ON THE “SMALLNESS OF LIFE”: NEW TEM EVIDENCE FROM BIOFILM IN HOT SPRINGS, VITERBO, ITALY

The generally accepted lower limit of “Life” is a volume equivalent to a sphere of 200-250 nm. This has led to continuing scorn of the claims of “nannobacteria” in Martian meteorites, earthly minerals, and medical conditions. TEM data on algal/bacterial slime collected from the sulfurous springs at Viterbo show a continuous gradation from “normal” cells in the 1 micron range with cell walls and ribosomes, down to spherical cells with ribosomes measuring 150 nm or less. Below that size, objects visible in section have cell walls and electron-clear interiors that range continuously down to 50 nm or even smaller. The important point is that no break occurs in this sequence around the “200 nm” boundary, and one cannot draw any line separating “life” from “non-life” at this arbitrary point. Many larger cells up to 0.5 micron show no visible evidence of ribosomes either, so their lack in the 50-150 nm cells is not surprising.

Quantitative point-count data show that Viterbese cells that do contain ribosomes have a median size of 200 nm, range 130-300 nm. The ribosomes inside are 25-35 nm, and the cell walls/membranes are uniformly 25-30 nm thick. Other authors have described similar, smaller (90 nm) “round objects” with cell walls and internal structures, comparable with our data.

For the small, equant cells without evident ribosomes from Viterbo, the median size is 70 nm, two-thirds of them are 55-120 nm, and the range is 40 to 300 nm. This raises a problem: the smallest “cells” have a central light spot of only 15-20 nm; much too small for “life”. Strictly defining the lower limit of life thus becomes as difficult as counting the number of angels on a pinhead. Viterbo data apparently extends the limit well below 200 nm, but where should one stop?