Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 367

native environment have been developed only recently, so we are just be-
ginning to study microbial communities from classical ecological view-
points (White, 1995). In this chapter, I describe aspects of the ecology of
microbes; Chapters 19 and 20 explore the ecology of macroscale plants
and animals. This chapter begins with a discussion of motility, provides a
general classification of interaction types, and then discusses species inter-
actions in microbial communities and how macroscopic organisms interact
with microscopic organisms.
BEHAVIOR OF MICROORGANISMS
The behavioral ecology of microbes is relatively simple. The ability to
control position in the environment is essential to the survival of many mi-
crobial species. The cues microorganisms use to move and the modes of
motility are discussed in the following sections.
Motility
Several modes of motility occur among microbes. In the bacteria, sim-
ple flagella are used for locomotion. The molecular biology of flagellar
motility has been well characterized for Escherichia coli (Glagolev, 1984;
Koshland, 1980), but other mechanisms are less understood. Cyanobacte-
ria have no flagella, but they are capable of gliding on solid substrata. In
addition, gas vacuoles allow bacteria and cyanobacteria to float or sink.
Behavioral aspects of this motility will be discussed later.
The amoeboid protozoa and Euglena are capable of moving across
solid surfaces by changing the shape of their cells. Other eukaryotic mi-
crobes, including diatoms and desmids (a green alga), can glide across solid
surfaces; the exact mechanisms for this gliding are poorly understood. Eu-
karyotic microorganisms also use a variety of strategies to swim through
open water, including flagella (protozoa and algae), paddles or other swim-
ming appendages in larger multicellular organisms such as Cladocera, or
by undulating their body (Copepoda).
In general, the copepods are the aquatic organisms capable of the
greatest relative speeds (200 body lengths per second), with the flagellated
bacteria coming in a distant second (Table 18.1). Although fish are capa-
ble of the greatest absolute speeds, they are less impressive when velocity
is scaled to body size. It is even more amazing that microbial species are
so fast given that viscosity is great at low Reynolds numbers.
Taxis
Moving toward or away from objects or environmental stimuli is an
important part of many organisms' survival. Many strategies are used, with
more complex strategies common in more complex animals. Any strategy
requires determining the relative change in signal and coupling this to pas-
sage of time or change in position. Movement of organisms in response to
stimuli is called taxis. Chemotaxis, phototaxis, and magnetotaxis are move-
ments of organisms stimulated by chemicals, light, and magnetic fields, re-