Osmotic stress tolerance in semi-terrestrial tardigrades

Nanna W T Heidemann,Nadja Møbjerg,Aslak Jørgensen,Lykke K B Clausen,Daniel K Smith,Claus Hélix-Nielsen,Lilian Stapane,Thomas L Hygum

doi:10.1111/zoj.12502

Abstract

Little is known about ionic and osmotic stress tolerance in tardigrades. Here, we examine salt stress tolerance in Ramazzottius oberhaeuseri and Echiniscus testudo from Niva (Denmark) and address whether limno-terrestrial tardigrades can enter a state of quiescence (osmobiosis) in the face of high external osmolyte concentrations. Direct transfers into NaCl solutions showed an upper tolerance level of around 600 mOsm kg−1 in R. oberhaeuseri and 200 mOsm kg−1 in E. testudo. During salt exposures, R. oberhaeuseri contracted into a ‘tun’, whereas E. testudo remained active leaving it more susceptible to acute effects of the ions. Further experiments focused on the more resilient R. oberhaeuseri, which entered a tun and readily regained activity when directly exposed to polyethylene glycol and sucrose of up to 872 ± 0 and 813 ± 3 mOsm kg−1, respectively, revealing a higher tolerance towards non-ionic osmolytes as compared to NaCl. Ramazzottius oberhaeuseri furthermore readily regained activity following gradual increases in non-ionic osmolytes and NaCl of up to 2434 ± 28 and 1905 ± 3 mOsm kg−1, respectively, showing that short-term acclimation promoted salt stress tolerance. Our results suggest that the limno-terrestrial R. oberhaeuseri enters a state of quiescence in the face of high external osmotic pressure and that it, in this state, is highly tolerant of ionic and osmotic stress.

Highlights

Tardigrades are renowned for their abilities to survive in extreme environments and many recent investigations address the limits of their tolerance, for example in relation to dehydration, freezing, heating and radiation (e.g. J€onsson & Bertolani, 2001; Horikawa et al, 2006; J€onsson et al, 2008; Møbjerg et al, 2011; Persson et al, 2011; Wełnicz et al, 2011)
Salts dissociate into ions that build osmotic pressures and osmobiosis would allow tardigrades to enter a state of quiescence in the face of extreme perturbations in external ionic strength and osmotic pressure
We investigate salt stress tolerance in two limno-terrestrial species, namely the eutardigrade Ramazzottius oberhaeuseri (Doyere, 1840) and the heterotardigrade Echiniscus testudo (Doyere, 1840), and address whether limno-terrestrial tardigrades can enter a state of osmobiosis

Summary

INTRODUCTION

Tardigrades are renowned for their abilities to survive in extreme environments and many recent investigations address the limits of their tolerance, for example in relation to dehydration, freezing, heating and radiation (e.g. J€onsson & Bertolani, 2001; Horikawa et al, 2006; J€onsson et al, 2008; Møbjerg et al, 2011; Persson et al, 2011; Wełnicz et al, 2011). The phylum Tardigrada is divided into two main evolutionary lineages, separated by distinct molecular and morphological traits: Heterotardigrada and Eutardigrada (Sands et al, 2008; Jørgensen et al, 2010) Members of both lineages thrive in marine, limnic and terrestrial environments, yet all species need a film of water to be in their active state.

MATERIAL AND METHODS

RESULTS AND DISCUSSION

C NaCl 100