Effect of elevated PCO2 on optical properties of the coccolithophorid Emiliania huxleyi grown under nitrate limitation

Side scatter and red fluorescence properties of the coccolithophore Emiliania huxleyi
were investigated by flow cytometry when NO3-limited continuous cultures
were submitted to a CO2 partial pressure (pCO2) increase from 400 to 700 ppm.
Cultures renewed at the rate of 0.5 d-1 and were submitted to saturating light
level. pCO2 was controlled by bubbling CO2-rich or CO2- free air in the cultures.
Most of the analyses were repeated 5 times and the average SD were < 1.6%, 0.1
and 0.2% for counting, fluorescence and side scatter respectively. Considering the
possible decalcification induced by the increase of CO2 in the chemostat atmosphere,
the maximum variation that can be expected for side scatter is that provided
by the coccolith depletion upon acidification of the cell suspension. The acidification
induced a large (36%) decrease of the side scatter signal but had no
detectable effect on the red fluorescence. A control was run with a non-calcifying
species, Dunaliella tertiolecta, where acidification induced no detectable change,
both on fluorescence and side scatter. During the time of the experiment, the
decline of side scatter in chemostat 1 never approached the potential 36% change
observed when coccoliths are fully dissolved. Interestingly, the specific chl a fluorescence
of E. huxleyi slightly increased during the period of high CO2 level. At
the end of the experiment this increase amounted to a significant 2.8% of the initial
signal. Furthermore, it progressed linearly with time over the period of observation.
However, the experiment did not last enough to know if the fluorescence
increase had already reached its maximum value. The acidification experiment
supported the use of side scatter as a relevant parameter to trace potential changes
in calcification. Since the estimated 25% decrease in calcification induced by the
rise in CO2 atmosphere did not result in dramatic changes in side scatter values,
we can conclude that the number of cocoliths and the overall shape and granulosity
of cells was not significantly affected by this decrease. Changes must have
only affected tiny structure details of the coccoliths which is supported by scanning
electron microscopy observations. The small but significant increase of the
fluorescence signal can be considered as a physiological response to the CO2 rise.
This suggests a more dynamic photosynthetic process that would result in a higher
rate of organic matter production providing that the system is not nutrient limited
as in the present situation.