Ammonia was necessary for the origin of life. However, NH3
would not have been a significant component of the
neutral or mildly reducing (N2, CO2, H2O)
atmosphere which characterized the Hadean earth (>3·8 Gyr ago),
especially in view of the u.v. lability of NH3, the greater
u.v. output of the young sun than pertains today, and the
absence of an atmospheric u.v. screen. Heterogeneous phase reactions,
following atmospheric chemistry, have
been proposed as a prebiotic NH3 source. Lightning, or
bolides (meteorites and comets), generated NOx.,
and Fe2+
in the sea could have reduced NO2−
resulting from the NOx. to NH4+;
this disequilibrium NH4+ level could have
supported the production of the nitrogenous organic building blocks of
life.
NOx. and NHy thus could both have
had important roles in the origin and evolution of life. Burgeoning
biota could soon have depleted abiotically
generated NH4+, and biological N2 fixation
could have evolved in its present Fe-demanding, O2-sensitive
forms
in the Archaean O2-free, Fe2+-rich environment.
Organic matter could have driven biological denitrification
reactions based on NO2− and NO3−
generated abiologically using some redox components which evolved in earlier
chemolithotrophs and photolithotrophs, regenerating atmospheric
NOx. and producing N2O. Any atmospheric
NH3 leaking from oceanic biology would have been subject to
u.v. breakdown and rain-out. Although O2-evolving
photosynthesis probably began in the Archean some 3·5 Gyr ago,
any O2 accumulation was local until 2·0 Gyr ago
(Proterozoic) due to consumption by oxidation of Fe2+
and S2−. However, localized O2 accumulation
before
2·0 Gyr ago could account for the observed early evolution of
cytochrome oxidase and the possibility of O2-consuming
chemolithotrophic and chemoorganotrophic nitrification, with further possibilities
of
NOx. production.
Oxygen accumulation globally from 2·0 Gyr onwards coincided
approximately with the evolution of eukaryotes,
which contributed phagotrophy to the reactions of the N cycle as well
as the nutrification-like aerobic production
of NO. by nitric oxide synthetase, while lightning and
bolides could now generate NO. from N2 and O2.
Evidence for terrestrial ecosystems is found from 1·0 Gyr onwards;
NOx. and NH3 generated by terrestrial
biota
stands a greater chance of escaping to the atmosphere than do these
compounds generated in the sea where
recycling within the water body is likely. As CO2 levels fell
and O2 levels rose, NH3 cycling in the photorespiratory
carbon oxidation cycle might have been evident as early as 1 Gyr ago,
although this does not seem to be a major
contributor to atmospheric NH3 today. Embryophyte evolution
on land 450 Myr ago, together with symbionts and
biophages, increased primary productivity and N cycling on land, with
greater quantitative possibilities for NOx.
and NH3 escape to the atmosphere. The evolution of lignin
(and related phenylpropanoids) at least 400 Myr ago,
with associated NH3 recycling in vascular land plant, does
not seem (on present evidence) to increase NH3 loss
to the atmosphere significantly. Biomass burning occurred at least 350
Myr
ago with lightning as the likely ignition
source; such burning yielded NO., NH3, N2O
and N2 from organic N. As in earlier times, the existence of
terrestrial embryophyte vegetation has been punctuated by major
bolide impacts, with the Cretaceous-Tertiary
boundary impact generating perhaps 104 as much NO.
as a year's thunderstorms do today, although the toxicity
of NOx.per se to vegetation might
not have been the major effect of the NOx. on
biota. Terrestrial plants suffered
fewer extinctions at this time than did many other major taxa.
Despite such very significant generators of
atmospheric combined N as major bolide impacts, the mean
levels of NHy, NOx. and N2O
(and O3) in the
atmosphere during the 450 Myr existence of embryophyte vegetation was
lower than the current, globally
averaged, anthropogenically influenced values. Current globally averaged
levels of atmospheric combined N are
thus not without precedent in the history of life, globally for
NOx., and locally for NHy, so vegetation
has had
evolutionary experience of high atmospheric combined N. However, this
should not make us complacent about
the impact of current wide-spread and continuing anthropogenic inputs
of combined N, in view of the rate and
extent of the inputs, and their combination with other aspects of
local anthropogenic influence (e.g. SO2 from
burning of high-S coal), stratospheric O3 depletion, and
global environmental change (increasing CO2, temperature and
sea-level).
