The major focus of research in my laboratory is upon application of the principles of ecological stoichiometry to microbial food webs.
Element stoichiometry of a model freshwater bacterium as a function of temperature and growth rate (determined by its C:N:P ratio)
The macromolecular composition of bacteria shifts as an exponential function of growth rate. When normalized to cell mass (as dry weight), protein content decreases slightly as growth rate increases, DNA remains essentially constant and RNA content increases.
It has been well established that there is a close correlation between RNA content and growth rate, and that the variation in RNA content is due almost exclusively to changes in ribosome content. RNA is approximately 9% P (by wt). Therefore we can expect that rapidly growing cells have an increased demand for P and that the cellular C:P ratio would fall as growth rate increases.
In their classic paper, Schaechter et al. (1958) reported that, for “a given medium, cell size and composition are almost independent of growth temperature. The characteristics of the cells would therefore seem to be determined primarily by the pattern of biochemical activities imposed by the medium.”
This position argues that the cellular stoichiometry of an organism would be the same when grown in any combination of resources that allows the organism to attain a specific growth rate. In ecological stoichiometry terminology, the cells are homeostatic.
However, Tempest and Hunter (1965) used chemostats to vary temperature independently of growth rate. They found that the macromolecular composition of cells changes as a function of temperature. For a given growth rate, RNA content increases as temperature decreases.
The physiology behind this finding appears straightforward; the rate at which a ribosome processes information is constant, therefore to maintain a given growth rate in the face of decreasing temperature and polymerizing activity, ribosome content must increase.
Working from their data and making the conservative assumption that C is 50% of the dry weight of a cell and that P is 9% of RNA, then C:P ratios are found to fall dramatically with a 15 degree change in temperature and that the nature of the limiting substance influences the cellular stoichiometry.
The feeding behavior of protozoan predators is altered by the food quality of prey
Food quality has been shown to influence greatly the rate at which metazoan herbivores exploit prey (Sterner et al. 1993). Daphnia, feeding upon Scenedesmus whose element composition was altered by regulating growth conditions, was found to have lower clearance and feeding rates when food quality was poor.
Lower clearance rates and feeding rates were associated with longer gut-passage times for poor-quality food-items. Similar predator-prey associations exist in microbial food webs where protozoan predators feed almost exclusively upon bacterial prey.
However, analogous studies of predator-prey food quality interactions are lacking due to the technical difficulty working with protozoans that are mixotrophic or obligate bacterivores. The flagellate protozoan Ochromonas may be grown heterotrophically and will grow exclusively on bacteria. In many varieties, autotrophic growth is exceptionally poor.
In this work, axenic Ochromonas danica is fed bacterial prey (Pseudomonas fluorescens) of known food quality (as C:N:P ratio). Ingestion, digestion, and clearance rates are determined for the protozoan during short term feeding experiments. Ultimately, we seek to describe the relationships among food quality, feeding behavior, growth rate, and element excretion for this predator-prey couple. These data will be extended into multi-element predator prey models.
Nutrient recycling by predators affects the resource base of their prey and thereby exerts indirect effects of competition.
The relationships among population dynamics, species interactions, and nutrient dynamics have long been of interest to ecologists.
Some two decades ago, Pomeroy (1976) established that active populations of heterotrophic bacteria and protozoa contribute significantly to biomass and secondary productivity. Azam et al. (1983) further contradicted then prevailing views, by suggesting that heterotrophic bacterioplankton consume dissolved inorganic nutrients, instead of mineralizing them from organic forms.
Placed in direct competition with bacteria for nutrients, phytoplankton would suffer growth limitation that might be partially relieved by nutrients recycled through protozoa that prey on bacteria. Since then, the view has emerged that competition for nutrients is sharpest in nutrient-poor systems, favoring competitively superior bacteria, and decreasing the relative abundance of algae.
Bacterial losses to flagellates, or perhaps to viruses, are likely higher in productive systems, with high rates of nutrient recycling that enhance the abundance of algae. The dominance of bacteria that are largely inedible to planktonic metazoa in nutrient poor systems could then impoverish grazing food chains and reduce export production.
Under this paradigm predation on bacteria and the associated nutrient recycling control the critical issue of whether energy and nutrients dissipate within a microbial food web, or flow to higher organisms.
This research employs simple microbial foods webs to illuminate mechanisms and processes structuring open-water ecosystems. Our simplified food webs use two bacterial prey species and a single protozoan predator in replicated two-stage chemostat systems to address prey selection and nutrient regeneration by the protozoan predator.
Bacterial prey are grown in the first stage of the coupled chemostat system. The growth conditions are set to create cells of known food quality (element composition).
These cells are subsequently fed to a protozoan predator in the second stage of the coupled chemostat system and, through mass-balance approaches, the element composition of the predator and element recycling is determined.
During more advance studies, the predator will be supplied with two prey species of differing food quality (element composition) and prey-selection and nutrient regeneration determined.