Effects of temperature and light on the population dynamics of the Asterionella-Rhizophydium association

Abstract

Growth parameters of the diatom Astenonella formosa Hass, and its fungal parasite Rhizophydium planktoniacum Canter emend, were measured at five temperatures and six light intensities with a 15·9 h light:dark cycle, using laboratory cultures of both organisms. With the parameter values obtained, threshold host densities were calculated in order to estimate the effects of light and temperature on survival and epidemic development of the parasite The uninfected host reached light-saturated growth rates between 0.917 day −1 at 21°C and 0 285 day 1 at 2°C. Under light limitation the optimum growth temperature for Asterionella decreased because of a reduced growth efficiency Growth inhibition at high irradiances was only observed at 2°C The parasite reached the highest zoospore production at 2°C and saturating irradiances: 30 2 zoospores per sporangium. This value was consistently reduced by lower irradiances and higher temperatures to only 2.2 zoospores at the opposite light and temperature extremes Low light conditions depressed also the infectivity of the zoospores At very low irradiances, they became completely uninfective The light dependence of zoospore production and infectivity was restricted to light intensities that limited the growth rate of the host. The development time of the sporangia and the mfecti ve lifetime of the zoospores were not affected by light but only by temperature, and ranged from 19.0 and 12 1 days respectively at 2°C to 1.9 and 2 1 days at 21°C- These effects result in optimal conditions for the development of a Rhizophydium epidemic at 11°C and a moderate light limitation of Astenonella At temperature above 7°C, the possibilities for epidemic development are only slightly affected by light and temperature, except for very low irradiance levels, when the zoospores of the parasite become uninfective. However, below 5°C the development of an epidemic is only possible at limiting light levels. Conditions for survival of the parasite at low host densities are optimal at low temperatures and high irradiance levels