Science - USA (2022-01-07)

INSIGHTS | PERSPECTIVES

28 7 JANUARY 2022 • VOL 375 ISSUE 6576

GRAPHIC: N. CARY/

SCIENCE

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derived fluxes of the potent greenhouse gas
N 2 O, which is a side product of their metabo-
lism ( 11 , 12 ). However, their presence in an-
oxic environments has remained perplexing.
Despite the global biogeochemical impor-
tance of AOA, their physiology and metabo-
lism are poorly understood. This is partly
because of their limited homology to geneti-
cally tractable model microorganisms and
because of the practical challenges in culti-
vation and biomass production for studies.
Kraft et al. add a dimension to the meta-
bolic repertoire and adaptation of marine
AOA, invoking a pathway for O 2 and N 2
production. By using sensors that can de-
tect O 2 at concentrations as low as 1 nano-
mole per liter, and^15 N stable isotope label-

ing, the authors show that N. maritimus
continues to oxidize ammonia to nitrite
under anoxic conditions while simultane-
ously reducing nitrite to N 2 O and N 2. After
external O 2 was exhausted, these microbes
would begin producing O 2 and bring the O 2
concentration in the surrounding media
back up to 50 to 200 nanomoles per liter,
indicating the concurrent production and
consumption of O 2. The authors performed
an extensive set of control experiments to
allow for correction of interference from
nitric oxide (NO) and exclude interferences
from other conceivable reactive nitrogen
and oxygen species. Although the ammonia
oxidation activity under anoxic conditions
was low relative to activity with external
O 2 and relied on an external supply of ni-

trite, a comparison with AOA cell numbers

and rate measurements shows that AOA in

oxygen minimum zones could contribute

to N 2 production in a similar range as the

previously known N 2 -producing processes,

denitrification and anaerobic ammonia

oxidation (see the figure).

Current biogeochemical models assume

that canonical nitrification cannot proceed

in anoxic environments. The results of Kraft

et al. suggest that the nitrogen cycle in ma-

rine oxygen minimum zones could be more

complex than previously thought. Notably,

a mechanism of oxygen production by ni-

trite disproportionation was recently sug-

gested to account for NOB activity in these

anoxic marine zones ( 1 ). However, these

theoretical mechanisms remain to be ex-

perimentally verified. Detection of free O 2

in the N. maritimus culture raises the ques-

tion of whether AOA-produced O 2 could

serve nitrite oxidation in oxygen minimum

zones. Further disentangling of the complex

microbial nitrogen cycling network in low-

oxygen marine environments is needed for

a more robust understanding of major ni-

trogen loss processes in the global oceans.

N. maritimus appears to produce O 2 and

N 2 using a biochemical process yet to be

understood. Known biological pathways for

O 2 production include the oxidation of H 2 O

in the photosystem II protein complexes

found in plants, algae, and cyanobacteria;

the detoxification of reactive oxygen species

(ROS) by catalase or superoxide dismutase;

the dismutation of chlorite to O 2 and chlo-

ride ions during microbial (per)chlorate res-

piration; and the dismutation of NO to O 2

and N 2 during nitrite-dependent anaerobic

methane oxidation ( 13 ). However, besides

two putative nitrite reductase paralogs, the

N. maritimus genome encodes no enzyme

implicating any of these processes, and ROS

detoxification under anoxic conditions was

ruled out experimentally.

Lacking biochemical precedent, Kraft et

al. could only speculate on the potential

mechanism. Based on the isotopic evidence,

they postulate the most parsimonious, ther-

modynamically favorable pathway involving

nitrite reduction and dismutation of NO to

O 2 and N 2 O, followed by the reduction of

N 2 O to N 2. Although this model provides a

canonical function for the putative nitrite

reductase found in all mesophilic AOA, it

adds questions to the unresolved ammonia

oxidation pathway by invoking further un-

precedented biochemistry. Future studies

should reveal whether this metabolism is

confined to anoxic conditions or if it is in-

tegral to ammonia oxidation in general, and

whether this metabolism can be found in

other AOA, such as those that live in soils,

sediments, or marine and terrestrial subsur-

face ecosystems. The detection of internal

oxygen-producing pathways in distantly re-

lated nitrite-dependent methane-oxidizing

bacteria and ammonia-oxidizing archaea

suggests that internal oxygen production

might be more widespread. Internal oxygen

production could have had a key role in en-

abling nitrification and accelerating the evo-

lution of the modern nitrogen cycle before

the rise of molecular O 2 after the great oxi-

dation event 2.4 billion years ago ( 14 , 15 ). j

REFERENCES AND NOTES

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ACKNOWLEDGMENTS

The authors thank B. Ward for reviewing an earlier version of

this manuscript.

10.1126/science.abn0373

Marine oxygen

minimum zones

200

Depth (m)^1000

Oxygen

Nitrate

Nitrite

NO 2

_

SO 4 2–

NH 4 +

H 2 O

H 2 S

CO 2

NO 2

_

+ NO 2

_

NO 3

_

NO 3 N 2

_

NH 4 + NH 4 + + H 2 O

NO

N 2

N 2 O

NO 2

_

NO

N 2

N 2 O

O 2 H 2 O

AOA

Denitrifying

microbes NOB

Anammox (anaerobic

ammonium oxidation)

bacteria

Organic

carbon

O 2

Nitrogen cycle in the oxygen

minimum zones
Conceptual diagram of the role of ammonia-oxidizing
archaea (AOA) and nitrite-oxidizing bacteria (NOB)
in the marine nitrogen cycle inside oxygen minimum
zones. Nitrification occurs below the photic zone
in the oceanic water column. But thus far, it was
believed not to be possible in anoxic zones. The paper
by Kraft et al. changes that possibility.