Microbial Energy Conversion

The Proceedings of a Seminar Sponsored by the UN Institute for Training and Research (UNITAR) and the Ministry for Research and Technology of the Federal Republic of Germany Held in Göttingen, October 1976

H. G. Schlegel, J. Barnea

• Publisher: Pergamon
• Year: 2013
• Print ISBN: 9780080217918
• eBook ISBN: 9781483139128
• Topic: Technology & Engineering
• Subtopic: Anthropology
• Index: Technology & Engineering

Microbial Energy Conversion documents the proceedings of a seminar in Gottingen in October 1976. This book discusses the potential of microorganisms to use solar energy or convert biomass produced by solar energy in such a way that new microbial energy sources can supplement or partially replace conventional sources. This compilation reviews biomass production and elaborates on in detail the microbial processes that are involved in the conversion of the primary biomass—either freshly harvested or disposed of as waste—into energy sources that are similar to hydrogen, methane, propane, gasoline, Diesel oil, methanol, ethanol, or electricity. The microbial processes that contribute to the development of known energy resources, such as mining of low grade ores of copper, zinc, and uranium; reclamation of oil from oil shale; and recovery of conventional and heavy oil and gas, are also deliberated. This text likewise elaborates on the study of photosynthetic enzyme systems, hydrogenase, immobilization of enzymes and pigments on membranes, and construction of artificial photosynthetic units. This book is beneficial to students and researchers conducting work on microbial energy conversion.

Methane formation and cellulose digestion

ACETATE, A KEY INTERMEDIATE IN METHANOGENESIS

R.A. Mah, Center for the Health Sciences, University of California, Los Angeles, California 90024, USA

R.E. Hungate and Kyoko Ohwaki, Department of Bacteriology, University of California, Davis, California 95616, USA

Abstract

Acetate is the chief precursor of methane in anaerobic fermentations. Methane can be formed from H₂ and CO₂ by most methanogenic bacteria, but these substrates can also be converted to acetate by Clostridium aceticum, isolated from sludge. Its activity may increase the fraction of methane arising from acetate. Anaerobic 2% Ca acetate mineral enrichments forming methane were transferred weekly with a one-sixth volume inoculum. The rate of methane production was 9 μmoles ml⁻¹ day⁻¹. Methanosarcina barkeri was the chief methanogen in the enrichment but numerous non-methanogens were present, three of which were isolated. In pure cultures of M. barkeri in medium supplemented with trypticase and yeast extract the rate of methanogenesis was 20 ymoles ml⁻¹ day⁻¹, and similar supplementation of the enrichment culture increased the rate from 9 to 23 μmoles ml⁻¹ day⁻¹. The rate of methane production was affected not only by M. barkeri but also by the non-methanogens in the enrichment. Microbiological analyses of methanogenic ecosystems is recommended as a means to provide clues of value in practical applications.

It is a pleasure to participate in a symposium exploring possibilities to augment world energy sources by exploiting microbial activities. We are greatly indebted to Dr. Schlegel and the Institut für Mikrobiologie for organizing this meeting and appreciate the financial support of the United Nations Institute for Training and Research and the Bundesministerium für Forschung und Technologie.

The preceding papers have reviewed the photosynthetic storage of renewable light energy in chemical form by various ecosystems. An inventory of energy resources shows (table 1) that sunlight constitutes more than 99.94% of the total energy (excluding nuclear) available annually on Earth.

Table 1

Potential Non-nuclear World Power

Kind	Amount in Kcal year^−1
Geothermal	0.008
Tidal	0.09
Water power	20
Coal, oil and gas	80
Sunlight	170000

Although only a small fraction, 0.12%, of the light is used in photosynthesis the energy stored in chemical form (table 2) is about twice the annual fuel consumption.

Table 2

Maximum World Agricultural Production

Kind	Amount in kcal year^−1
Arable lands	120
Grazing lands	25
Forests	23
Seas	32
Total	200

This session on methanogenesis will explore potentials and problems in microbial generation of methane from available agricultural products. The processes to be considered are anaerobic. Aerobic microbial metabolism of organic matter converts energy chiefly into the form of microbial cell bodies whereas anaerobically cells are minor and the reduced carbon in the waste fermentation products contains most of the substrate energy, available through oxidation. In a complete anaerobic breakdown of organic matter carbon atoms are converted to their most reduced or most oxidized form, i.e., to the gases, CH₄ or CO₂.

