Changes in N:P Supply Ratios Affect the Ecological Stoichiometry of a Toxic Cyanobacterium and Its Fungal Parasite.

Thijs Frenken,Alena S Gsell,Joren Wierenga,Ellen Van Donk,Dedmer B Van De Waal,Thomas Rohrlack

doi:10.3389/fmicb.2017.01015

Abstract

Human activities have dramatically altered nutrient fluxes from the landscape into receiving waters. As a result, not only the concentration of nutrients in surface waters has increased, but also their elemental ratios have changed. Such shifts in resource supply ratios will alter autotroph stoichiometry, which may in turn have consequences for higher trophic levels, including parasites. Here, we hypothesize that parasite elemental composition will follow changes in the stoichiometry of its host, and that its reproductive success will decrease with host nutrient limitation. We tested this hypothesis by following the response of a host–parasite system to changes in nitrogen (N) and phosphorus (P) supply in a controlled laboratory experiment. To this end, we exposed a fungal parasite (the chytrid Rhizophydium megarrhizum) to its host (the freshwater cyanobacterium Planktothrix rubescens) under control, low N:P and high N:P conditions. Host N:P followed treatment conditions, with a decreased N:P ratio under low N:P supply, and an increased N:P ratio under high N:P supply, as compared to the control. Shifts in host N:P stoichiometry were reflected in the parasite stoichiometry. Furthermore, at low N:P supply, host intracellular microcystin concentration was lowered as compared to high N:P supply. In contrast to our hypothesis, zoospore production decreased at low N:P and increased at high N:P ratio as compared to the control. These findings suggest that fungal parasites have a relatively high N, but low P requirement. Furthermore, zoospore elemental content, and thereby presumably their size, decreased at high N:P ratios. From these results we hypothesize that fungal parasites may exhibit a trade-off between zoospore size and production. Since zooplankton can graze on chytrid zoospores, changes in parasite production, stoichiometry and cell size may have implications for aquatic food web dynamics.

Highlights

Human activities have substantially increased the flux of nutrients from land into receiving waters (Smith, 2003)
This may be a result of an imbalanced nutrient supply (Carpenter et al, 1996; Sterner et al, 2007), as well as an increased nutrient demand associated to high phytoplankton densities (Carpenter et al, 1996)
Nutrient limitation will alter the elemental composition of phytoplankton, and may increase carbon:nutrient ratios (Sterner and Elser, 2002)

Summary

Introduction

Human activities have substantially increased the flux of nutrients from land into receiving waters (Smith, 2003) This nutrient enrichment enhances aquatic primary production, and may lead to dramatic changes in the composition and structure of aquatic food webs (Schindler and Fee, 1974; Smith et al, 2006). Nutrient loading has increased, primary production in aquatic ecosystems is often still limited by nitrogen (N) and/or phosphorus (P) (Elser et al, 2007; Bracken et al, 2014). Nutritional quality of the phytoplankton decreases, thereby possibly constraining higher trophic levels (Sterner and Elser, 2002; Hessen et al, 2013) This may apply to parasites that solely depend on their host as a food source (Smith, 2007)

Methods

Results

Conclusion