Estimating the impact of fungal parasites on phytoplankton populations

Abstract

A population-dynamic model is formulated to estimale the impact of chytrid and related fungal parasites on phytoplankton populations. The specific loss rate of uninfected host cells due to infection is used as a measure of the impact. Calculating this loss rate requires information about four parameters: prevalence of infection (i.e. the proportion of the host cells infected), development time of the sporangia of the parasite, specific growth rate of the uninfected host, and difference between loss rates of infected and uninfected host cells due to loss factors other than parasitism. When only cell counts of the host population are available instead of growth and loss rate values, the model can be used to calculate a minimum estimate of the impact of the parasite. The impact of the parasite at a given prevalence of infection can vary considerably as a consequence of environmental effects on the latter three parameters. Therefore, prevalence is not a good parameter to judge the severity of fungal epidemics. A phytoplankton population starts to decline when prevalence exceeds a certain threshold value. This critical prevalence of infection can be calculated with the model and is also highly dependent on external conditions.The model is applied to epidemics of the chytrid Rhizophydium planktonicum Canter emend. in ,a population of the diatom Asterionella formosa Hass. in Lake Maarsseveen 1. The end of both the spring and summer 1984 bloom of the diatom was caused by epidemics of tlhe chytrid. Model results suggested also that higher sinking rates, caused by an increasing proportion of dead cells in the colonies, resulted in an additional loss factor due to parasitism for this colcnrial diatom.

Food uptake and fine structure of Cryothecomonas longipes sp. nov., a marine nanoflagellate incertae sedis feeding phagotrophically on large diatoms

“…Evidence is increasing that most of the common and bloom-forming diatoms in the North Sea are prey to these small predators. Thus, the nanoflagellates play an important but often underestimated role in the control of phytoplankton populations (Sommer 1994) as do fungal infections (Bruning et al 1992). …”

Parasitism of photosynthetic dinoflagellates in a shallow subestuary of Chesapeake Bay, USA

“…Lytic viruses are now thought to play a major role in the mortality of marine bacteria and phytoplankton, with the potential of causing population level changes in host organisms on time scales of hours to days (Bratbak et al 1993, Nagasaki et al 1994, Suttle 1994, Suttle & Chan 1995. Similarly, fungal parasites of freshwater diatoms and dinoflagellates have been implicated in mass mortalities of host organisms, suppression or retardation of phytoplankton blooms, and selective effects on species composition leading to successional changes in plankton comnlunities (Canter & Lund 1948, Reynolds 1973, Youngman et al 1976, Van Donk & Ringelbei-g 1983, Sommer et al 1984, Heaney et al 1988, Kudoh & Takahashi 1990, Bruning et al 1992. Fewer studies have considered protozoan parasites of planktonic organisms, but at least 1 group of protists, the parasitic dinoflagellates, appears sufficiently common and widespread to be of ecological significance (Drebes 1984, Cachon & Cachon 1987.…”

Effects of two strains of the parasitic dinoflagellate Amoebophrya on growth, photosynthesis, light absorption, and quantum yield of bloom-forming dinoflagellates

“…Resale or republication not permitted without written consent of the publisher mura et al 1995, Kamiyama 1997, Jeong et al 1999a,b, Matsuyama et al 1999, and eukaryotic parasites including fungi and certain flagellates (Taylor 1968, Bruning et al 1992, Coats 1999, Norén et al 1999, ErardLe Denn et al 2000 all have the potential to exert controlling influences on HAB species.…”