Enumeration of Parasitic Chytrid Zoospores in the Columbia River via Quantitative PCR

Through lethal infection, fungal parasites of phytoplankton ("chytrids") repackage organic material from the large, effectively inedible, colonial diatoms they infect into much smaller zoospores, which are easier for zooplankton to consume. However, their small size and lack of distinguishing morphological features render it difficult to distinguish zoospores from other small flagellates in mixed assemblages in the natural environment. In this study, we developed and tested a method to quantify chytrid zoospores in field studies using quantitative PCR (qPCR) targeting the internal transcribed spacer 2 (ITS2) region within the rRNA gene cluster. To achieve accurate quantification, the assay was designed to be highly specific for a parasite (Rhizophydium planktonicum) of the diatom Asterionella formosa; however, the approach is applicable to additional host-parasite systems. Parasitic zoospores were detected and quantified in the freshwater reaches of the lower Columbia River, as well as in the salt-influenced estuary and river plume. The coincidence between zoospore abundances and a prevalence of small zooplankton during blooms of large, colonial diatoms in the spring suggests that Columbia River zooplankton may be poised to benefit nutritionally from chytrid zoospores, thus providing a mechanism to retain organic carbon within the system and reduce losses to downstream export. We estimate that ∼15% of the carbon biomass tied up in blooms of the dominant diatom species is transformed into zoospores through the parasitic shunt during spring.

The small size of the parasitic fungi that infect phytoplankton makes it difficult to identify and quantify them in natural systems. We developed and tested a method to quantify these organisms (chytrid zoospores) using a molecular technique that targets the internal transcribed spacer region within the rRNA gene cluster. Using this method, we quantified the abundance of the motile stage of a specific parasite in the freshwater and saltwater-influenced regions of the Columbia River in the U.S. Pacific Northwest. Parasitic chytrid zoospores were found to be present throughout the year and at higher abundances during the spring, when phytoplankton blooms occur. The presence of these organisms indicates not only that they may be responsible for the death of host phytoplankton cells but that they may also provide a readily available food source to small consumers (zooplankton) in the food web of the Columbia River.