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

Abstract

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