『Abstract
This article suggests a theme for future nitrogen studies, involving
the functioning and evolution of the biosphere, together with
certain aspects of human biology. It is hoped that this theme
could be developed into a multi-authored book at some point in
the future, although numerous new case studies may be required.
The biosphere can be considered to be a single interactive system,
comprised of numerous component parts, including the atmosphere,
rain, rivers, lakes, oceans, continental ecosystems, marine ecosystems,
soils and sediments. In order to study the internal complexity
of each of the components, and the relationships that exist between
them, one can choose a common theme. The author believes that
nitrogen is a good prospect because of its ubiquitous nature and
complex chemistry.
The burial of sedimentary volatiles in the continental crust
has contributed to the long-term chemical evolution of the exterior
environments of the Earth. The most marked effects, notably the
accumulation of atmospheric oxygen, have been due to the burial
of reduced carbon. An understanding of the passage of sedimentary
volatiles through the crust will provide important constraints
on the evolution of the biosphere, and may lead to a more meaningful
comparative planetology. Carbon is difficult to trace through
crustal processes because its high temperature form (graphite)
is incompatible with the crystal structures of silicate minerals.
However, the high temperature form of sedimentary nitrogen (ammonium)
readily enters into potassic minerals, and it can be easily traced
through crustal processes.
Herein, the fate of sedimentary nitrogen is traced from wet sediments,
through metamorphic rocks, into granites and other crustal melts.
Without the trapping of nitrogen in the crust, and the liberation
of oxygen that is a consequence of biological nitrogen fixation,
the N2/O2 of the atmosphere
may have been about 82.9％ to 16.3％, and the total atmospheric
pressure about 1.2 atm. Most of the changes in the oxygen content
is due to dilution by nitrogen, with only about 5％ of the present
atmosheric oxygen being a consequence of nitrogen fixation.
The ancient continental surfaces would have been volcanic deserts
containing little (＜1 ppm) or no nitrogen. Today, nitrogen-rich
soils support continental ecosystems. One can use nitrogen as
a ‘window’ to speculate on the colonisation of the continents
by land plants. Herein, nitrogen fixation by legumes is considered
in detail, and by drawing analogies with the present day, the
author speculates on the colonisation of the continents by land
plants.
Having speculated on the co-evolution of the atmosphere, soils
and continental ecosystems, one can place a human being in the
centre of a tropical forest, and begin to examine how they relate
to the modern biosphere. The chosen example is the metabolism
of plant protein, and the roles of glutamate dehydrogenase (GDH),
vitamin B2, and flavin mononucleotide (FMN).
The realisation of this theme would imply numerous new case studies,
notably those concerning the continental crust, where nitrogen
studies were linked to mineralogy, petrology and other geochemical
tracers, including Sr, O, Al, and K, and where the short-range
order of δ15N were taken into consideration.
Keywords: Biosphere; Biogeochemistry; Continental crust; Nitrogen
cycle; Nitrogen fixation; Land plants; Human metabolism』
1. Introduction
1.1. General
1.2. Introduction to the framework
1.2.1. The functioning of the present day biosphere
1.2.2. Nitrogen and biospheric evolution
1.2.3. Nitrogen and the origin of modern ecosystems
1.2.4. Nitrogen, the biosphere and human biology
1.3. Outline
2. Relationships between the biosphere and the continental crust
2.1. A modern shallow shelf sea
2.1.1. Inputs to the marine basin
2.1.1.1. Rain/direct deposition
2.1.1.2. Biological nitrogen fixation
2.1.1.3. Rivers/run-off
2.1.1.4. Continental weathering
2.1.2. Inputs to marine basins in the geological past
2.1.2.1. Continental ecosystems in the geological past
2.1.2.2. Anoxia in the Archean and Hadean
2.1.3. Losses of nitrogen from the marine basin
2.1.3.1. Degradation of organic matter and denitrification
2.1.3.2. Sedimentation
2.2. Degradation of organic nitrogen during diagenesis
2.2.1. Low temperature
2.2.1.1. Oxygen-rich
2.2.1.2. Oxygen-poor
2.2.2. High temperature
2.2.3. Net loss of ammonium from the biosphere
2.3. The behaviour of nitrogen during metamorphism
2.3.1. Detection of ammonium in potassic minerals
2.3.2. Distribution of ammonium between co-existing minerals
2.3.3.Partitioning of δ15N between co-existing minerals
2.3.4. Ammonium content, mineralogy and petrology
2.3.5. Nitrogen loss and isotope fractionation during metamorphism
2.3.6. Returns of nitrogen during metamorphism
2.4. Granites
2.4.1. Anatexis
2.4.2. Assimilation
2.4.3. Hydrothermal alteration
2.4.4. Isotope studies of granites and related rocks
2.5. A simple model for the cycling of nitrogen in the Phanerozoic
2.6. Atmosphere / biosphere / mantle relations
2.7. The long-term chemical evolution of the atmosphere
2.8. The need for a data base
2.9. Towards a more meaningful comparative planetology
3. Evolution of continental ecosystems
3.1. Nitrogen fixation in legumes
3.1.1. Basic metabolism
3.1.2. Symbiosis within the root nodules of legumes
3.1.2.1. General
3.1.2.2. Biochemical aspects of nitrogen fixation in legumes
3.1.2.3. The basic reaction of nitrogen fixation
3.1.2.4. The liberation of oxygen during nitrogen fixation
3.2. The colonisation of the continents by plants
3.2.1. Present day organisms and the colonisation of new ground
3.2.1.1. Colonisers
3.2.1.2. Nitrogen-fixing higher plants
3.2.1.3. Non-fixing higher plants
3.2.1.4. The effect of fixed NH4+
3.2.1.5. The effect of ancient crustal nitrogen on modern ecosystems
3.2.2. Mid-Ordovician / mid-Silurian
4. Aspects of human biology
4.1. General
4.2. Human metabolism of plant protein
4.2.1. Nitrogen removal
4.2.2. Disposal of nitrogen and oxidation of the carbon skeletons
5. Conclusions
Acknowledgements
References