Water content and freezing tolerance of Antarctic mosses - indicators of climate change
Metadata record for data from AAS (ASAC) Project 3061.
Water availability constrains plant growth in the Antarctic. Cross-disciplinary application of state-of-the-art techniques to determine dynamic composition of water in different states during freezing and thawing will enable unprecedented insight into water relations and mechanisms of freezing tolerance, and how they relate to the ... functional biodiversity of mosses in Antarctica.
The objectives of this project are:
1) To characterise water relations in Antarctic mosses during freezing and thawing
2) To assess functional biodiversity in the water relations of Antarctic mosses during freezing and thawing
3) To assess environmental influences on the dynamics of freezing and thawing
4) To link moss water relations during freezing and thawing with carbon gain and growth
5) To develop a biophysical model of water relations during freezing and thawing
In the dry and cold Antarctic climate, water availability is the primary limitation of moss colonisation and growth (Kennedy 1993). In order for water to be taken up and support physiological processes in plants, it needs to be in liquid form. In Antarctica, this aggregate state of water is the exception. In near-coastal areas where mild temperatures during part of the year allow snow or ice to melt, 'windows of opportunity' are created for mosses to activate their physiology and acquire carbon. The frequency and duration of these windows depend on external temperature, water availability, cushion size (Zotz et al. 2000), regeneration speed and absorptivity of mosses, and the state of water surrounding and within cells. Increased frequency of freeze-thaw events can not only increase time available for carbon gain, but also impose a stress to mosses (Melick and Seppelt 1992; Lovelock et al. 1995a,b)
Climatic change in Antarctica will determine whether more or fewer, longer or shorter 'windows' will be created for moss carbon gain and growth. In peninsular and maritime Antarctica, temperatures have increased dramatically over the last 30-50 years (Vaughan et al. 2003, Vaughan 2006), providing greater opportunity for ice to melt, while the temperatures along the continental Antarctic coast have shown no consistent trend (Melick and Seppelt 1997, Vaughan et al. 2003). For the Windmill Islands region, Melick and Seppelt (1997) have suggested no temperature change but a long-term drying pattern which is consistent with a decline in moss vegetation and an increase in lichen-covered areas.
Temperature is only one of the factors which determines the state of water and its availability for biological processes. Equally important is the relative humidity of the surroundings, which determines the osmotic pressure within the tissue, and ultimately controls the amount of water inside cells. In dry environments tissues will dehydrate, thus concentrating solutes (salts, sugars etc). Cells lose water, thereby maintaining osmotic balance with their surrounding, which in turn leads to an increase in the intracellular solute concentration. This dehydration-induced concentration has a number of consequences (for reviews see Bryant et al. 2001, Wolfe and Bryant 1999):
(i) the freezing point of the solution is depressed, so tissue in dry environments can often survive low sub zero temperatures without freezing.
(ii) if freezing occurs in the tissue, this leads to further freeze induced dehydration of the cells, further lowering the freezing point. Thus cells may be able to avoid intracellular ice formation at low sub zero temperatures.
(iii) the solute concentration can become so high that the solution vitrifies - ie forms a glass - an amorphous solid with no long range order. If the solution inside a cell or organelle vitrifies, there can be no further dehydration of the system. Moreover, formation of ice crystals (freezing) is avoided. These mechanisms are common in a number of plants and animals (eg resurrection species), and in structures such as seeds. The amount of dehydration in tissue will be determined by a complex interplay between the temperature, the availability of liquid water, and the local relative humidity.
AAD project 2780 (Ball, Schortemeyer, Robinson) will monitor the seasonal course of moss temperature and water availability for three species growing at or near Casey. The project will also manipulate temperatures, and investigate biological responses to temperature and water availability in mosses. The current project will be synergistic with the above project and work with these authors, using the same infrastructure, to address above objectives.
Taken from the 2008-2009 Progress Report:
Progress against objectives:
Work has focussed on development of techniques and experimental protocols using local populations of congeneric and conspecific mosses to those in Antarctica. Comparative measurements will be made on samples brought back from Antarctica in collaboration with Dr Mary Skotnicki. These data will be used to compare behaviour of local and closely related Antarctic species, and to establish experimental designs in preparation for field work in Antarctica.
Three methods have been developed for achievement of the research goals. A DSC was purchased with the grant money plus additional funds. This field portable instrument gives data on temperatures of transition in water states and is being calibrated against a more sophisticated lab-based DSC in preparation for field work in Antarctica. A complementary technique using solid-state NMR spectroscopy has been developed to determine the quantitative distribution of water in different states within mosses during an artificially imposed freeze/thaw event. This technique requires infusion of the sample with deuterated water. Tests showed that dried mosses readily absorbed the label and remained physiologically competent for at least eight hours, as judged by monitoring time-dependent changes in chlorophyll a fluorescence characteristics. Finally, cryo-SEM has been used to quantitatively assess changes in the dimensions of cells and tissues of mosses subject to a freeze/thaw event. This technique required cryo-planing of individual moss thalli to prepare a polished surface for imaging.
Measurements to date with the DSC on freezing behaviour of fully hydrated local moss specimens have revealed exotherms consistent with interspecific differences in hydraulic anatomy in local populations of Schistidium apocarpum and Ceratodon purpureus. Schistidium has no specialised vascular tissue whereas a core of hydroids occurs within thalli of Ceratodon. DSC measurements of fully hydrated specimens revealed a single exotherm at -9.0C in Schistidium, consistent with intracellular freezing, whereas two exotherms occurred in Ceratodon: one at -11.2C, consistent with freezing of apoplastic water in hydroids, and another at -12.6C, reflecting intracellular freezing. However, when similar measurements were made with moss samples in contact with external ice, then no exotherms were measured. Cryo-SEM of the samples revealed that, when in contact with external ice, the mosses readily dehydrated as water diffused from the mosses to external sites of ice formation. While thick walled cells of the epidermis retained much of their original shape during freeze-induced dehydration, the other living cells shrank with no loss in contact with cell walls. In Ceratodon the hydroids were fully embolised at -4C and fully collapsed at -12C. Thus, given the temperatures for internal freezing and the ease of water loss in response to external freezing, there seems little possibility that internal freezing would ever occur under natural conditions. We are now repeating DSC and NMR measurements to determine whether a glass forms during extreme dehydration. The data from the 3 methods are being combined to develop a quantitative model of water relations during freeze/thaw events.
Download point for the data
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The values provided in temporal and spatial coverage are approximate only.
Taken from the 2008-2009 Progress Report:
Variations to work plan or objectives:
Field work for 2008-9 had to be postponed due to several logistical difficulties.
Copies of the data, including detailed readme documents are available for download from the provided URL.
However, the winter re-activation dataset is not yet publicly available, as it has not yet been published.
Data Set Progress
+61 3 9925 2139
gary.bryant at rmit.edu.au
Royal Melbourne Institute of Technology
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Jack.Egerton at anu.edu.au
Plant Science Division
Australian National University
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+61 3 6232 3244
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dave.connell at aad.gov.au
Australian Antarctic Division
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