Osmotic adjustment and requirement for sodium in marine protist thraustochytrid

Abstract

A non-invasive ion-selective microelectrode technique was used to elucidate the ionic mechanisms of osmotic adjustment in a marine protist thraustochytrid. Hypoosmotic stress caused significant efflux of Na + , Cl -and K + from thraustochytrid cells. Model calculations showed that almost complete osmotic adjustment was achieved within the first 30 min after stress onset. Of these, sodium was the major contributor (more than half of the total osmotic adjustment), with chloride being the second major contributor. The role of K + in the process of osmotic adjustment was relatively small. Changes in Ca 2+ and H + flux were attributed to intracellular signalling. Ion flux data were confirmed by growth experiments. Thraustochytrium cells showed normal growth patterns even when grown in a sodium-free solution provided the medium osmolality was adjusted by mannitol to one of the seawater. That suggests that the requirement of sodium for thraustochytrid growth cycle is due to its role in cell osmotic adjustment rather than because of the direct Na + involvement in cell metabolism. Altogether, these data demonstrate the evidence for turgor regulation in thraustochytrids and suggest that these cells may be grown in the absence of sodium providing that cell turgor is adjusted by some other means.

An estuarine species of the alga Vaucheria (Xanthophyceae) displays an increased capacity for turgor regulation when compared to a freshwater species

“…Based on this, the inhibition of turgor recovery in our experiments could be due to the inhibition of Na + uptake through NSCC, considering that Na + uptake was shown to be the major contributor to cell turgor in a marine species of the stramenopile, Thraustochytrium (Shabala et al. ).…”

Screening for Salt Tolerance in Eight Halophyte Species from Yellow River Delta at the Two Initial Growth Stages

“…In addition, salt stress generated an increase in reactive oxygen species (ROS), which leaded to decreases in plant photosynthetic capacity [38]. Despite the absence of data of salt stress effects on leaf photochemistry in this study, the fact that NaCl treatment caused a very significant reduction in net CO 2 assimilation, being closely correlated with biomass and plant height, has been proven [39]. Therefore, it appeared that stomatal limitation of photosynthesis was an important factor reducing halophytes growth in saline conditions.…”

Cadmium has contrasting effects on polyethylene glycol – Sensitive and resistant cell lines in the Mediterranean halophyte species Atriplex halimus L.

“…As far as cell lines are concerned, previous study showed that the maintenance of PEG-resistant cell line on a medium completely free of Na induced a PEG-sensitive phenotype, while an exogenous application of Na increased the ability of PEG-sensitive cell line to cope with water stress (Martínez et al, 2005). Sodium was recently shown to play a key role in osmotic adjustment in some lower marine fungi (Shabala et al, 2009) but such a function is unlikely in our cell lines since exogenous NaCl induces only a small decrease inCs (despite a strong increase in endogenous Na) comparatively to Cd in the absence of NaCl (which only slightly increased endogenous Na) (Tables 1 and 2). Small amounts of Na are required for efficient photosynthesis in C4 plant species, mainly in relation to phosphoenolpyruvate regeneration, but this should not be considered in our heterotrophic cell lines maintained in dark conditions and further works are thus required to identify the specific functions of Na in A. halimus.…”