Ecophysiological, morphological and genetic differences between two Southern Ocean morphotypes of the coccolithophorid Emiliania huxleyi (Lohm.) Hay and Mohler (Haptophyta)

The coccolithophorid Emiliania huxleyi has long been considered a cosmopolitan species occurring from the tropics to polar waters. This keystone marine phytoplankton through its shedding of delicate and intricately beautiful calcium carbonate coccoliths is a major contributor to the global geochemical cycling of carbon and is attracting widespread interest as to whether it will increase photosynthesis or reduce calcification in response to current climate change. To gain an understanding of the ecology of little studied Southern Hemisphere populations of E. huxleyi 423 single cell isolates were established from 10 sampling locations encompassing 5 ocean currents/systems around southern Australia, Tasmania and the Southern Ocean, including below the Antarctic Polar Front. Two distinct coccolith morphotypes were recognised which remained stable during long-term culturing: type A and type B/C. DNA was extracted from these strains and each was amplified with 8 microsatellite markers developed for the species. The amplification success of the microsatellites was low (65%) but comparable between this and a previous study using the same markers with E. huxleyi (61%). Low to moderate genetic differentiation (from pairwise comparisons between populations F\\(_{ST}\\) range = 0.01 - 0.09) was apparent among type A populations despite considerable admixture suggesting that gene flow is not sufficient to completely prevent differentiation. Moderate to high levels of genetic differentiation (F\\(_{ST}\\) range 0.14 - 0.16) were observed between the Southern Ocean morphotype B/C and all type A populations. The genetic differences may arise from an ecological or environmental constraint to this unique Southern Ocean B/C morphotype. Pigment analyses of the two morphotypes revealed significant differences in fucoxanthin pool composition and size. The 19'-hexanoyloxyfucoxanthin:chlorophyll ˜í¬± ratio (Hex:chl ˜í¬±) in type A strains was always <1, but > 1 in type B/C strains indicating an important role for Hex in managing light harvesting in type B/C strains. Based on non-Chl ˜í¬± normalized pigment data type B/C strains had a larger fucoxanthin pool due to 11 x greater Hex:fucoxanthin ratio and a higher ratio of carotenoids to chlorophylls. Type A strains had an 8x higher concentration of fiicoxanthin. Type A possessed the carotenoid 4-keto-19'- hexanoyloxyfucoxanthin (4-keto-hex) but this pigment was never detected in over 30 type B/C strains. Related physiological differences between the two morphotypes were evident in the response to light, both in short and long term exposure experiments. Non-photochemical quenching (NPQ) of chlorophyll fluorescence and xanthophyll de-epoxidation were induced twice as rapidly in type A than in type B/C. Recovery of photosynthetic yield from short-term high light exposure was 12.5 x faster in type A than in type B/C. The Ek value of neither morphotype reflected either low or high steady state growth irradiance level (i.e. having an E\\(_k\\) value near the ambient irradiance) nevertheless in high growth irradiance the light saturation index (E\\(_k\\)) for type A was higher than type B/C. Upon exposure to simulated midday sea surface irradiance, the high light grown type B/C more than doubled its E\\(_k\\) value to surpass type A. Low light acclimated strains of both morphotypes suffered long term photodamage when exposed to sea surface irradiance levels. The two E. huxleyi morphotypes, A and B/C, use and are affected by light differently according to the light level to which they have previously been acclimated. With its higher fucoxanthin content, rapid xanthophyll de-epoxidation activity and recovery/repair mechanisms, type A is adapted to a narrower light intensity range but is more efficient under sustained high light conditions than type B/C. Accelerated xanthophyll cycling that increases the short-term photoprotective capacity of cells may be an important factor contributing to the ecological success of E. huxleyi type A during surface blooms at high light intensities. In contrast, type B/C, although possessing the capacity to acclimate to high light, has slower light response rates and takes longer to recovery and therefore may be compromised in its ability to adapt and optimise photosynthesis to reach the rapid growth rates required to form blooms. Furthermore, if as predicted under climate change models, ocean stratification and shallow mixing increase, then these conditions will have serious implications for the survival of the type B/C as it will potentially be exposed to longer periods of high light conditions which may prove permanently damaging over a sustained period of time. The degree of genetic differentiation along with the difference in pigment composition and photophysiological properties between the two morphotypes reflects an adaptive evolutionary divergence due to isolation of type B/C by the Antarctic Circumpolar Current. It is proposed that morphotype B/C be recognized as variety aurorae var. nov distinct from the more widespread type A (var. huxleyi). Recognition of strain variation of this previously considered cosmopolitan taxon is critical in order to predict the future success of this key ocean plankton.