Mireille Consalvey PhD Thesis - University of St Andrews

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However, overall a significant positive correlation was found to exist between
light level and F,, 15. It is impossible to deten-nine if it is related to incomplete QA
oxidation during the 15 min dark adaptation period (see General Discussion), is
merely a co-incidence with an endogenous rhythm matching the PPFD cycle, or
reflects a real trend. The latter two points could be re-examined by repeating the
work under different PPFD conditions. On cores from the Tagus, the maximum
rate of increase occurred in between 120 and 180 min, in advance of the maximal
light level and a considerable time into the exposure period. At this time of day
only 10 gMol M-2 s- I of light would reach cells at 200 [tm depth, to effect
increases in the F,, 15 yield, cells would have to migrate from deeper than this,
therefore, providing evidence that cells are moving towards the surface
independently of a light stimulus. The transition from a positive to a negative rate
of change between 420 and 480 min occurred at the same time as the largest
increase in PPFD (859 ýtMol M-2 S-1). It is possible that the cells migrated into the
sediment to reduce their exposure to potentially damaging irradiances. The light
attenuation co-efficients obtained for this study demonstrated that by moving
down into the sediment by only 50 [im, the cells would experience half of the
surface PPFD. To detennine the extent of the influence of PPFD would require
further manipulation (e. g. examination of days with different low tides,
manipulation of light conditions over daytime/night-time periods). The maximal
rate of decline occurred at a time co-incident with the time of immersion at the
site and may again provide some evidence of a tidally driven microphytobenthic
migratory rhythm on the Tagus Estuary.
The decrease in maximum light utilisation efficiency over the course of
the day in the Tagus cores was likely to be indicative of stress associated with
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