The Cell Division Rates of Some Marine Phytoplankters as a Function of Light and Temperature

Abstract

An apparatus is described for exposing phytoplankton cultures to 64 different combinations of light and temperature. The light was of a known spectral energy resembling sunlight after passage through a few metres of clear coastal sea water and was measured in the energy units of cals/cm²/min (langlies/min or ly/min). The temperature range used varied in each experiment, the range of illumination was from about 0.005 to 0.4 ly/min, the latter being equivalent to bright summer sunlight at a depth of only a few metres in the sea.The growth of five bacteria-free cultures, Dunaliella tertiolecta, Amphidinium carteri, Monochrysis lutheri, Skeletonema costatum and Thalassiosira nordenskiöldii was followed for a period of 3–5 days using a cycle of 16 hr illumination and 8 hr of darkness every 24 hr. Growth was measured by counting cell numbers and the rate constants for cell division were evaluated at standard times.The resulting growth-response curves are reported and discussed in relation to their ecological significance. Except for T. nordenskiöldii cell division was relatively insensitive to temperature over a range exceeding 10 °C. Cell division showed a "stress" response, the growth rate being more susceptible to extremes of illumination when the cells were near to the extremes of temperature for a given species and to extremes of both temperature and light if the chemical nature of the medium was unsatisfactory. At optimum temperatures there was no inhibition of cell division at intensities of 0.4 ly/min of photosynthetically active light.