Doctor of Philosophy
School of Biological Sciences, Faculty of Science
Clarke, Laurence John, Resilience of the Antarctic moss Ceratodon purpureus to the effects of elevated UV-B radiation and climate change, Doctor of Philosophy thesis, School of Biological Sciences, Faculty of Science, University of Wollongong, 2008. https://ro.uow.edu.au/theses/1919
Climate change is predicted to affect the biota of high latitudes first and most severely, and significant changes in temperature and wind speed have already been observed in parts of the Antarctic. Similarly, springtime ozone depletion since the late 1970s has led to an increase in potentially mutagenic UV-B radiation over the Antarctic, and elevated mutation rates have been reported for several Antarctic moss species. This thesis explores the resilience of Antarctic mosses to the effects of elevated UV-B radiation and climate change, with special reference to populations of the cosmopolitan moss Ceratodon purpureus from the Windmill Islands region, East Antarctica. Radiocarbon analyses were used to determine the growth rates of seven moss shoot ‘cores’ representing four species, including C. purpureus, from the Windmill Islands and Vestfold Hills, East Antarctica. My results show that the oldest core sections predate the 1960s peak in atmospheric 14C due to nuclear testing, indicating average growth rates between 0.6 and 1.3 mm yr-1. This is the first study to describe the profile of the 1960s 14C bomb-pulse in Antarctic plants and importantly radiocarbon results indicate that individual colonies are more than 50 years old and that growth rates have varied significantly over this time. These data allow the influence of environmental variables on growth rates and water relations of Antarctic mosses to be explored over time periods not possible using other techniques. Correlation analysis was used to determine whether growth rate and 13C of three C. purpureus and one Bryoerythrophyllum recurvirostre core from East Antarctica were associated with year-to-year variations in summer temperature, wind speed and stratospheric ozone depth over a 40 year period. Temperature was positively correlated with growth rate in all cores (r=0.22 to 0.61), whereas the 13C data indicated temperature and wind speed were consistently positively (r=0.20 to 0.74) and negatively (r=-0.14 to -0.67) correlated with water availability, respectively. Correlations of ozone depth with growth rate and 13C varied between cores and may be driven by covariance of ozone depth with wind speed and temperature. The significant positive correlation between 13C and growth rate indicates that growth of C. purpureus is water-limited. Warming and increased precipitation predicted for the Antarctic over the 21st century are likely to increase moss growth rates, whereas the already observed increase in wind speeds may reduce water availability and negatively impact the growth of these mosses. The timing and balance between the positive influence of warming and the negative influence of high wind speeds may determine the fate of East Antarctic moss communities. Previous studies of Antarctic clonal moss populations using Random Amplified Polymorphic DNA markers have described extraordinarily high levels of variation in banding profiles within single colonies. This has been claimed to reflect somatic mutation, possibly resulting from elevated UV-B radiation due to the Antarctic ozone hole. I used microsatellite markers to compare the genetic variation present within continental Antarctic, sub-Antarctic and temperate C. purpureus populations and in contrast to previous studies found no evidence of elevated mutation rates in the Antarctic samples. Indeed continental Antarctic C. purpureus displays significantly less intra-population genetic diversity than populations from sub-Antarctic and temperate sites. Moreover, continental Antarctic sites displayed greater levels of allelic differentiation (PT=0.205, P=0.001) than temperate sites (e.g. PT=0.006, P=0.34), suggesting that they are less interconnected by gene flow than lower latitude sites and are effectively closed to immigration. However, a more in-depth analysis of the distribution of genotypes within the Windmill Islands suggests that individual genotypes have dispersed over more than ten kilometres, and indeed that some propagules originate outside the region. Taken together these results imply that climate change will present ongoing challenges for continental Antarctic moss populations with less potential than temperate populations to adapt to environmental change. Previous studies of UV-induced DNA damage in the three moss species that occur in the Windmill Islands have shown C. purpureus to be the most UV-tolerant although it possesses lower concentrations of methanol soluble UV-screening compounds than the co-occurring Bryum pseudotriquetrum. I used alkali extraction of cell wall-bound phenolics combined with methanol extraction of soluble phenolics to determine whether higher levels of cell wall-bound UV-screens could explain the greater UV tolerance of C. purpureus. The combined pool of UV-screens was similar in C. purpureus and B. pseudotriquetrum but whilst B. pseudotriquetrum had equal concentrations of MeOH-soluble and alkali-extractable cell wall-bound UV-screening compounds, C. purpureus had almost six times the concentration of cell wall-bound compared to MeOH-soluble UV-screens, offering this species a more uniform and potentially more effective UV screen. The Antarctic endemic Schistidium antarctici possessed half the combined pool of UV-screens of the other species, indicating this species may be disadvantaged under continuing springtime ozone depletion. These data suggest that while cell wall compounds have not previously been quantified in bryophytes they may be an important component of the UV defences of lower plants. The wide distribution of C. purpureus provided the opportunity to test whether moss colonies growing in different UV environments differ in their UV tolerance. I tested the hypothesis that sun forms from different UV environments (Wollongong, Australia and Lake Waiau, Hawaii) and sun and shade forms (Wollongong) differ in their UV tolerance by using chlorophyll fluorescence to compare the photosynthetic health of C. purpureus from each site over a four hour UV irradiation. Photosynthetic and UV-screening compound concentrations were compared between Australian and Hawaiian C. purpureus in an attempt to explain variations in UV tolerance. The effective yield of PSII photochemistry declined by 50% in the UV-treated Wollongong shade samples, significantly more than the 25% decline in the Wollongong sun samples, but no difference was found in the response of the two sun forms, despite an almost two-fold greater annual UV flux at the Hawaiian site. Ceratodon purpureus colonies from Australia, Hawaii and Antarctica have similar concentrations of UV-screening compounds, indicating production of UV-screening compounds may be constitutive in this species. Observed differences in UV tolerance appear to reflect acclimation rather than adaptation, which may benefit Antarctic C. purpureus as its life history reduces its capacity for adaptive change in response to recent changes in the UV environment. In conclusion, Antarctic C. purpureus is slow growing, long-lived and well protected from current levels of UV-B but potentially vulnerable to other disturbances, such as changes in water availability. Furthermore, Antarctic C. purpureus populations are weakly interconnected by gene flow and display low genetic diversity compared to populations from temperate and sub-Antarctic sites, reducing their capacity for adaptive change in the face of predicted regional climate change.
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