Tardigrades are microscopic animals found worldwide in aquatic as well as terrestrial ecosystems. They belong to the invertebrate superclade Ecdysozoa, as do the two major invertebrate model organisms: Caenorhabditis elegans and Drosophila melanogaster. We present a brief description of the tardigrades and highlight species that are currently used as models for physiological and molecular investigations. Tardigrades are uniquely adapted to a range of environmental extremes. Cryptobiosis, currently referred to as a reversible ametabolic state induced by e.g. desiccation, is common especially among limno-terrestrial species. It has been shown that the entry and exit of cryptobiosis may involve synthesis of bioprotectants in the form of selective carbohydrates and proteins as well as high levels of antioxidant enzymes and other free radical scavengers. However, at present a general scheme of mechanisms explaining this phenomenon is lacking. Importantly, recent research has shown that tardigrades even in their active states may be extremely tolerant to environmental stress, handling extreme levels of ionizing radiation, large fluctuation in external salinity and avoiding freezing by supercooling to below -20 °C, presumably relying on efficient DNA repair mechanisms and osmoregulation. This review summarizes the current knowledge on adaptations found among tardigrades, and presents new data on tardigrade cell numbers and osmoregulation.

[1] The uptake rates of atmospheric CO 2 in the Nordic Seas are among the highest in the world's oceans. This has been ascribed mainly to a strong biological drawdown, but chemical processes within the sea ice itself have also been suggested to play a role. The importance of sea ice for the carbon uptake in the Nordic Seas is currently unknown. We present evidence from 50 localities in the Arctic Ocean that dissolved inorganic carbon is rejected together with brine from growing sea ice and that sea ice melting during summer is rich in carbonates. Model calculations show that melting of sea ice exported from the Arctic Ocean into the East Greenland current and the Nordic Seas plays an important and overlooked role in regulating the surface water partial pressure of CO 2 and increases the seasonal CO 2 uptake in the area by approximately 50%.Citation: Rysgaard, S., J. Bendtsen, L. T. Pedersen, H. Ramløv, and R. N. Glud (2009), Increased CO 2 uptake due to sea ice growth and decay in the Nordic Seas,

Changes in the trehalose level in the tardigrade Adorybiotus coronifer were observed during induction of, and arousal from, anhydrobiosis. A trehalose accumulation surpassing 1.6% dry weight (d.w.) in anhydrobiotic animals collected dry on their biotope was rapidly reduced on rehydration, reaching the level of active animals (0.1-0.2% d.w.) after 6 hours. Tardigrades dried while embedded in sand in the laboratory accumulated trehalose from 0.1% to 2.3% d.w. within 5-7 hours.Induction of aerobic acidosis in arousing tardigrades by CO, perfusion reversibly arrested reactivation for at least 36 hours and induced a reduction in anabolic and catabolic activities, measured as a significant reduction in trehalose degradation, and a sevenfold reduction in the rate of protein synthesis.These data support the hypotheis that trehalose generally serves a protective role in desiccationtolerant Metazoa, but indicate that tardigrades require only a moderate level for efficient protection. Trehalose accumulationA positive correlation between the survival of the cryptobiotic state and the accumulation of the disaccharide trehalose has been demonstrated for a number of anhydrobiotic organisms, and concentrations as high as 13-18% dry weight have been reported in anhydrobiotic cysts of the crustacean Artemia salina' (Clegg, '64; '65) and in the nematode Aphelenchus avenae (Madin and Crowe, '75; Crowe et al., '77). Moreover, trehalose reduces damage, induced by freeze-drying, to isolated liposomes (Crowe et al., '861, microsomes from sarcoplasmatic reticulum (Crowe et al., '83), and the enzyme phosphofructokinase (Carpenter et al., '87; '88). Other data such as the physical properties of nearly dry Artemia salina cysts (Clegg et al., '82; Clegg, '86) and computerized modeling of molecular interactions between trehalose and a phospholipid bilayer (Gaber et al., '86) indicate that trehalose is a likely candidate for the polyhydroxy compound that, according to the water replacement hypothesis (Webb, '65; Crowe, '711, replaces the structural water of the cellular compounds during desiccation.Anhydrobiosis is widespread in the phylum Tardigrada, but quantification of trehalose accumulation during the induction of anhydrobiosis has not yet been made in this phylum. Crowe ('75) reported that paper chromatography analysis of ethanol extracts of the tardigrade areolatus showed more intense trehalose spots for anhydrobiotic animals than for active ones. The observations could not be quantified, and the author considered them tentative. In order to examine whether significant trehalose accumulation is a general feature of anhydrobiotic animals in different phyla, this paper is concerned with the changes (if any) of trehalose content during the induction of, and recovery from, anhydrobiosis in the tardigrade Adorybiotus coronifer. pHi-dependent regulation of metabolic statusThe intracellular pH (pH,) is known to play a regulatory role in transitions between different metabolic and developmental states in several biological systems ...

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Soil invertebrate survival in freezing temperatures has generally been considered in the light of the physiological adaptations seen in surface living insects. These adaptations, notably the ability to supercool, have evolved in concert with surface invertebrates' ability to retain body water in a dry environment. However, most soil invertebrates are orders of magnitude less resistant to desiccation than these truly terrestrial insects, opening the possibility that the mechanisms involved in their cold-hardiness are also of a radically different nature. Permeable soil invertebrates dehydrate when exposed in frozen soil. This dehydration occurs because the water vapor pressure of supercooled water is higher than that of ice at the same temperature. The force of this vapor pressure difference is so large that even a few degrees of supercooling will result in substantial water loss, continuing until the vapor pressure of body fluids equals that of the surrounding ice. At this stage, the risk of tissue ice formation has been eliminated, and subzero survival is ensured. Here we show that these soil invertebrates do not base their winter survival on supercooling, as do many other ectotherms, but instead dehydrate and equilibrate their body-fluid melting point to the ambient temperature. They can achieve this equilibration even at the extreme cooling rates seen in polar soils.