106
deeper in the photic zone, although it is not clear whether this is driven by differences in the feeding activities or the taxonomic composition of the grazer
community (Turk-Kubo et al. 2018). Dugenne et al. (2020) used imaging flow
cytometry to investigate the dynamics of Crocosphaera abundances and the abundances of protist microzooplankton grazers such as Strombidium and
Protoperidinium. More knowledge of grazing dynamics is needed for the different
marine diazotrophs, which will differ among species due to differences in size, toxicity and other factors.
In addition to grazing, there have been some reports of other losses of diazotrophs, such as viral interactions with cyanobacteria (Brown et al. 2013; Hewson
et al. 2004; Ohki 1999) or programmed cell death (Spungin et al. 2019). There is
much yet to learn about the losses and trophic transfers of diazotroph fixed N
(Bonnet et al. 2016).
6.10 Conclusions
Physical and chemical factors affect N 2 fixation in the same ways as they do microbial activities in general. O 2 plays a special role in N 2 fixation as the enzyme nitrogenase is rapidly inactivated by O 2 . Microbes have many strategies to avoid O 2 ,
including avoidance in the anaerobes. Cyanobacteria have evolved spatial and temporal strategies to avoid oxygen inactivation due to the O 2 from oxygenic photosynthesis. Inorganic nutrients, in particular N and P, are particularly important, although
the presence of fixed N can select against N 2 -fixers or inhibit N 2 fixation.
In the marine environment much is yet to be elucidated about how physical,
chemical and biological factors control when and where N 2 fixation occurs and how
different species are selected. Factors include temperature, light and turbulence, and
all of these factors interact in defining the distribution of diazotrophs and N 2 fixation
activity in the sea (see Chaps. 7 and 8). Perspectives are changing on how temperature and fixed N availability affect N 2 fixation in the oceans. There continues to be
a controversy regarding how low O 2 waters may affect the relative availabilities of
N and P that may explain regional differences in N 2 fixation (see Chap. 8). Fe availability is particularly important in the oceans as it is in generally low concentrations
due to its insolubility, but there are still uncertainties as to the chemical form and
relative importance of P and Fe at different times and different places. Learning
more about how the factors determine species composition and diversity and the
biogeography of N 2 fixation continues to be a challenge for the future. Furthermore,
most is known about how factors control N 2 fixation in the water column, much yet
needs to be learned about controlling factors of this process in the benthos and
deep sea.
6 Factors Controlling N 2 Fixation
deeper in the photic zone, although it is not clear whether this is driven by differences in the feeding activities or the taxonomic composition of the grazer
community (Turk-Kubo et al. 2018). Dugenne et al. (2020) used imaging flow
cytometry to investigate the dynamics of Crocosphaera abundances and the abundances of protist microzooplankton grazers such as Strombidium and
Protoperidinium. More knowledge of grazing dynamics is needed for the different
marine diazotrophs, which will differ among species due to differences in size, toxicity and other factors.
In addition to grazing, there have been some reports of other losses of diazotrophs, such as viral interactions with cyanobacteria (Brown et al. 2013; Hewson
et al. 2004; Ohki 1999) or programmed cell death (Spungin et al. 2019). There is
much yet to learn about the losses and trophic transfers of diazotroph fixed N
(Bonnet et al. 2016).
6.10 Conclusions
Physical and chemical factors affect N 2 fixation in the same ways as they do microbial activities in general. O 2 plays a special role in N 2 fixation as the enzyme nitrogenase is rapidly inactivated by O 2 . Microbes have many strategies to avoid O 2 ,
including avoidance in the anaerobes. Cyanobacteria have evolved spatial and temporal strategies to avoid oxygen inactivation due to the O 2 from oxygenic photosynthesis. Inorganic nutrients, in particular N and P, are particularly important, although
the presence of fixed N can select against N 2 -fixers or inhibit N 2 fixation.
In the marine environment much is yet to be elucidated about how physical,
chemical and biological factors control when and where N 2 fixation occurs and how
different species are selected. Factors include temperature, light and turbulence, and
all of these factors interact in defining the distribution of diazotrophs and N 2 fixation
activity in the sea (see Chaps. 7 and 8). Perspectives are changing on how temperature and fixed N availability affect N 2 fixation in the oceans. There continues to be
a controversy regarding how low O 2 waters may affect the relative availabilities of
N and P that may explain regional differences in N 2 fixation (see Chap. 8). Fe availability is particularly important in the oceans as it is in generally low concentrations
due to its insolubility, but there are still uncertainties as to the chemical form and
relative importance of P and Fe at different times and different places. Learning
more about how the factors determine species composition and diversity and the
biogeography of N 2 fixation continues to be a challenge for the future. Furthermore,
most is known about how factors control N 2 fixation in the water column, much yet
needs to be learned about controlling factors of this process in the benthos and
deep sea.
6 Factors Controlling N 2 Fixation