C. B. van Niel

The Concept of a Bacterium

Since the earliest days of microbiology, the biological nature and relationships of the bacteria have been subjects of perennial discussion. Why have these questions obsessed some members of each succeeding generation of microbiologists ? There can be no doubt about the principal reason. Any good biologist finds it intellectually distressing to devote his life to the study of a group that cannot be readily and satisfactorily defined in biological terms; and the abiding intellectual scandal of bacteriology has been the absence of a clear concept of a bacterium. Our first joint attempt to deal with this problem was made 20 years ago (STANIER and VAN NIEL 1941). At that time, our answer was framed in an elaborate taxonomic proposal, which neither of us cares any longer to defend. But even though we have become sceptical about the value of developing formal taxonomic systems for bacteria (see VAnNIEL 1946, for an exposition of the reasons), the problem of defining these organisms as a group in terms of their biological organization is clearly still of great importance, and remains to be solved. A great deal of relevant information has emerged in recent years, and the time therefore seems opportune for a fresh analysis of this question. I t is not our intention to review here the development of ideas about the nature and relationships of the bacteria ; readers who are interested in the historical aspects of the problem can find authoritative and very full accounts in PRINeSI~EI~ (1923, i949) and VAN NIEL (1955).

On the photochemical reduction of nitrate by algae

At high light intensity suspensions of Chlorella pyrenoidosa, supplied with non-limiting concentrations of CO2, produce oxygen at a greater rate when nitrate is simultaneously present. In that case the photosynthetic quotient, CO2/O2, is considerably lower than in the absence of nitrate, even though the rate of CO2 assimilation is not reduced. From these results it is concluded that the photochemical nitrate reduction, discovered by Warburg and Negelein in 1920, cannot be explained on the basis of a dark reaction which yields CO2 by the oxidation of cell material with the concomitant reduction of nitrate. It can best be interpreted as a process in which nitrate acts directly as an alternate and additional hydrogen acceptor in photosynthesis.

On the metabolism of the thiorhodaceae

After having established, in 1929, the photosynthetic act ivi ty of green bacteria and Thiorhodaceae with the simultaneous oxidation of I-I 2 S to S and H 2 S O a, I remarked tha t perhaps the difference between these organisms and the Athiorhodaceae might be tha t the lat ter use organic compounds instead of H 2 S as the normal reducing substances in the assimilation process (1). Later experiments then showed tha t also the Thiorhodaceae in the light are capable of anaerobic development in the presence of organic substances, even when oxidizable sulfur compounds are completely lacking (2, 3), and the quanti tat ive investigation by Muller (4) of the reactions occurring under such conditions seemed to establish definitely tha t the function of hydrogen donor for the photochemical C02-reduction, normally carried out by H2S or other oxidizable sulfur compounds, can be fulfilled by simple organic substances.

On the mechanism of ballistospore discharge

SummaryStudies on the discharge of ballistospores by Sporobolomyces holsaticus have shown that the ejection mechanism resides exclusively in the spore. They support Olives contention that the spore is propelled by the sudden rupture of a gas-filled vesicle which is formed between the inner and outer layers of the wall at the hilar end of the spore. Evidence based on microscopic observations, micromanipulation, and electron microscopy is presented.In addition to gas, liquid is also released during spore discharge.An explanation of the discharge mechanism is presented which accommodates earlier and conflicting views.

Studies on the pigments of the purple bacteria

Summary1.A method for the isolation and purification of spirilloxanthin, the purple pigment of Spirillum rubrum Esmarch, has been described.2.The analytical results indicate that its empirical formula is C48H66O3; that its molecule contains 15 double bonds; that no more than one of its oxygen atoms is present in a hydroxyl group; and that no free carboxyl group is present.3.The absorption spectrum of the pigment substantiates the high degree of unsaturation found by catalytic hydrogenation.4.Other carotenoid pigments seem to be present in this organism, but these have not yet been isolated in sufficient quantity to be examined chemically.

A note on the apparent absence of azotobacter in soils

SummaryIt is shown that soils which fail to give Azotobacter growth when inoculated in the original elective medium may still contain viable Azotobacter cells. These develop into normal cultures when 0.00005% Na2MoO4 is added to the liquid medium.This phenomenon is explained as a result of the low molybdenum content of the soils used.In view of the possible influence of molybdenum on soil fertility the soil plaque method was tried successfully for the demonstration of a molybdenum deficiency. of soil samples.

Alexander Ivanovitch Oparin and the Origin of Life: A Recollection and Appreciation

In 1928, shortly before leaving Holland to assume a position at the Hopkins Marine Station of Stanford University, I went to pay a visit to Professor M. W. Beijerinck, whose courses at the Technological University in Delft had introduced me to the science of microbiology and kindled my fascination with that field. Following his compulsory retirement in 1921, he had moved to a home in the country, and I had not seen him since then. Now I wanted to express my deep appreciation for the great and lasting influence his teaching had had on my life.