157
© 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_9
Chapter 9
Marine N 2 Fixation, Global Change
and the Future
Knowledge of N 2 fixation in the oceans has greatly expanded in recent decades.
Diazotroph communities are much more diverse than previously known, including
uncultivated symbiotic cyanobacteria and heterotrophs (Chap. 4). Although the role
of the symbiotic UCYN-A cyanobacteria is clear, it is still not known what the relative contributions of different diazotrophs are, what controls their distribution and
activities, or what is their fate in the food web or vertical flux (sinking). It is clear
that N 2 fixation occurs in habitats not previously thought to harbor N 2 fixation such
as coastal waters, cooler waters including at high latitudes, and deeper waters containing fixed N including ODZs, although the rates and significance in these habitats
remains to be determined. In contrast to prior speculation, more recent research has
found that there is likely not an imbalance between global ocean denitrification
(including anammox) and N 2 fixation and furthermore, that sites of N 2 fixation are
not closely linked to local denitrification (Bonnet et al. 2017; Marconi et al. 2017;
Moore et al. 2009).
Against this backdrop, environmental changes are challenging microbial populations of the oceans in many ways, ranging from upper ocean warming due to increasing atmospheric CO 2 concentrations and the resultant increases of dissolved CO 2 in
surface waters (Jiang et al. 2019) and subsequent acidification (Gruber 2011;
Rykaczewski and Checkley 2008; Rykaczewski and Dunne 2010)(Table 9.1).
Surface ocean warming will likely lead to stronger upper stratification in some areas
of the ocean and therefore less exchange with deep ocean nutrient pools (Yamaguchi
and Suga 2019). Other changes in characteristics of the ocean environment are
occurring, including in nutrient availability due to increasing atmospheric deposition of inorganic nutrients, nutrient runoff and loading (including organics) in
coastal waters, and expanding hypoxia and shifts in aeolian dust inputs (Bograd
et al. 2008; Gruber 2011; Hutchins and Fu 2017; Jickells et al. 2017). Another recent
area of environmental concern is the accumulating load of plastics across the size
spectrum from nanoparticles to large fragments (Rochman 2018), which affect
microorganisms and food webs and may have impacts on diazotroph species and
ecology as well.
© 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_9
Chapter 9
Marine N 2 Fixation, Global Change
and the Future
Knowledge of N 2 fixation in the oceans has greatly expanded in recent decades.
Diazotroph communities are much more diverse than previously known, including
uncultivated symbiotic cyanobacteria and heterotrophs (Chap. 4). Although the role
of the symbiotic UCYN-A cyanobacteria is clear, it is still not known what the relative contributions of different diazotrophs are, what controls their distribution and
activities, or what is their fate in the food web or vertical flux (sinking). It is clear
that N 2 fixation occurs in habitats not previously thought to harbor N 2 fixation such
as coastal waters, cooler waters including at high latitudes, and deeper waters containing fixed N including ODZs, although the rates and significance in these habitats
remains to be determined. In contrast to prior speculation, more recent research has
found that there is likely not an imbalance between global ocean denitrification
(including anammox) and N 2 fixation and furthermore, that sites of N 2 fixation are
not closely linked to local denitrification (Bonnet et al. 2017; Marconi et al. 2017;
Moore et al. 2009).
Against this backdrop, environmental changes are challenging microbial populations of the oceans in many ways, ranging from upper ocean warming due to increasing atmospheric CO 2 concentrations and the resultant increases of dissolved CO 2 in
surface waters (Jiang et al. 2019) and subsequent acidification (Gruber 2011;
Rykaczewski and Checkley 2008; Rykaczewski and Dunne 2010)(Table 9.1).
Surface ocean warming will likely lead to stronger upper stratification in some areas
of the ocean and therefore less exchange with deep ocean nutrient pools (Yamaguchi
and Suga 2019). Other changes in characteristics of the ocean environment are
occurring, including in nutrient availability due to increasing atmospheric deposition of inorganic nutrients, nutrient runoff and loading (including organics) in
coastal waters, and expanding hypoxia and shifts in aeolian dust inputs (Bograd
et al. 2008; Gruber 2011; Hutchins and Fu 2017; Jickells et al. 2017). Another recent
area of environmental concern is the accumulating load of plastics across the size
spectrum from nanoparticles to large fragments (Rochman 2018), which affect
microorganisms and food webs and may have impacts on diazotroph species and
ecology as well.