97
stored during the day, which fuels N 2 fixation by respiration at night (Schneegurt
et al. 1994). A circadian rhythm is involved, which at least partially controls the
timing of synthesis of proteins. Interestingly, when experimentally grown under
constant illumination Crocosphaera continues a pattern of nifH expression that suggests involvement of a circadian rhythm (Pennebaker et al. 2010). How O 2 is avoided
under these conditions is not really known, but may involve cell to cell variability in
timing of photosynthesis and N 2 fixation, at least in aggregate forms of Crocosphaera
(Inomura et al. 2019).
Although unicellular cyanobacteria species generally fix N 2 at night, thus avoiding the O 2 evolved during the day (Berman-Frank et al. 2003; Fay 1992), the recently
discovered unicellular UCYN-A appears to fix N 2 only during the day (Church et al.
2005a). UCYN-A does not directly evolve O 2 itself, but is believed to be an endosymbiont of a photosynthetic unicellular planktonic haptophyte (Prymnesiophyte)
alga that produces O 2 (Thompson et al. 2012). How UCYN-A protects itself from
the algal photosynthetic O 2 is not known, but it may involve hopanoids in its cell
membrane (Cornejo-Castillo and Zehr 2019) which are important for N 2 fixation in
the cell walls of terrestrial bacteria such as Frankia (Berry et al. 1993).
Table 6.1 (continued)
Type
Source or control
Factors &
variability
Fluctuations
& gradients
Effects
pH
H
+ from
bicarbonate
equilibrium with
atmosphere CO 2
Oxidation
reduction states
and microbial
activities,
reduced oxygen
zones
ODZs, oxic
vs anoxic
sediments,
particles
Calcification,
respiration
Inorganic
nutrients
(NO3, PO4,
NH4, SiO4)
Decomposition,
recycling, runoff
from land,
atmospheric
deposition
Winds, terrestrial
sources,
upwelling and
mixing
Depth,
coastal-open
ocean,
latitude
Growth rate,
nutrient stress
Required
metals (Fe,
Ni, Mo)
Dust,
hydrothermal
vents, mixing
Winds, terrestrial
sources,
currents,
upwelling and
mixing
Depth,
coastal-open
ocean,
latitude
Nutrient
limitation,
nitrogenase
Toxic metals
(e.g. Cu, Hg,
As)
Runoff,
depositon
Toxicity
Organics
Photosynthesis,
heterotrophic
metabolism,
decomposition in
sediments, runoff
from land,
natural
petroleum seeps
Runoff,
photoxidation,
heterotrophic
metabolism,
recalcitrance
Depth,
coastal,-open
ocean
Heterotrophic
metabolism,
chemical
complexation
and reactions
6.1 Oxygen
stored during the day, which fuels N 2 fixation by respiration at night (Schneegurt
et al. 1994). A circadian rhythm is involved, which at least partially controls the
timing of synthesis of proteins. Interestingly, when experimentally grown under
constant illumination Crocosphaera continues a pattern of nifH expression that suggests involvement of a circadian rhythm (Pennebaker et al. 2010). How O 2 is avoided
under these conditions is not really known, but may involve cell to cell variability in
timing of photosynthesis and N 2 fixation, at least in aggregate forms of Crocosphaera
(Inomura et al. 2019).
Although unicellular cyanobacteria species generally fix N 2 at night, thus avoiding the O 2 evolved during the day (Berman-Frank et al. 2003; Fay 1992), the recently
discovered unicellular UCYN-A appears to fix N 2 only during the day (Church et al.
2005a). UCYN-A does not directly evolve O 2 itself, but is believed to be an endosymbiont of a photosynthetic unicellular planktonic haptophyte (Prymnesiophyte)
alga that produces O 2 (Thompson et al. 2012). How UCYN-A protects itself from
the algal photosynthetic O 2 is not known, but it may involve hopanoids in its cell
membrane (Cornejo-Castillo and Zehr 2019) which are important for N 2 fixation in
the cell walls of terrestrial bacteria such as Frankia (Berry et al. 1993).
Table 6.1 (continued)
Type
Source or control
Factors &
variability
Fluctuations
& gradients
Effects
pH
H
+ from
bicarbonate
equilibrium with
atmosphere CO 2
Oxidation
reduction states
and microbial
activities,
reduced oxygen
zones
ODZs, oxic
vs anoxic
sediments,
particles
Calcification,
respiration
Inorganic
nutrients
(NO3, PO4,
NH4, SiO4)
Decomposition,
recycling, runoff
from land,
atmospheric
deposition
Winds, terrestrial
sources,
upwelling and
mixing
Depth,
coastal-open
ocean,
latitude
Growth rate,
nutrient stress
Required
metals (Fe,
Ni, Mo)
Dust,
hydrothermal
vents, mixing
Winds, terrestrial
sources,
currents,
upwelling and
mixing
Depth,
coastal-open
ocean,
latitude
Nutrient
limitation,
nitrogenase
Toxic metals
(e.g. Cu, Hg,
As)
Runoff,
depositon
Toxicity
Organics
Photosynthesis,
heterotrophic
metabolism,
decomposition in
sediments, runoff
from land,
natural
petroleum seeps
Runoff,
photoxidation,
heterotrophic
metabolism,
recalcitrance
Depth,
coastal,-open
ocean
Heterotrophic
metabolism,
chemical
complexation
and reactions
6.1 Oxygen