Water content and freezing tolerance of Antarctic mosses - indicators of climate change

Metadata record for data from AAS (ASAC) Project 3061.Public 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.Project objectives:The objectives of this project are:1) To characterise water relations in Antarctic mosses during freezing and thawing2) To assess functional biodiversity in the water relations of Antarctic mosses during freezing and thawing3) To assess environmental influences on the dynamics of freezing and thawing4) To link moss water relations during freezing and thawing with carbon gain and growth5) To develop a biophysical model of water relations during freezing and thawingBackgroundIn 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.As mentioned above, these data were also collected under the auspices of project 2780. Information from that project is presented here:Metadata record for data from AAS (ASAC) project 2780.Public SummaryThe distribution of plants in Antarctica is chiefly limited by the availability of water and sufficiently high temperatures. This project assesses and simulates variation in these factors as experienced by Antarctic moss species, measures how mosses physiologically respond to temperature and moisture changes, and how they will fare in possible future climate scenarios.Project objectives:The objectives of this project are:1) To assess and monitor the seasonal and inter-seasonal variation in temperature and moisture regimes of moss vegetation in continental Antarctica2) To assess the response of Antarctic moss species to the interaction of moisture and temperature, and different cycles of freezing/thawing and drying/wetting3) To assess the physiological response of Antarctic moss species to simulated climate change by experimental warming in the field4) To provide baseline data for modelling the productivity of moss vegetation in response to moisture/temperature interactions, and the possible response of vegetation to short- and long-term changes in climatic patterns in continental AntarcticaBackgroundThe distribution of plants in Antarctica is chiefly limited by the availability of water, nutrients, and temperatures that are sufficiently high to allow the plant to physiologically operate, as well as to provide water in liquid form. Water availability and temperature are tightly linked. Where plants have access to liquid water, a 'window' is created where the plant can acquire carbon and grow.In the arid climate of eastern continental Antarctica, mosses can occur in areas where mild temperatures during part of the year allow snow or ice to melt and provide the necessary water for carbon acquisition and growth. When water becomes scarce, mosses desiccate and usually survive dry periods until the next 'window of opportunity' opens.Moss growth is limited by the number and duration of such 'window' periods. There are, however, trade-offs; adjusting to repeated freezing and thawing or drying and re-wetting often reduces the photosynthetic performance of mosses (Kennedy 1993, Lovelock et al. 1995a,b, Robinson et al. 2000). It has been suggested for mosses from other xeric environments that carbon balance limits the distribution of desiccation-tolerant mosses where repeated drought alternates with short wetting periods (Alpert and Oechel 1985). Studies of photosynthetic performance during dehydration (Robinson et al. 2000) or after re-wetting (Schlensog et al. 2004, Wasley 2004, Wasley et al submitted, Schortemeyer, Siebke, Medek and Ball, unpublished data from AAD project 2544) show considerable differences between moss species in the timing of the decline or increase in photosynthesis during drying or after re-wetting, respectively. Some species recover their photosynthetic competence after re-wetting more rapidly than others, and some species lose their photosynthetic competence faster during drying. In addition, cushion size will affect drying and wetting patterns and has been shown to influence the response of photosynthesis to drying and wetting (Zotz et al. 2000). Differential responses of moss species to freezing and drying cycles will influence the comparative performance of species and ultimately species distribution and vegetation composition.There are good temperature records that extend for more than half a century for a number of sites in continental Antarctica. However, temperature data gathered by weather stations often do not reflect the temperatures of soil or ice surfaces, and importantly, of moss cushions or turfs, which can be substantially warmer than the ambient air temperature (Melick and Seppelt 1997).While maritime Antarctica shows clear warming trends over the last 50 years (Turner et al. 2005), the patterns for continental Antarctica are less clear. Melick and Seppelt (1997) have suggested a long-term drying pattern for the Windmill Islands region, consistent with a decrease in moss and an increase in lichen vegetation. Inter-annual variation in temperature and moisture can be highly variable and often obscure long-term trends. Whichever way temperature, precipitation and wind patterns develop, they will greatly affect vegetation that is at the edge of its distribution, in a tightly balanced system in the world's most marginal sites for terrestrial plant life.Mosses are (together with lichens) the principal component of continental Antarctic vegetation. To assess the response of mosses to changes in temperature and moisture during a season, these factors must be monitored at the moss level. This project proposes to monitor moisture and temperature in several moss species along moisture gradients near Casey Station (Wilkes Land). We will measure the physiological response of the different species to different regimes of freezing, thawing, drying, and wetting, in field-based free-air heating experiments as well as in controlled laboratory environments.Taken from the 2009-2010 Progress Report:Progress against objectives:All preparations for the planned experiments were made, including construction, purchase and testing of equipment. Unfortunately, the research program had to be postponed because unusually warm temperatures caused the flights to be cancelled and consequently we were not able to travel to Casey.Taken from the 2010-2011 Progress Report:Progress against objectives:All preparations for the planned experiments were made, including testing of equipment. Unfortunately, the research program had to be postponed because unusually warm temperatures caused the flights to be cancelled and consequently we were not able to travel to Casey.