Table 1 Organisms, Environments, and Presenters

Organism

Environment

Speaker

Viruses

Animal or plant cells

Van Etten

Nanobacteria

Animal serum

Kajander

Attached bacteria

Soils, sediments, rocks

Staley

Hyperthermophiles

Hot springs and vents

Stetter

Hyperthermophiles

Hot springs and vents

Adams

Aquatic bacteria

Lakes and oceans

Button; DeLong

Table 2 Size Ranges of Organisms or Organelles, and Niches Where They Are Found

Organism

Diameter Range (nm)

Life Style

Virus

30 to 200

Host-dependent

Nanobacteria

100 to 200

Host-dependent

Marine bacteria

100 and larger

Free-living

Attached forms

100 and larger

Free-living

Hyperthermophiles

200 and larger

Free-living

Mitochondria

200 and larger

Host-dependent

Chloroplasts

200 and larger

Host-dependent

presentations were specifically made in these areas. The size ranges shown in Table 2 represent the consensus values reached in the presentations and in ensuing discussions by the assembled group. In many cases it was hard to reach consensus on firm estimates for the smallest organisms or organelles encountered, and the reader is referred to specific arguments in the individual papers. For example, there was considerable debate with regard to the nanobacteria, as summarized by Dr. Kajander. While such nanobacteria have been reported to be smaller than 100 nm in diameter, Dr. Kajander was of the opinion that the only organisms for which growth could be established with certainty were those of 100-nm diameter or larger. This represents an area of considerable importance in terms of being able to search for and recognize very small organisms (e.g., Are there organismal fragments that appear to have similar morphologies, but are not actually viable, growing entities?).

A point of interest with regard to this area is that virtually all of the microbiologists present had encountered structures resembling cells in the size range of 100 to 200 nm, but whether or not these could be demonstrated to be viable or cultivable microbes had usually not been established. The timeworn method of filtration through a 200-nm (0.2 micrometer) pore-size filter was still very dependable in terms of delineating cultivable bacteria.

Environmental Parameters and Size

What are the environmental issues that may impose or relieve restrictions on the smallest sizes that can be achieved by organisms? In pursuit of this question, the speakers considered a variety of different environmental factors that might lead to organisms adopting a smaller size. These included:



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