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

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Cook, Suellen Saidee (2010) Ecophysiological, morphological and genetic differences between two Southern Ocean morphotypes of the coccolithophorid Emiliania huxleyi (Lohm.) Hay and Mohler (Haptophyta). PhD thesis, University of Tasmania.

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Abstract

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 FsT 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 sTrange 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 a ratio (Hex:chl a)
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 a 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
Ek value near the ambient irradiance) nevertheless in high growth irradiance the light
saturation index (Ek) 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 Ek 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.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Additional Information:

Available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:58
Last Modified: 11 Mar 2016 05:53
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