270
KENNETH V. THIMANN AND GEORGE M. CURRY
long ago as 1882-1888 Engelmann observed that these anaerobic organisms, if they swim into a region of lower light intensity, undergo a
"shock reaction" which changes their direction of movement. Since this
occurs whenever they cross the boundary into a darker zone, the result is
that the organisms accumulate in the lighted area ("phobo-phototaxis").
If now they are illuminated with a small spectrum the bacteria will
move towards, and collect in, the following specific regions of the spectrum: the infrared from 800 to 900 ηΐμ, a band in the yellow near 590
m/Λ, and bands in the green later specified by Buder as 530, 490 and
470 τημ. The absorption spectrum of a suspension of the cells shows
bands in just the same places, with an additional one at 550 τημ. Those
at 880 and 590 τημ have since been identified as belonging to the green
pigment bacteriochlorophyll, the others to a series of carotenoids (73,
74). The deduction is, then, that all of these pigments act as photoreceptors for phototaxis. A complication is that the extracted and purified pigments do not show peaks at quite the same wavelengths as in
the living cell, due to their combination with protein in vivo. Nevertheless the spectral bands of the pigments could be fairly well identified
with those in the action spectrum.
The significance of the chlorophyll and carotenoids as photoreceptors lies in the fact that the purple bacteria, as Engelmann believed
and as was proved almost fifty years later by van Niel (75) are photosynthetic, and both the chlorophyll and the carotenoids (with the apparent exception of spirilloxanthin, the one which absorbs at 550 m/x)
can absorb light active in photosynthesis. The action spectrum for
photosynthesis of Rhodospirillum rubrum, indeed, shows peaks at about
880, 590, 525, 490, and 460 m^ (76) which agree perfectly with the
figures for phototaxis. Also the high light intensity causing decreased
phototactic sensitivity coincides with that causing saturation of the
photosynthetic system (77). Thus the phototactic stimulus is considered
to result from a sudden decrease in the photosynthetic rate (74, 78).
Engelmann's less extensive experiments under anaerobic conditions
with the ciliate Paramecium bursaria, which contains endozootic cells
of the green alga Chlorella, showed a similar behavior, but in this case
the cells collected in the region of maximum absorption of chlorophylls
a and b, namely around 650-700 τημ. Since these chlorophylls are the
main photosynthetic pigments of the green algae, the observations again
indicate that the stimulus for phototaxis is a change in the rate of photosynthesis. Indeed Links (79) has generalized this conclusion to apply
also to chemotaxis and osmotaxis: "The common factor in the chain of
processes . . . which give rise to the characteristic shock-reaction is the
sudden lowering of the energy consumption in the motility apparatus of
KENNETH V. THIMANN AND GEORGE M. CURRY
long ago as 1882-1888 Engelmann observed that these anaerobic organisms, if they swim into a region of lower light intensity, undergo a
"shock reaction" which changes their direction of movement. Since this
occurs whenever they cross the boundary into a darker zone, the result is
that the organisms accumulate in the lighted area ("phobo-phototaxis").
If now they are illuminated with a small spectrum the bacteria will
move towards, and collect in, the following specific regions of the spectrum: the infrared from 800 to 900 ηΐμ, a band in the yellow near 590
m/Λ, and bands in the green later specified by Buder as 530, 490 and
470 τημ. The absorption spectrum of a suspension of the cells shows
bands in just the same places, with an additional one at 550 τημ. Those
at 880 and 590 τημ have since been identified as belonging to the green
pigment bacteriochlorophyll, the others to a series of carotenoids (73,
74). The deduction is, then, that all of these pigments act as photoreceptors for phototaxis. A complication is that the extracted and purified pigments do not show peaks at quite the same wavelengths as in
the living cell, due to their combination with protein in vivo. Nevertheless the spectral bands of the pigments could be fairly well identified
with those in the action spectrum.
The significance of the chlorophyll and carotenoids as photoreceptors lies in the fact that the purple bacteria, as Engelmann believed
and as was proved almost fifty years later by van Niel (75) are photosynthetic, and both the chlorophyll and the carotenoids (with the apparent exception of spirilloxanthin, the one which absorbs at 550 m/x)
can absorb light active in photosynthesis. The action spectrum for
photosynthesis of Rhodospirillum rubrum, indeed, shows peaks at about
880, 590, 525, 490, and 460 m^ (76) which agree perfectly with the
figures for phototaxis. Also the high light intensity causing decreased
phototactic sensitivity coincides with that causing saturation of the
photosynthetic system (77). Thus the phototactic stimulus is considered
to result from a sudden decrease in the photosynthetic rate (74, 78).
Engelmann's less extensive experiments under anaerobic conditions
with the ciliate Paramecium bursaria, which contains endozootic cells
of the green alga Chlorella, showed a similar behavior, but in this case
the cells collected in the region of maximum absorption of chlorophylls
a and b, namely around 650-700 τημ. Since these chlorophylls are the
main photosynthetic pigments of the green algae, the observations again
indicate that the stimulus for phototaxis is a change in the rate of photosynthesis. Indeed Links (79) has generalized this conclusion to apply
also to chemotaxis and osmotaxis: "The common factor in the chain of
processes . . . which give rise to the characteristic shock-reaction is the
sudden lowering of the energy consumption in the motility apparatus of