Effects of salinity, pH, organic solutes, anaerobic conditions, and the presence of other microbes on production and survival of Lagenidium giganteum (Oomycetes: Lagenidiales) zoospores

The effects of several environmental factors on the production of viable zoospores by Lagenidium giganteum were determined by counts of germlings produced after induction of zoosporogenesis by suspension of mycelium in various substances. NaCl at a concentration of 0.6 g/liter virtually eliminated zoosporogenesis. At 0.2 g/liter NaCl there was a significant reduction in four of the five fungal isolates tested. Transmission of fungus between mosquito larvae at 0.8 g/liter NaCl suggests that estimates of solute effects from in vitro studies are exaggerated. Zoosporogenesis took place from pH 4.5 to 8.4 by three isolates and from 4.5 to 8 by two other isolates (±0.2). Anaerobic conditions halted zoosporogenesis. Reintroduction of oxygen within 7 days allowed the process to resume, but with reduced production for each day under anaerobic conditions. Five species of bacteria inhibited zoosporogenesis at concentrations of 10⁷–10⁸ cells/ml. The yeast, Saccharomyces cerevisiae, was inhibitory at 10⁶ cells/ml. Three sugars, a sugar alcohol, three amino acids, and peptone all reduced zoosporogenesis at levels that generally correlated positively with their nutritional values for the fungus. Peptone, which can support lush vegetative growth of L. giganteum in the absence of other nutrients, was far more effective in inhibiting zoosporogenesis than the other solutes.