Dunaliella salina (Chlorophyceae) Affects the Quality of NaCl Crystals

The green alga Dunaliella salina is a common inhabitant of crystallizers in solar saltworks. Although its presence has been associated with low salt quality, little is known about the mechanism through which Dunaliella causes such effect. It has been suggested that the release of organic matter from healthy D. salina cells interferes with NaCl crystallization. With the intention to cast some light on this matter, we grew D. salina cells at different [NaCl] (0.3, 1.0, 2.0 and 3.0 M), in the presence of either NO₃⁻ or NH4⁺, in conditions that, although strongly hyperaline, were mild enough to minimize cell lysis and substantial spill over of cytosolic sap. The two N treatments were done because the N-source was shown to influence organic release from D. salina. We then attempted to link this physiological information to the information to the quality of NaCl crystals. In order to reduce the impact of inter-crystal inclusion and focus on the structure of the crystalline reticulum, the volume of water used for the production of crystals was as small as possible. Photosynthesis (as chlorophyll variable fluorescence associated with photosystem II), cell organic (as determined by Fourier Transform Infrared spectroscopy, FTIR) and elemental (measured by total reflection X ray fluorescence spectrometry, TXRF) composition were analyzed at the beginning and in the middle of the exponential growth phase, and in the stationary phase. The quality of the crystals obtained from sterile media and from D. salina growth media was studied by optical microscopy (to determine the shape of the crystals) and by X-ray diffractometry.Our results demonstrate that the presence of D. salina cells is directly connected to alterations of NaCl crystalline structure (specifically of crystallite size) and with the quantity and quality of contaminants in the salt.