105
organisms, including diazotrophic cyanobacterial taxa (Doney 2009) (see Chap. 9).
Under laboratory conditions, certain isolates of Trichodesmium and Crocosphaera
show increased rates of growth, CO 2 fixation, and N 2 fixation in response to elevated
CO 2 conditions (Fu et al. 2008; Hutchins et al. 2007). However, field-based experiments have not demonstrated CO 2 stimulation of N 2 fixation in natural diazotrophic
assemblages (Böttjer et al. 2014; Eichner et al. 2014; Law et al. 2012), likely due to
the variable species-specific responses to elevated CO 2 (Hutchins et al. 2013)(see
Chap. 9 for further consideration of pH and CO 2 ).
6.8 Turbulence
The fact that heterocyst-forming cyanobacteria are less common in estuaries and
coastal seas than inland water bodies, was the focus of much speculation. One set of
hypotheses was that turbulence of estuaries had various effects, including preventing the formation of anoxic microzones in aggregates or cyanobacteria cell surfaces
(Paerl 1985), but experimental results in mesocosms suggested this was not the case
(Howarth et al. 1993; Marino and Howarth 2016). The effects of turbulence are
more important at small scales and affect the diffusion of O 2 in particles and aggregates (Moisander et al. 2002; Paerl et al. 1995).
For Trichodesmium, quiescent physical conditions can lead to surface accumulations (Carpenter and Capone 1992). In contrast, strong turbulence can result in inhibition of N 2 fixation (Carpenter and Price 1976).
6.9 Fate of Diazotrophs
The fate of diazotrophs, through death, viral, lysis, sinking or grazing determines
the roles of diazotrophs in ecosystem function and recent modeling studies suggest
grazing may play an important role in controlling N 2 fixation in the upper ocean
(Wang et al. 2019). Very little is known about the fates of individual species or
groups, or the dynamics of loss terms specifically for diazotrophs (Dugenne et al.
2020). The diatom-cyanobacteria symbiosis is known to sink, resulting in rapid
export of fixed N and C to the mesopelagic (Anderson et al. 2018; Böttjer et al.
2017; Karl et al. 2012; Subramaniam et al. 2008). However, even smaller, freeliving species such as Crocosphaera have been suggested to be exported (Wilson
et al. 2017). Early studies suggested that harpacticoid zooplankton consumed the
diazotroph Trichodesmium (O’Neil 1998) and some more recent observations have
suggested that the smaller species are grazed, based on microscopic observation or
nif gene sequences associated with zooplankton (Conroy et al. 2017; Hunt et al.
2016; Scavotto et al. 2015). Grazing dilution experiments suggest that microzooplankton grazing rates on UCYN-A and UCYN-C are taxa specific and are higher
6.9 Fate of Diazotrophs
organisms, including diazotrophic cyanobacterial taxa (Doney 2009) (see Chap. 9).
Under laboratory conditions, certain isolates of Trichodesmium and Crocosphaera
show increased rates of growth, CO 2 fixation, and N 2 fixation in response to elevated
CO 2 conditions (Fu et al. 2008; Hutchins et al. 2007). However, field-based experiments have not demonstrated CO 2 stimulation of N 2 fixation in natural diazotrophic
assemblages (Böttjer et al. 2014; Eichner et al. 2014; Law et al. 2012), likely due to
the variable species-specific responses to elevated CO 2 (Hutchins et al. 2013)(see
Chap. 9 for further consideration of pH and CO 2 ).
6.8 Turbulence
The fact that heterocyst-forming cyanobacteria are less common in estuaries and
coastal seas than inland water bodies, was the focus of much speculation. One set of
hypotheses was that turbulence of estuaries had various effects, including preventing the formation of anoxic microzones in aggregates or cyanobacteria cell surfaces
(Paerl 1985), but experimental results in mesocosms suggested this was not the case
(Howarth et al. 1993; Marino and Howarth 2016). The effects of turbulence are
more important at small scales and affect the diffusion of O 2 in particles and aggregates (Moisander et al. 2002; Paerl et al. 1995).
For Trichodesmium, quiescent physical conditions can lead to surface accumulations (Carpenter and Capone 1992). In contrast, strong turbulence can result in inhibition of N 2 fixation (Carpenter and Price 1976).
6.9 Fate of Diazotrophs
The fate of diazotrophs, through death, viral, lysis, sinking or grazing determines
the roles of diazotrophs in ecosystem function and recent modeling studies suggest
grazing may play an important role in controlling N 2 fixation in the upper ocean
(Wang et al. 2019). Very little is known about the fates of individual species or
groups, or the dynamics of loss terms specifically for diazotrophs (Dugenne et al.
2020). The diatom-cyanobacteria symbiosis is known to sink, resulting in rapid
export of fixed N and C to the mesopelagic (Anderson et al. 2018; Böttjer et al.
2017; Karl et al. 2012; Subramaniam et al. 2008). However, even smaller, freeliving species such as Crocosphaera have been suggested to be exported (Wilson
et al. 2017). Early studies suggested that harpacticoid zooplankton consumed the
diazotroph Trichodesmium (O’Neil 1998) and some more recent observations have
suggested that the smaller species are grazed, based on microscopic observation or
nif gene sequences associated with zooplankton (Conroy et al. 2017; Hunt et al.
2016; Scavotto et al. 2015). Grazing dilution experiments suggest that microzooplankton grazing rates on UCYN-A and UCYN-C are taxa specific and are higher
6.9 Fate of Diazotrophs