Journal of Applied Phycology 13: 493–499, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 493 Effect of salinity on photosynthetic activity of Nodularia spumigena Peter Hobson & Howard Fallowfield ∗ Department of Environmental Health, School of Medicine, Flinders Medical Centre, Flinders University of South Australia, Bedford Park, South Australia, 5042 ( ∗ Author for correspondence; e-mail howard.fallowfield@flinders.edu.au) Received 21 November 2000; revised 6 April 2001; accepted 6 April 2001 Key words: Nodularia spumigena, photosynthesis-irradiance response, salinity Abstract The aim of the study was to determine the influence of total dissolved solids/salinity (mg L -1 TDS) on photo- synthetic activity of Nodularia spumigena strain 001E isolated from Lake Alexandrina, South Australia, using photosynthesis-irradiance (PI) curves. N. spumigena 001E cultures were grown in ASM medium at a range of TDS concentrations (360, 6,600, 13,200, 19,800, 26,400 mg L -1 ) at an irradiance of 30 μmol m -2 s -1 (PAR, 400–700 nm) at 25 ◦ C. The PI relationship was determined at 25 ◦ C for irradiances between 0 and 500 μmol photon m -2 s -1 (PAR). The initial slope of PI curve, α, a function of light harvesting efficiency and photosynthetic energy conversion, decreased proportionally with an increase in salinity from 360 to 26,400 mg L -1 TDS. The maximum rate of photosynthesis (P max ), occurred at 6,600 mg L -1 TDS. No influence of salinity on I k , the irradiance at which P max was measured, or on R d , the dark respiration rate, was identified. Abbreviations: P max – maximum rate of photosynthesis (μgO 2 μg chl a -1 h -1 ); P s – light saturated rate of photosynthesis (μgO 2 μg chl a -1 h -1 ); α – the initial rate of photosynthesis (μgO 2 μg chl a -1 h -1 μmol photon m -2 s -1 ); I k – irradiance at which P max was determined (μmol photon m -2 s -1 ); R d – dark respiration rate (μg O 2 μg chl a -1 h -1 ); TDS – total dissolved solids; β – photoinhibition rate constant (μgO 2 μg chl a -1 h -1 μmol photon m -2 s -1 ); PI – photosynthesis-irradiance; POC – particulate organic carbon Introduction The potential hazards associated with human exposure to cyanobacterial toxins in drinking water supplies and recreational waters have focused research on the en- vironmental factors affecting cyanobacterial growth. There is limited information on the effect of envir- onmental factors on the photosynthetic activity of cyanobacteria and published work refers largely to non-toxin producing species. The relationship between photosynthesis, net gas exchange, and irradiance in cyanobacteria can be described by a non-linear function known as a photosynthesis-irradiance (PI) curve (Geider & Os- borne, 1992). A PI curve provides a rapid measure- ment of the rate of photosynthesis over a range of irra- diance. The relationship between photosynthesis and irradiance can be divided into three regions (Geider & Osborne, 1992). The light-limited region where the photosynthetic rate is proportional to the irradiance intercepted by the photosynthetic apparatus is defined by the constant α. This constant is a function of light harvesting efficiency and photosynthetic energy con- version. Secondly, the light-saturated region where the photosynthetic rate is independent of changes in irra- diance. The maximum rate of photosynthesis (P max ) occurs in this region of the curve at the irradiance I k . The third region of the curve identified is where the photosynthetic rate decreases with increasing irradi- ance resulting in photoinhibition. The dark respiration rate, R d can be determined from the point at which the PI curve intersects the y axis. Equations have