixation is found in extremophiles at both ends of the temperature spectrum. N 2 -fixation was demonstrated in Archaea isolated from hydrothermal vents at 95 °C (Mehta and Baross 2006) and in Antarctic lakes (Vincent
2000). Thus, it was not completely surprising when N 2 fixation and N 2 -fixing microorganisms were found in cold polar waters (Harding et al. 2018; Shiozaki et al.
2018; Shiozaki et al. 2020), although polar waters had not previously been assumed
to be a site for N 2 fixation.
Trichodesmium is found at relatively warm temperatures and generally is constrained to above 20 °C (Breitbarth et al. 2007). Relationships between temperature
and the distribution of other N 2 -fixing species have been demonstrated in a number
of studies (Church et al. 2008; Moisander et al. 2010), and see review in Stal (2009).
One of the effects of temperature is its influence on the relative solubility of
gases, most importantly O 2 . The observation that heterocyst-forming cyanobacteria
are less common in the marine environment than the nonheterocyst-forming species
Trichodesmium (and now Crocosphaera and UCYN-A), has been the subject of
speculation and multiple hypotheses (Marino and Howarth 2016; Marino et al.
2006). One hypothesis involves the physiological response to O 2 and O 2 solubility
as a function of water temperature (Staal et al. 2003; Stal 2009). As water temperature increases above 25 °C, solubility decreases which reduces the advantage of the
heterocyst cell wall in preventing O 2 diffusion (Staal et al. 2003).
Thus, temperature ranges can directly, or by interacting with O 2 solubility, select
for different species that have different optima or different strategies for avoiding O 2
(i.e heterocyst-forming species). In addition, temperature in the environment correlates with other factors that control N 2 fixation, such as N depletion in the warm
oligotrophic gyres.
6.4 Nutrients
Microbial growth in aquatic environments requires basic major macronutrients and
trace elements. One or more may be in the shortest supply relative to nutritional
requirements, and is called the limiting nutrient (Moore et al. 2013) (see Chaps. 7
and 8). In marine environments, N, P and Fe are often the primary nutrients of greatest relevance and whose relative availability may limit primary productivity
(Falkowski et al. 1998). Other trace elements may be a factor for specific sub-groups
(Morel and Price 2003) such as silicon for Diatom-Diazotroph Associations (DDAs).
Diazotrophs have the same fundamental nutritional requirements as non- diazotrophs,
but possibly in different relative proportions. For example, the additional investment
6 Factors Controlling N 2 Fixation