Carbon, Nitrogen And Phosphorus Stoichiometry Of A Mixotrophic Protist

Abstract

The microbial loop is one of two major pathways in aquatic systems in which nutrient and energy flow from dissolved nutrients to higher trophic levels. Protozoan-bacteria predator-prey relationships lie at the base of this food web. It is through this interaction that dissolved nutrients sequestered in the bacterial biomass become available to higher trophic levels. Heterotrophic nanoflagellates are the main consumers of bacteria in aquatic systems (Porter et al.1985). Consequently, limitation of dissolved nutrients in the bacterial biomass could effect protozoan stoichiometry and the amounts of excreted waste products. These excreted waste products are a valuable resource to bacteria; therefore protists play a critical role not only in the transfer of nutrients and energy to higher trophic level but also in nutrient resource regeneration. In this work, we explored how bacteria prey (Pseudomonas fluorescens) grown in four different nutrient conditions (Balanced, C-Limited, N-Limited and P-Limited media) effected growth dynamics, nutrient quotas (concentrations of particulate: carbon, nitrogen, and phosphorous) and consequently nutrient recycling when fed to a protozoan predator (Ochromonas danica). Protozoan nutrient quotas are difficult to determine due to the presence of undigested bacteria in protozoan food vacuoles. Knowledge of the bacterial carbon, nitrogen and phosphorus quotas are required to distinguish undigested bacterial biomass from protozoan biomass. Filtration methods were used to separate the protozoan predator from their bacterial prey population to acquire more accurate quota measures. By doing so, four analytical methods were used to calculate the protozoan quotas. Each method revealed a similar tend in that Ochromonas contained the highest element quotas when feeding on P-Limited bacteria, and the lowest element quotas when feeding on C-Limited bacteria. Element ratios based on nutrient concentrations revealed that predator element ratios varied as a linear function of prey element ratios. Estimates of nutrient recycling were based on protozoan and bacterial nutrient quotas and dissolved nutrient measurements. Nitrogen and phosphorus recycling by the flagellate was greatest when feeding on C-Limited bacteria.