Growth and Osmoregulation of Chlorella emersonii in NaCl and Neutral Osmotica

Abstract

Cell division of C. emersonii ceased for 21 h following transfer of cells from 1 to 335 mM NaCl [osmotic potential (¿*s) = - 1.64 MPa], while the cells became bigger and the increase in volume of the algal material was reduced by only 30%. After cells had grown for 5-8 days in 335 mM NaCl, relative growth rates for both cell numbers and protein were 30-40 % lower than in 1 mM NaCl. These adapted cells were about fourfold bigger, and contained 3-4.5 times more protein per cell, than cells grown in 1 mM NaCl. Growth was reduced to a similar extent by NaCl and by slowly permeating mannitol and raffinose at a ¿*s of - 1.64 MPa. In contrast, rapidly permeating ethylene glycol (mol. wt 62) inhibited growth to a much smaller extent. Cells grown in NaCl (at a ¿*s of - 1.64 MPa) accumulated proline and to a lesser extent sucrose. Proline and sucrose accumulation was the same in raffinose and NaCl, less in mannitol, and did not occur at all in cells grown in rapidly permeating ethylene glycol. Thus, NaCl exerted few specific ion effects. Furthermore, growth reductions and accumulation of proline and sucrose were somehow related to the required osmoregulation, i.e. the control of turgor potential and/or volume, rather than to low internal water potentials. NaCl did not affect starch levels and exogenous glucose and glutamate did not stimulate growth of cells adapted to 335 mM NaCl. Thus, the growth reductions were presumably due to a restraint on the metabolic machinery, rather than to a lack of organic carbon and nitrogen sources.