Journal Club - Warming on chytrid infections in lake phytoplankton

To start this fall's journal club, I could not resist introducing my colleagues to the fantastic world of parasites... To fit the interests of my research group at LOG, I decided to present the work done by Frenken et al. (2016 Global Change Biology) on the role of water temperature on chytrid parasites epidemics in phytoplankton spring blooms.

Chytrids are fungi mostly infamous for contributing to the worldwide decline in amphibians. However, most chytrid species actually live in the oceans or in freshwater ecosystems, where they infect phytoplankton. Their life-cycle involves a free-living stage, the zoospore, which is infective to host cells to which is attaches in order to reproduce.

Using mesocosms, the authors investigated how 4°C rises in water temperature could influence phytoplankton spring bloom through its effects on one of its major factors of termination, chytrids. After introducing a range of organisms (including zooplankton, bacteria, phytoplankton and the parasites) filtered from pond water, they followed the plankton community over 4 months, including weekly samples of the phytoplankton community.

Their results indicate that warming affected chlorophyll-a concentration in the mesocosms so that values were generally higher in the control treatment reflecting regular temperatures, with a lower bloom maximum in the warm treatment. Earlier bloom termination in warm waters were associated with a more rapide increase in chytrid prevalence in the phytoplankton community. A rapide decline in prevalence was also found at warm temperatures to be associated with low host cell densities. However, this led to a longer persistence of the bloom and of the epidemic.

Changes in the water chemistry during the experiment also led the authors to question the effects of phosphorus limitation for chytrid epidemics, suggesting that high C:P ratio may have constrined the growth of the parasite. Importantly, their results also indicate that at warmer temperatures, zooplankters such as rotifers have an early development time and that under these circumstances, zooplankter population growth follows the rise of chytrid prevalence in the phytoplankton community. Because zoospores can be preyed upon by a number of organisms composing the zooplankton, it seems likely that these organisms used chytrid zoospore as an alternative food of resources at warm temperatures.

Overall, their results stress the need to understand the complex effects of global warming on phytoplankton bloom dynamics and emphasize the importance of considering parasites in such questions.





source for picture: http://cfb.unh.edu/phycokey/Choices/Anomalous_Items/fungi/Chytrid_05_500x317_on_Ast.jpg