This is the first in a series of posts to be made by graduate and undergraduate students to discuss chapters of Ecological Stoichiometry and how they relate to our field work in China, Norway, and Mexico. This post is by Michelle and Angie:
Summary of Chapter 1: Stoichiometry and Homeostasis
To get started, we will go over a few definitions:
Stoichiometry – a branch of chemistry that deals with the application of the laws of definite proportion and conservation of mass and energy.
Ecological Stoichiometry (ES) – the balance of multiple chemical substances in ecological interactions and processes, or the study of this balance.
Homeostasis – ability of most organisms to keep their chemical composition constant, despite changes in the chemical compositions of their environment/food.
Strict Homeostasis – consumer stoichiometry does not vary with resource’s stoichiometry.
Balanced – when considering stoichiometric systems (consumer & resource), they are balanced if they have a similar stoichiometry.
Elemental Imbalance – measure of the dissimilarity in relative supply of an element between an organism and its resource.
The proper mindset for ES
Think of organisms as complex evolved chemical substances that interact with each other and the abiotic world in a way that resembles a complex, composite chemical reaction. When organisms interact mass must be conserved. In other words, no elements are created or destroyed.
Focus on the elements
In order to exists, all life requires the elements C,H,O,N, and P, as well as trace elements. Stoichiometric analysis is concerned with relative abundances. An organism’s physiology must take complex chemical resources containing multiple elements, absorb some, metabolize some, rearrange many, and excrete others. This book is about the way organisms do this and the consequences for ecological dynamics. Consider element content relative to carbon, e.g C:element ratio.
Reiners (1986) provides a logical structure for ecological stoichiometry. He assembled a series of statements ranging from a cellular level to global level into a logical framework using axioms to derive theorems. “ES is not a hypothesis or series of hypotheses but a way of organizing thoughts. It is a hypothesis generating machine, a window to interesting connections in the biotic and abiotic worlds.”
Relationship to work in Norway
In our Norway work, we are interested in understanding how elemental imbalance caused by atmospheric N loading has influenced aquatic organisms in lakes, with focus on the organisms’ need for P. The organisms we will study tend to be more homeostatic than autotrophs/plants, although not strictly homeostatic.
First, we are collecting sediments from lakes at the high and low ends of the N deposition gradient. Michelle is interested in how the supply of excess N has altered the activity of sediment microoganisms. She will conduct experiments where key resources (C, nitrate (NO3-), and P) are given to sediments and measure the activity of a group of microbes – denitrifiers – in response to the resource additions. Based on previous work in Norway, she expects denitrifiers from all lakes to show the greatest response to added NO3-. Also, high deposition lakes may show a response to P during multi-day experiments. Second, we are collecting zooplankton from lakes across the N deposition gradient. Angie will work with Marcin at UiO to analzye the zoops for alkaline phosphotase (AP). AP activity is an indicator of P limitation in zooplankton. We expect that zoops collected from the high N deposition lakes will have greater AP activity than zoops from low N deposition lakes.