Evaluation of Daphnid Grazing on Microscopic Zoosporic Fungi by Using Comparative Threshold Cycle Quantitative PCR

Lethal parasitism of large phytoplankton by chytrids (microscopic zoosporic fungi) may play an important role in organic matter and nutrient cycling in aquatic environments by shunting carbon away from hosts and into much smaller zoospores, which are more readily consumed by zooplankton. This pathway provides a mechanism to more efficiently retain carbon within food webs and reduce export losses. However, challenges in accurate identification and quantification of chytrids have prevented a robust assessment of the relative importance of parasitism for carbon and energy flows within aquatic systems. The use of molecular techniques has greatly advanced our ability to detect small, nondescript microorganisms in aquatic environments in recent years, including chytrids. We used quantitative PCR (qPCR) to quantify the consumption of zoospores by Daphnia in laboratory experiments using a culture-based comparative threshold cycle (CT) method. We successfully quantified the reduction of zoospores in water samples during Daphnia grazing and confirmed the presence of chytrid DNA inside the daphnid gut. We demonstrate that comparative CT qPCR is a robust and effective method to quantify zoospores and evaluate zoospore grazing by zooplankton and will aid in better understanding how chytrids contribute to organic matter cycling and trophic energy transfer within food webs.

The study of aquatic fungi is often complicated by the fact that they possess complex life cycles that include a variety of morphological forms. Studies that rely on morphological characteristics to quantify the abundances of all stages of the fungal life cycle face the challenge of correctly identifying and enumerating the nondescript zoospores. These zoospores, however, provide an important trophic link between large colonial phytoplankton and zooplankton: that is, once the carbon is liberated from phytoplankton into the parasitic zoospores, the latter are consumed by zooplankton and carbon is retained in the aquatic food web rather than exported from the system. This study provides a tool to quantify zoospores and evaluate the consumption of zoospores by zooplankton in order to further our understanding of their role in food web dynamics.