Age and fluorescence characteristics in some species of Athiorhodaceae.

Nonsulfur photosynthetic bacteria (Athiorhodaceae) exhibit a time-variable fluorescence in addition to a constant fluorescence. All species examined show upon aging a remarkable gain in the variable component at the expense of the constant component while the total fluorescence remains essentially invariant. This result can be rationalized by supposing a change in distribution of bacteriochlorophyll in photosynthetic units as cells age. Alternatively, one may assume operation of two photochemical systems-one cyclic and predominant in young cells, the other noncyclic and predominant in old cells. It is also noted that a hitherto unreported minor fluorescence with maximum emission at approximately 860 nm exists in addition to the well-known main fluorescence band at approximately 890 nm. The rise in variable fluorescence is associated with the main band, a result in accord with the notion that the bacteriochlorophyll component responsible and absorbing at 870 nm is directly in contact with the energy trap.