Research interests

How do populations evolve to cope with changing biotic and abiotic environments? What traits are affected by adaptation to (for example) temperature? How do trait correlations promote or constrain adaptation to novel environments? Broadly, I am interested in the evolutionary ecology of microbes (primarily phytoplankton) in the context of the ecological community. I use ecological and evolutionary theory to inform empirical trait-based approaches to the study of microbial adaptation to novel temperature, nutrient, light, and zooplankton grazer environments. My research relies heavily on experimental evolution, ecological (e.g. competition) and physiological experiments, and meta-analysis of (co)variation in microbial physiological traits.

Evolutionary responses of marine diatoms to temperature selection
My primary project at present focuses on evolutionary adaptation of the marine diatom Thalassiosira pseudonana to temperatures above and below its thermal optimum. I culture T. pseudonana at the lower and upper extremes of its thermal niche and track the evolution of traits such as the lower critical temperature, upper critical temperature, thermal optimum, thermal niche width, and also physiological traits associated with growth and nutrient uptake kinetics, over time. Recently, we detected significant evolutionary change in thermal the optima of several of our experimentally evolving cultures (see "Big News", left)!

I use competition experiments to assess relative fitness of evolved v. ancestral strains of T. pseudonana at a given temperature, and also of strains evolved at different temperatures. A major innovation facilitating these experiments is the competition chamber (below), which uses a semi-permeable, polycarbonate membrane to separate populations, allowing them to compete for a common resource pool without coming into direct contact with one another.

Covariation of microbial thermal traits across the thermal tolerance range of life on Earth

I am currently working on a meta-analysis of microbial thermal traits (thermal optimum, upper/lower critical temperatures, niche width) across the entire range of temperatures at which life is known to exist. In particular, I am interested in how these traits co-vary, and how variation and covariation differ among various functional and taxonomic groups (e.g. marine and freshwater phytoplankton, bacteria, archaea). I will eventually incorporate phylogenetic techniques to elucidate historical patterns in thermal trait evolution. I presented my preliminary findings at the 2015 Astrobiology Science Conference in Chicago.

Link to AbSciCon 2015 poster

Danny O'Donnell examining a dense culture of Thalassiosira pseudonana in the laboratory at Kellogg Biological Station.

A hypothetical phytoplankton thermal performance curve (population growth rate as a function of temperature). Four traits can be quantified from this curve: 1. the thermal niche breadth; 2. the thermal optimum; 3. the lower critical temperature; and 4. the upper critical temperature.

Thermal performance curves for T. pseudonana strain 1335 selected at (black) 16 °C and (red) 31 °C for ~350 generations. Each curve represents a single replicate selection line. All selection lines were acclimated for at least 10 generations (2 weeks) at the assay temperature. Curves are the Norberg equation, fit by maximum likelihood estimation (Nelder-Mead method).

SEM of Thalassiosira pseudonana. Photo taken by N. Kröger, Alfred Wegener Institut, Germany. Color polarity has been reversed relative to original, and the scale bar added. Link to original photo

The competition chamber. The chamber consists of two 50 ml tissue culture flasks, each with a side removed. Separating the open sides is a 1μ polycarbonate filter membrane, fixed in place using silicone aquarium sealant.

Copyright 2011 by Daniel R. O'Donnell. All rights reserved. Last modified: GMT