For anaerobic reactions to occur the substrate carbon atoms must be in an intermediate oxidation state, and energywise the most favorable condition for rapid and complete conversion of the organic matter to CO₂ and CH₄ is provided when the C, H, and O atoms are in the ratio of CH₂O, i.e. carbohydrate. The conversion of carbohydrate to CH₄ and CO₂ is accompanied by a ten percent decrease in the potential energy, but ninety percent of the energy available through oxidation of the initial carbohydrate can be derived through oxidation of the reduced fermentation product, methane.

1

2

Proteins, fats and many other compounds can also be converted to CO₂ and CH₄ but they are not as abundant as carbohydrates, and the proteins are too valuable to be used as fuel. Lignins, comprising up to 30% of woody plant material, are a handicap to complete anaerobic conversion of plant materials. Lignins are relatively indigestible anaerobically, and their intimate association with carbohydrates reduces the digestibility of the carbohydrate components in the lignified plants. Thus carbohydrates, and possibly fats, are the chief renewable substrates suitable for anaerobic conversion to CO₂ and CH₄.

A carbohydrate such as hexose can be converted to methane in the overall process summarized in equation 1, but this includes two separate pathways by which methane originates. The two triose molecules derived from the hexose are fermentated in a fashion such that methane arises either by reduction of CO₂ (Barker, 1936a)or by the splitting of acetate (Buswell and Sollo, 1948). Equations 3 to 5 illustrate how the hydrogen to reduce CO₂ is obtained during breakdown of the two triose molecules derived from hexose.

3

4

The hydrogen made available in these reactions is used to reduce CO₂ to CH₄.

5

In summary,

6

using one-third of the available hydrogen to produce one methane molecule. The acetate is then split to produce the remaining two-thirds of the methane.

7

Many fermentations give rise to products such as ethanol, or propionic and butyric and higher acids, converted in sludge to methane, in part by reduction of CO₂ with the hdyrogen obtained by dehydrogenation of the substrate to acetate, in part by splitting of the acetate into methane and CO₂. With these fermentations also, acetate constitutes two-thirds of the carbohydrate initially attacked, and is the source of two-thirds of the methane. If a fat such as stearate is the initial substrate it can be converted by β-dehydrogenation to acetate, with the hydrogen used to reduce exogenous CO₂ to methane. In this process the fraction of methane arising from acetate is slightly less than 69% of the total.

8

In the sludge fermentation of municipal and domestic wastes, tracer studies indicate that the fraction of methane arising from acetate is about 73% (Jeris and McCarty, 1965; Smith and Mah, 1966), 4 and 6% higher than the theoretical values for fats and carbohydrates, respectively. A possible explanation for this higher value is the occurrence in sludge of the acetigenic reaction discovered by Wieringa (1938) in which H₂ and CO₂ are converted into acetic acid by a spore-forming bacterium, Clostridium aceticum, isolated from Dutch canal mud.

9

One of us (KO) has isolated presumably similar bacteria from sewage sludge (Ohwaki, 1975) at Davis. Mylroie (1953) earlier showed at Pullman that H₂ and CO₂ added to sewage sludge were actively converted to acetate with little formation of methane. The numbers of acetigenic bacteria at Pullman were not large, ranging from 10⁶ to 10⁸ ml⁻¹. In pure culture this species grows rapidly, a ml of culture absorbing as much as 130 μmoles of H₂ day⁻¹. Yeast extract is required for growth, and vitamin B₁₂, folate, Se, Fe and Mo are stimulatory. With this supplemented medium growth on 80% H₂-20% CO₂ is two to three times as great as on 80% N₂-20% CO₂.

Growth on the yeast extract is very rapid, and i...

Table of contents

• Cover image
• Title page
• Table of Contents
• Other Pergamon Titles of Interest
• Copyright
• PREFACE
• INTRODUCTION TO RECOMMENDATIONS
• WORKING GROUP I: Biomass
• WORKING GROUP II: Recycling of Wastes
• WORKING GROUP III: Methane Production
• WORKING GROUP IV: Photoproduction of Hydrogen/Purple Membrane
• WORKING GROUP V: Microbial Recovery of Hydrocarbons
• WORKING GROUP VI: Prices of important substrates and economics of chemical interconversions
• Primary Production of Biomass
• Methane formation and cellulose digestion
• Hydrogen formation
• Reclamation of hydrocarbons
• Engineering, operation and economics of biodigesters
• Ecological problems and waste recovery
• Ecological problems, secondary implications included
• Economic considerations
• Sociological implications in developed and developing countries
• Authors Index