166
generally true, though more striking in surface and very deep samples,
and was reported originally by Yentsch (1962).
The fluorescence excitation spectra for this station, normalized
to chlorophyll ~ are presented. in figure 8. Spectral shifts attributable
to the addition of accessory pigments are again clearly present. Also,
the magnitude of the fluorescence per chlorophyll increases with depth.
Spectral shifts and increased magnitudes are also seen in the fluorescence
efficiency spectra in figure 9. Additionally, the efficiency spectra
clearly indicate that in all cases, light ~bsorbed by accessory pigments
is more efficient in exciting chlorophyll ~ fluorescence than light absorbed by chlorophyll ~ itself.
IV. DISCUSSION
We propose that the changes in magnitude and shape of the chlorophyll ~ excitation spectra are caused by photoadaptation of the viable
phytoplankton. The fluorescence we measure at 683 nm. could be from
either phaeopigments or chlorophyll a. However, the relatively small
contribution at 416 nm. excitation, where phaeopigments absorb maximally,
implies that phaeopigment fluorescence is unimportant in interpreting
these spectra. Even in the bottom of the euphotic zone where the fluorometrically determined chlorophyll:phaeopigment ratio is about one, this
appears to be a valid assumption.
If the degradative processes which
convert chlorophyll to phaeopigment are sufficiently harsh, they may
disrupt the energy transfer coupling between the accessory pigments and
any residual fluorescing molecules.
Yentsch and Yentsch (1979) have proposed that changes in uncorrected
excitation spectra are attributable to taxonomic changes.
It is certainly true that for unialgal cultures, the shapes of the spectra can distinguish differences in taxonomic groups. However, when one induces photoadaptation in cultures, the results are generally similar: both accessory pigments absorption, and their contribution to chlorophyll ~ fluorescence increase. The accessory pigments absorb predominantly between
460 nm. and 550 nm. although at wavelengths longer than 500 nm., the
fucoxanthin in diatoms and peridinin of dinoflagellates are the main
accessory pigments.
In open ocean stations as the one presented here,
these taxa are relatively rare so that the accessory pigments absorption
bands for the taxa present are mostly in a narrow range from 460-490 nm.
Thus, taxonomic discrimination would be difficult using these absorption
bands, and any photoadaptive effects would act cumulatively to produce
similar spectral shifts. Furthermore, changes comparable to those presented here have been induced in field cultures incubated under light
generally true, though more striking in surface and very deep samples,
and was reported originally by Yentsch (1962).
The fluorescence excitation spectra for this station, normalized
to chlorophyll ~ are presented. in figure 8. Spectral shifts attributable
to the addition of accessory pigments are again clearly present. Also,
the magnitude of the fluorescence per chlorophyll increases with depth.
Spectral shifts and increased magnitudes are also seen in the fluorescence
efficiency spectra in figure 9. Additionally, the efficiency spectra
clearly indicate that in all cases, light ~bsorbed by accessory pigments
is more efficient in exciting chlorophyll ~ fluorescence than light absorbed by chlorophyll ~ itself.
IV. DISCUSSION
We propose that the changes in magnitude and shape of the chlorophyll ~ excitation spectra are caused by photoadaptation of the viable
phytoplankton. The fluorescence we measure at 683 nm. could be from
either phaeopigments or chlorophyll a. However, the relatively small
contribution at 416 nm. excitation, where phaeopigments absorb maximally,
implies that phaeopigment fluorescence is unimportant in interpreting
these spectra. Even in the bottom of the euphotic zone where the fluorometrically determined chlorophyll:phaeopigment ratio is about one, this
appears to be a valid assumption.
If the degradative processes which
convert chlorophyll to phaeopigment are sufficiently harsh, they may
disrupt the energy transfer coupling between the accessory pigments and
any residual fluorescing molecules.
Yentsch and Yentsch (1979) have proposed that changes in uncorrected
excitation spectra are attributable to taxonomic changes.
It is certainly true that for unialgal cultures, the shapes of the spectra can distinguish differences in taxonomic groups. However, when one induces photoadaptation in cultures, the results are generally similar: both accessory pigments absorption, and their contribution to chlorophyll ~ fluorescence increase. The accessory pigments absorb predominantly between
460 nm. and 550 nm. although at wavelengths longer than 500 nm., the
fucoxanthin in diatoms and peridinin of dinoflagellates are the main
accessory pigments.
In open ocean stations as the one presented here,
these taxa are relatively rare so that the accessory pigments absorption
bands for the taxa present are mostly in a narrow range from 460-490 nm.
Thus, taxonomic discrimination would be difficult using these absorption
bands, and any photoadaptive effects would act cumulatively to produce
similar spectral shifts. Furthermore, changes comparable to those presented here have been induced in field cultures incubated under light