Responses of the benthic diatom Nitzschia ovalis Arnot ex Grunow (strain Mono Lake, California) to changes of salinity in alkaline conditions

Nitzschia ovalis was isolated from Mono Lake, a hypersaline (90ppt) and alkaline (pH = 9.8) water body, and studied for its biochemical responses from 5 to 120ppt salinity. At 150ppt it forms resting spores. The cyclitol 1,4/2,5 cyclohexanetetrol was isolated and identified by NMR and GC-MS. Cyclitol, Proline and Lysine increase in concentration with increasing salinity. The response of N. ovalis to increases in salinity could be divided in four steps: (1) Homeostasis in the intracellular ion concentration (Na+, K+, Ca++ and Mg++) between 5--70ppt. (2) Increase of intracellular ion concentration without causing stress at 90ppt. (3) Saline concentration that causes disruptive stress: Increase in the intracellular ion concentration, the antioxidant DSMP (beta-dimethylsulphonium propionate) and changes in the xanthophyll cycle at 120ppt. (4) Formation of resting spores at 150ppt. The intracellular Na+:Ca++ ratio was maintained over the salinity range. N. ovalis may increase the synthesis of carbonic anhydrase as a response to the increase in salinity as suggested by the increase of Zn++ concentration in the cell. The growth and photosynthesis rates decreased with increasing salinity but the intracellular ionic concentration remained constant over a wide range of the salinity. It is possible that an end product of photosynthesis could be inhibiting it. The growth ceased above of 120ppt. This coincided with the lowest Pmax, the highest intracellular ion concentration, a sudden increase in DMSP (antioxidant), and changes in the xanthophyll cycle. Therefore the combination of low Pmax (possible a low activity of Rubisco) and high intracellular cation concentration at high salinity limited the growth rate. N. ovalis (Mono Lake strain) represents a good model organism for saline studies because it shows a very efficient ion homeostatic mechanism. The photosynthesis rate and intracellular ion concentration are not related over most of the salinity range. Therefore this is an ideal organism with which to study the salinity effect on photosynthesis without the problems on ionic inhibition in comparison to higher plants where the problem of the closing of the stomata at high salinity makes it difficult to separate both effects.