Mireille Consalvey PhD Thesis - University of St Andrews

Chaoter S General Discussion
the effect of self shading on the fluorescence yield; cells near the surface may
potentially exhibit NPQ whilst cells deeper in the sediment have no need.
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Sediment surface
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The dark level minimum fluorescence, F, is subject to variation and
several idiosyncrasies that seem specific to microphytobenthos. F, can be
affected in two opposing directions by non-photochemical quenching (NPQ; see
chapter I and 7) and the photochemical system. NPQ is a strategy evolved to
cope with potentially damaging irradiances (e. g. the conversion of diadinoxathin
to diatoxanthin). Diatoxanthin is less efficient at transferring energy and
therefore plays a vital role in preventing photodamage. The presence of
diatoxanthin may additionally affect the fluorescence yields, as not all of the
measuring beam will strike the reaction centre. Therefore, if diatoxanthin is not
converted back to diadinoxathin during the dark adaptation period, a decrease in
F, could be falsely attributed to a downwards migration. NPQ can be measured
but there are problems inherent with measuring a true F, and furthermore the
time lag inherent in measuring F,, and F,, ' complicates measurements of NPQ
for motile species. In addition it has been shown that diatoms are able to
maintain a proton gradient in the dark (Caron et al. 1987; Ting and Owens 1993;
Schreiber et al. 1995), therefore NPQ will remain switched on. This phenomenon
was shown to be dependent upon light conditions as well as time in the dark, and
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