Ayaaki Ishizaki

Department of Food Science and Technology, Faculty of Agriculture,
Kyushu University, Hokozaki, Higashi-ku, Fukuoka 812-81, Japan

1. Introduction

Today, we are faced with serious global environmental problems. Among them, counter-measure against the elevation of carbon dioxide in the atmosphere must be an urgent tasks. Microbial technology can contribute to the solution of this problem. First, we must note that the composition of the atmosphere over the globe has undergone drastic changes in the past 4.5 billion years of the earth’s his­tory which has resulted in the development and diversification of living organisms and changes in the biota of the earth. However, these changes have been occurred in a very limited part of the earth i. e. biosphere in which the atmosphere and oceans are included. These changes have been accompanied by changes in the flora and fauna as well. The ecosystem of the globe and the environment influ­ence each other in the recycling of atoms in the biosphere to renew the composi­tion of the air and to establish new equilibrium from time to time. However, the global problem of the environment today is a new subject which never arose in the past history of the earth and is caused by new technologies developed to con­trol nature and to ensure a comfortable life. The speed of these developments have been very rapid indeed. Therefore, there has not been enough time for hu­mans to adapt the environment brought by these technologies to suit. Such drastic change has never occurred in the past history of earth and this is the first experi­ence for all living organisms, thus the problem is becoming critical and difficult to solve.

The amount of the elemental carbon existing in the crust of the earth is great but this carbon is inactive since they are contained in the crust such that no chemical reactions occur resulting in no emission of carbon dioxide from the crust into the atmosphere. Thus, carbon dioxide which influences environment is restricted to the elemental carbon recycling in the biosphere. Since there is about 300 ppm of carbon dioxide in the air so the total amount of elemental carbon containing in the atmosphere is about 700 Gt (gigatons). On the land, heterotro­phic organisms including humans release about 50 Gt of elemental carbon (in the form of carbon dioxide) per year into air. If the same amount of elemental carbon

© 1997 American Chemical Society

was fixed into organic materials by autotrophic organisms, a constant concentra­tion of carbon dioxide would be maintained in the atmosphere. In the oceans, bal­ance would be maintained by exchange elemental carbon between autotrophs such as sea weeds and heterotrophs such as fish, resulting in a constant concentra­tion of dissolved carbon dioxide and bicarbonate in the mixing layer of ocean.

The amount of elemental carbon thus recycled between inorganic (carbon diox­ide) and organic matter both on the land and in the sea is a very small amount compared to the total amount of elemental carbon existing in the globe. However, the amount of elemental carbon recycling in the biosphere, 50 Gt, is big enough to influence the carbon dioxide concentration in air because the size of carbon di­oxide pool in biosphere is only 700 Gt.

The approximately 5 Gt of elemental carbon released from the combustion of fossil fuels is considerably large when compared to the amount of elemental carbon recycling in the biosphere, 50 Gt. It is therefore reasonable to attribute ele­vating levels of carbon dioxide concentration in the air to carbon dioxide emis­sions from combustion of fossil fuels. In order to prevent such increase of carbon dioxide concentration in the air, numbers of autotrophic organisms must increase so as to fix 5 Gt of elemental carbon into organic materials.

In the case of nitrogen, the effect of increased artificial nitrogen fixation (synthetic ammonia and urea) to the environment has not been observed. To date, the amount of nitrogen recycling through artificial fixation has reached 30 Gt per year which is almost equivalent to the amount of biologically fixed nitrogen (by
nitrogen fixing bacteria such as Rhizobium). The luck of influence of the rapid growth of artificially fixed nitrogen on the environment of the globe is due to the huge nitrogen pool size in the biosphere, where about 3,800,000 Gt of elemental nitrogen is presented.