Impact 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 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.
Side scatter of a single cell is considered to reflect not only its overall shape but mainly
its inner structure and content through a complex combination of optical properties
including absorption, diffusion, refractive index and refraction. Consequently, the
cultures of Emiliania huxleyi were monitored by flow cytometry to detect possible
changes in its optical properties at the single cell level under the effect of CO2
doubling in the atmospheric phase. The average SD for counting 5 replicates was less
than 1.6% over the period of the study. It was 0.1 and 0.2% for 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
as expected and had no detectable effect on the red fluorescence. To validate the
assignment of this change on coccolith dissolution, the same experiment was repeated
with a culture of a non-calcifying species, Dunaliella tertiolecta. The acidification
of D. tertiolecta suspension induced no detectable change, both on fluorescence and
side scatter. During the time of the experiment, the decline of side scatter 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 2.8% of the initial signal. The average SD of red fluorescence being 0.1%, this
increase must be considered as significant. 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 coccoliths 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 like those observed on
published electron micrographs.
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.