159
9.1 Effects of Environmental Changes on Species
As noted in Chap. 6, recent studies have suggested that upper ocean warming may
benefit the growth of some marine diazotrophs such as Trichodesmium (Breitburg
et al. 2018; Jiang et al. 2018). Increased CO 2 concentrations have been shown to
stimulate N 2 fixation and CO 2 fixation in Trichodesmium and some unicellular cyanobacteria (Barcelos e Ramos et al. 2007; Hutchins et al. 2013; Levitan et al. 2007),
but other reports did not observe such stimulation for UCYN-A dominated assemblages (Böttjer et al. 2014; Gradoville et al. 2014; Law et al. 2012; Stramma et al.
2008). As the UCYN A host is a calcifying haptophytic alga (Hagino et al. 2016),
increasing ocean acidification could affect calcification by the host, although experimental results on how increasing acidity will affect calcium deposition in other
haptophytes have shown evidence of both stimulation and inhibition in different
systems (Balch 2018; Feely et al. 2004; Hutchins and Fu 2017). Increases in CO 2
may differentially affect energy generating and energy requiring pathways in
Trichodesmium (Eichner et al. 2014a, b). A recent mesocosm experiment near the
Cape Verde islands found inhibition of N 2 fixation by increased pCO 2 at the outset
of the experiment before exogenous nutrients were added (Singh et al. in review).
The effect reversed upon addition of limiting nutrients.
The flux of combined N to the ocean through riverine and diffuse runoff and the
atmosphere is also rising rapidly with increasing human populations (Gruber and
Galloway 2008; Hong et al. 2017; Hutchins and Fu 2017; Jickells et al. 2017).
Increases in concentrations of surface combined N from this input have already
been detected in coastal waters (Kim et al. 2011, 2014). Deposition of N could suppress oceanic N 2 fixation (Kim et al. 2009, 2014; Knapp 2012; Knapp et al. 2012)
which might serve as a negative feedback (Jickells et al. 2017) and a shift of N
inputs from N 2 fixation to atmospheric deposition could have direct effects on the
composition of food webs assimilating the fixed N (Berthelot et al. 2016;
Krishnamurthy et al. 2010). However, modeled projections indicate this is unlikely
in the near future (Gobler and Baumann 2016; Jickells et al. 2017).
ODZs are increasing in their extent (Breitburg et al. 2018; Jickells et al. 2005)
and are potentially ideal habits for diazotrophs with reduced O 2 and combined N
levels. Indeed, as noted in Chaps. 4, 7 and 8, several recent studies have confirmed
measurable but low levels of N 2 fixation within the ODZ of the eastern tropical
south Pacific (ETSP) (Bonnet et al. 2013; Fernandez et al. 2011; Loescher et al.
2014). It is not clear what the significance of expanding ODZs will have on global
N 2 fixation.
Simultaneous changes in multiple physical (e.g. temperature and light) and
chemical (e.g. nitrate, phosphate and Fe concentration and availability relative to
each other) factors associated with climate change may induce synergistic or antagonistic physiological responses. As noted in Chap. 6, Trichodesmium can shift from
N 2 fixation to DON use under P or Fe limitation and high CO 2 (Walworth et al.
2018), demonstrating that there can be complex interactions resulting from increased
CO 2 . Higher temperatures appear to reduce the extent of Fe limitation (Hutchins
9.1 Effects of Environmental Changes on Species
9.1 Effects of Environmental Changes on Species
As noted in Chap. 6, recent studies have suggested that upper ocean warming may
benefit the growth of some marine diazotrophs such as Trichodesmium (Breitburg
et al. 2018; Jiang et al. 2018). Increased CO 2 concentrations have been shown to
stimulate N 2 fixation and CO 2 fixation in Trichodesmium and some unicellular cyanobacteria (Barcelos e Ramos et al. 2007; Hutchins et al. 2013; Levitan et al. 2007),
but other reports did not observe such stimulation for UCYN-A dominated assemblages (Böttjer et al. 2014; Gradoville et al. 2014; Law et al. 2012; Stramma et al.
2008). As the UCYN A host is a calcifying haptophytic alga (Hagino et al. 2016),
increasing ocean acidification could affect calcification by the host, although experimental results on how increasing acidity will affect calcium deposition in other
haptophytes have shown evidence of both stimulation and inhibition in different
systems (Balch 2018; Feely et al. 2004; Hutchins and Fu 2017). Increases in CO 2
may differentially affect energy generating and energy requiring pathways in
Trichodesmium (Eichner et al. 2014a, b). A recent mesocosm experiment near the
Cape Verde islands found inhibition of N 2 fixation by increased pCO 2 at the outset
of the experiment before exogenous nutrients were added (Singh et al. in review).
The effect reversed upon addition of limiting nutrients.
The flux of combined N to the ocean through riverine and diffuse runoff and the
atmosphere is also rising rapidly with increasing human populations (Gruber and
Galloway 2008; Hong et al. 2017; Hutchins and Fu 2017; Jickells et al. 2017).
Increases in concentrations of surface combined N from this input have already
been detected in coastal waters (Kim et al. 2011, 2014). Deposition of N could suppress oceanic N 2 fixation (Kim et al. 2009, 2014; Knapp 2012; Knapp et al. 2012)
which might serve as a negative feedback (Jickells et al. 2017) and a shift of N
inputs from N 2 fixation to atmospheric deposition could have direct effects on the
composition of food webs assimilating the fixed N (Berthelot et al. 2016;
Krishnamurthy et al. 2010). However, modeled projections indicate this is unlikely
in the near future (Gobler and Baumann 2016; Jickells et al. 2017).
ODZs are increasing in their extent (Breitburg et al. 2018; Jickells et al. 2005)
and are potentially ideal habits for diazotrophs with reduced O 2 and combined N
levels. Indeed, as noted in Chaps. 4, 7 and 8, several recent studies have confirmed
measurable but low levels of N 2 fixation within the ODZ of the eastern tropical
south Pacific (ETSP) (Bonnet et al. 2013; Fernandez et al. 2011; Loescher et al.
2014). It is not clear what the significance of expanding ODZs will have on global
N 2 fixation.
Simultaneous changes in multiple physical (e.g. temperature and light) and
chemical (e.g. nitrate, phosphate and Fe concentration and availability relative to
each other) factors associated with climate change may induce synergistic or antagonistic physiological responses. As noted in Chap. 6, Trichodesmium can shift from
N 2 fixation to DON use under P or Fe limitation and high CO 2 (Walworth et al.
2018), demonstrating that there can be complex interactions resulting from increased
CO 2 . Higher temperatures appear to reduce the extent of Fe limitation (Hutchins
9.1 Effects of Environmental Changes on Species