FUNGAL CONTROL OF POPULATION CHANGES OF THE PLANKTONIC DIATOM ASTERIONELLA FORMOSA IN A SHALLOW EUTROPHIC LAKE¹

Abstract

Parasitic fungi infected an Asterionella formosa Hass. population for 10 months of the year, during which time the population density of the alga was over 100 cells‐L−1. High injection rates of more than 20% of the cells were observed between February and July except on a few occasions such as late April and late June. Mortality of infected diatom cells was temperature dependent in dialysis tube culture experiments; the loss rate of field populations due to fungal injection also was affected by temperature. Loss rates of diatom cells were negligibly small m February even though infection percentages were more than 20%, but increased toward summer with an increase m temperature. High infection percentages and correspondingly high loss rates (e.g. 0.3 d−1) occurred periodically during the summer. The results suggest that this type of host‐parasite interaction is an important control of algal population size in natural waters.

Parasitic chytrids: their effects on phytoplankton communities and food-web dynamics

“…These fluctuations have been partly explained by abiotic factors, such as temperature (Barr & Hickman, 1967a;Masters, 1971a;Blinn & Button, 1973;Van Donk & Ringelberg, 1983;Sen, 1987a;Kudoh & Takahashi, 1990), light (Barr & Hickman, 1967a;Blinn & Button, 1973;Abeliovich & Dikbuck, 1977;Kumar, 1978b;Canter & Jaworski, 1979Bruning, 1991b, d;Kagami & Urabe, 2002), nutrients (Bruning, 1991a), pH (Sen, 1987a), turbulence (Doggett & Porter, 1996;Kuhn & Hofmann, 1999), and biotic factors, such as Daphnia grazing on fungal zoospores . There are two contrasting statements concerning the necessary environmental conditions for the development of a fungal epidemic.…”

“…This may be explained by the fact that several environmental parameters commonly change simultaneously in aquatic systems. In addition, host density effects may override the effects of other factors (Kudoh & Takahashi, 1990). Besides, several factors affect host-parasite dynamics in complex ways (Sen, 1988a;Bruning, 1991b;Doggett & Porter, 1996).…”

“…It should also be noted that host cell mortality was not proportional to the fungal infection rate, and that mortality effects of the parasite varied according to environmental factors such as temperature (Kudoh & Takahashi, 1990). Most previous studies have focused primarily on fungal dynamics (infection rates), and did not measure the mortality rate of the host algae (population decline rates).…”

Use of Calcofluor White for Detection, Identification, and Quantification of Phytoplanktonic Fungal Parasites

“…Some of them have been misidentified as protistan bacterivorous nanoflagellates, e.g., flagellates in the group of Bicosoeca, which are attached to phytoplankton but do not harm the algae (31-33). A few studies have demonstrated epidemic outbreaks of chytrids on phytoplankton, but mainly with particular emphasis on host-parasite interactions and coevolution (21,30,48). Thus far, observations of parasitic fungi were obtained by using phase-contrast light microscopy with live or Lugol's iodine preserved samples (30,39,48).…”

“…A few studies have demonstrated epidemic outbreaks of chytrids on phytoplankton, but mainly with particular emphasis on host-parasite interactions and coevolution (21,30,48). Thus far, observations of parasitic fungi were obtained by using phase-contrast light microscopy with live or Lugol's iodine preserved samples (30,39,48). Such conventional microscopy allows observation of fungal sporangia or similar forms (especially in laboratory cultures) but is a poor approach for characterizing chytrid parasites in natural samples, at the complex community level.…”