INFECTION OF THE DIATOM ASTERIONELLA BY A CHYTRID .1. EFFECTS OF LIGHT ON REPRODUCTION AND INFECTIVITY OF THE PARASITE

The influence of light limitation of the diatom Asterionella formosa Hass, on the growth-determining parameters of its fungal parasite Rhizophydium planktonicum Canter emend, was measured, using laboratory cultures of both organism. The experiments were carried out at 6-degrees-C under a 15:9 h light-dark cycle. At saturating light conditions, the mean zoospore production of the parasite was 23.4 zoospores sporangium-1, and the mean development time of the sporangia was 7.9 days. Light limitation of the host caused a substantial decrease of the zoospore production, while the development time was only slightly reduced. The improved zoospore production at high light intensities was mainly the result of incorporation of photosynthetic products generated by the host after infection. Under limiting light conditions, Asterionella cells were less susceptible to infection with fungal zoospores. No infection at all occured below 2-mu-E m-2 s-1, a light intensity that still supported some algal growth. The maximum infection rate indicated that chemotactic attraction of the parasite's zoospores by extracellular products of the host is involved. The infective lifetime of the zoospores of the parasite did not depend on light conditions, and was estimated at 8 days. The measured zoospore production rates, both under limiting and saturating light conditions, enable the parasite to exceed the specific growth rate of the host, and thus become epidemic, at sufficiently high host densities.