102
Hutchins 1995). It may be even more important in limiting diazotrophs (Rueter
et al. 1992), based on the requirements for multiple Fe atoms in FeS centers in both
nitrogenase structural proteins (Berman-Frank et al. 2001a; Kustka et al. 2003). Fe
availability in the oceans is affected by chemistry (pH) and redox state. As a result,
free dissolved Fe concentrations can be relatively high in coastal regions and sediments, but can be extremely low in large regions of the open ocean where its availability limits N 2 fixation (Wu et al. 2001, 2003).
Sources of Fe are mixing from deep water, release of reduced (Fe
2+
) from sediments, and atmospheric deposition (Mahowald et al. 2009; Wu et al. 2001). Fe in the
oxic ocean exists primarily as the ferric (Fe
3+
) form, which can rapidly form surfaceactive oxy-hydroxide complexes and be lost as particles from the surface ocean.
However, Fe can also be complexed with organic ligands. Some microorganisms
secrete specific Fe complexing siderophores which keep Fe dissolved in surface
waters (Rivers 2009). The competitive abilities of N 2 -fixing microorganisms may
depend on their ability to compete for these forms (Garcia et al. 2015).
Iron is important for growth and N 2 fixation in Trichodesmium (Berman-Frank
et al. 2001a; Kustka et al. 2003) and Crocosphaera (Garcia et al. 2015; Jacq et al.
2014). Switching Fe from one protein to another (PSI to nitrogenase) may help to
decrease Fe requirements in Crocosphaera which needs Fe for its photosynthetic
reaction centers during the day, but may reuse the Fe in nitrogenase at night (Saito
et al. 2011).
Nutrient availabilities and metabolism can have interactive effects. There is an
interaction between P and Fe availability and species responses, where low Fe limitation can actually increase relative growth rates of some cyanobacteria under P
stress (Garcia et al. 2015). Fe and P stress can result in aggregation in the colony
forming Trichodesmium (Tzubari et al. 2018). The H 2 evolved by nitrogenase may
also provide an electron source for reducing and obtaining Fe (Eichner et al.
2019a, b).
There are now species-specific molecular assays for gene expression and proteins that can be used to determine the Fe or P limitation of Trichodesmium and
Crocosphaera in the field (Rouco et al. 2016; Webb et al. 2001, 2007) which are
useful for validating biogeographic predictions. Key genes involved in Fe metabolism are involved in transport, such as FutA/idiA, which are more highly expressed
under Fe limitation (Webb et al. 2001). Phosphorus stress is indicated by expression
of high affinity transporters (pstS) (Orchard et al. 2003, 2009; Pereira et al. 2019),
and when there is an abundance of extracellular P, P can be stored as poly-P (Orchard
et al. 2010). Organic P compounds are comprised of multiple classes and assumed
to be a relatively labile source for microorganisms (Karl 2014) including diazotrophs. One class of organic P compounds are phosphonates, which have a very
strong C-P bond and have been shown to be abundant in Trichodesmium (Dyhrman
et al. 2009), but that reliance on methylphosphonates results in lower N 2 fixation
rates (White et al. 2010). Intriguingly, it was discovered that the degradation of
methylphosphonates results in methane production, which may at least partially
explain the presence of methane in oxic surface waters (Karl et al. 2008).
6 Factors Controlling N 2 Fixation
Hutchins 1995). It may be even more important in limiting diazotrophs (Rueter
et al. 1992), based on the requirements for multiple Fe atoms in FeS centers in both
nitrogenase structural proteins (Berman-Frank et al. 2001a; Kustka et al. 2003). Fe
availability in the oceans is affected by chemistry (pH) and redox state. As a result,
free dissolved Fe concentrations can be relatively high in coastal regions and sediments, but can be extremely low in large regions of the open ocean where its availability limits N 2 fixation (Wu et al. 2001, 2003).
Sources of Fe are mixing from deep water, release of reduced (Fe
2+
) from sediments, and atmospheric deposition (Mahowald et al. 2009; Wu et al. 2001). Fe in the
oxic ocean exists primarily as the ferric (Fe
3+
) form, which can rapidly form surfaceactive oxy-hydroxide complexes and be lost as particles from the surface ocean.
However, Fe can also be complexed with organic ligands. Some microorganisms
secrete specific Fe complexing siderophores which keep Fe dissolved in surface
waters (Rivers 2009). The competitive abilities of N 2 -fixing microorganisms may
depend on their ability to compete for these forms (Garcia et al. 2015).
Iron is important for growth and N 2 fixation in Trichodesmium (Berman-Frank
et al. 2001a; Kustka et al. 2003) and Crocosphaera (Garcia et al. 2015; Jacq et al.
2014). Switching Fe from one protein to another (PSI to nitrogenase) may help to
decrease Fe requirements in Crocosphaera which needs Fe for its photosynthetic
reaction centers during the day, but may reuse the Fe in nitrogenase at night (Saito
et al. 2011).
Nutrient availabilities and metabolism can have interactive effects. There is an
interaction between P and Fe availability and species responses, where low Fe limitation can actually increase relative growth rates of some cyanobacteria under P
stress (Garcia et al. 2015). Fe and P stress can result in aggregation in the colony
forming Trichodesmium (Tzubari et al. 2018). The H 2 evolved by nitrogenase may
also provide an electron source for reducing and obtaining Fe (Eichner et al.
2019a, b).
There are now species-specific molecular assays for gene expression and proteins that can be used to determine the Fe or P limitation of Trichodesmium and
Crocosphaera in the field (Rouco et al. 2016; Webb et al. 2001, 2007) which are
useful for validating biogeographic predictions. Key genes involved in Fe metabolism are involved in transport, such as FutA/idiA, which are more highly expressed
under Fe limitation (Webb et al. 2001). Phosphorus stress is indicated by expression
of high affinity transporters (pstS) (Orchard et al. 2003, 2009; Pereira et al. 2019),
and when there is an abundance of extracellular P, P can be stored as poly-P (Orchard
et al. 2010). Organic P compounds are comprised of multiple classes and assumed
to be a relatively labile source for microorganisms (Karl 2014) including diazotrophs. One class of organic P compounds are phosphonates, which have a very
strong C-P bond and have been shown to be abundant in Trichodesmium (Dyhrman
et al. 2009), but that reliance on methylphosphonates results in lower N 2 fixation
rates (White et al. 2010). Intriguingly, it was discovered that the degradation of
methylphosphonates results in methane production, which may at least partially
explain the presence of methane in oxic surface waters (Karl et al. 2008).
6 Factors Controlling N 2 Fixation