ORGANIC CARBON RELEASE BY DUNALIELLA SALINA (CHLOROPHYTA) UNDER DIFFERENT GROWTH CONDITIONS OF CO2, NITROGEN,AND SALINITY1

Two strains of Dunaliella salina (Dunal) Teod., UTEX 1644 and UTEX 200, were cultured under different growth regimes, including 10 mM NO₃^? or NH₄⁺, 1.5 or 3.0 M NaCl, and low (0.035%) or high (5%) CO₂ in air. The release of ¹⁴C-labeled dissolved organic carbon (DOC), expressed as a rate and as a percentage of photosynthetic ¹⁴CO₂ assimilation, was subsequently determined. The percentage of DOC released was inversely related to cell density in the assay medium, but photosynthesis on a per-cell basis was not. Release of DOC was low, in the range of 1–5% of photosynthesis, but during acclimation to growth on NH₄⁺, it rose to 11%. The presence of NH₄⁺ rather than NO₃^? in the growth medium increased the rate of release by both strains, but the percentage release was stimulated only in UTEX 200 cells, because their photosynthetic rate was depressed by NH₄⁺. For UTEX 1644, high, as compared to low, CO₂-grown cells, had somewhat higher rates and percentages of DOC release, but release from UTEX 200 cells was unaffected by the growth-CO₂. The rate of DOC release by high CO₂-grown cells was not enhanced at a low concentration of dissolved inorganic carbon, indicating that the released material did not originate from the photorespiratory pathway. The effects of NaCl on DOC release varied with strain and growth conditions. For UTEX 200, the cells in NO₃^?, but not NH₄⁺, exhibited a doubling or more in percentage of release with a doubling in NaCl concentration, irrespective of growth-CO₂. With UTEX 1644 the low CO₂-grown cells showed the greatest enhancement in 3.0 M NaCl. Organic matter accumulated on the external surface of the cell membrane and constituted a well-defined cell-coat, which was more dense in NH₄⁺ than in NO₃^?-grown cells. Microtubules, which may play a role in maintaining cell shape, were observed just below the plasma membrane. From a practical viewpoint, the presence of organic material in the hypersaline ponds of salt-works is detrimental to salt production. When D. salina cells become abundant in such ponds, the attendant, continuous release of DOC may make a significant contribution to the problem.