Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 386

protozoa and their bacterial endosymbionts can encyst and be transported
through oxic habitats to colonize other anoxic habitats. Such an associa-
tion of organisms may be responsible for much of the unwanted methane
production that occurs in landfills and it is likely common in anoxic sedi-
ments in all aquatic habitats.
Bacteria are often associated with cyanobacterial heterocysts (Fig.
18.15F). In this case, the photosynthetic cyanobacterium provides fixed
carbon and nitrogen. The bacteria respire and remove O 2 . Lowering the
O 2 tension around heterocysts promotes N2 fixation because nitrogenase
is damaged by O 2 . Thus, both microorganisms benefit from the interaction
(Paerl, 1990).
Macrophytes that are grazer-resistant may benefit from organisms that
remove algae and bacteria from their surface, and the grazers may benefit
from the macrophyte that provides growth substrata for their food and per-
haps protection from predation. Examples of this type of interaction include
snails that remove epiphytic bacteria and algae from Nostoc and possibly
epiphyte grazers that remove epiphytes from the filamentous green alga
Cladophora (Dodds, 1991). Grazing snails are commonly seen removing the
biofilm from macrophytes, and this may be a mutualistic interaction.
CHEMICAL MEDIATION OF MICROBIAL INTERACTIONS
Given that most microorganisms evaluate the environment around
them by sensing chemicals, it is not surprising that many of the interac-
tions among microorganisms (and among macroorganisms; Dodson et al.,
1994) are mediated by chemicals. Many studies exist on chemically medi-
ated interactions, including aspects such as attachment cues, toxic chemi-
cals excreted by bacteria, chemicals that alter morphology, and chemicals
that attract and repel other microbes (Aaronson, 1981a). Chemically me-
diated interactions are documented for all major groups of microorgan-
isms, including the bacteria, protozoa, fungi, algae (Aaronson, 1981b), and
rotifers (Snell, 1998).
Examples of chemically mediated interactions among phytoplankton
species are rare, but such interactions are likely important (Wetzel, 1991).
The work of Keating (1977, 1978), discussed previously, shows how spe-
cific ecological predictions can be made if the details of chemically medi-
ated interactions are documented.
An interesting example of chemically mediated interactions involving
microbes has been described for Daphnia diving in response to fish. It has
been demonstrated that a compound that causes Daphnia to move to
deeper water is excreted by bacteria growing on fish that prey on Daphnia
(Ringelberg and Van Gool, 1998). This positive effect on Daphnia proba-
bly has no effect on the bacteria, but the bacteria have an indirect negative
effect on the fish.
The ramifications of directional water flow on chemically mediated in-
teractions are obvious. Generally, such interactions are more likely to be
reciprocal among species when they occur within the diffusion boundary layer
than when they occur outside it (Dodds, 1990). Thus, Reynolds number,
flow dynamics, and Fick's law can be used to make predictions about the
type of chemically meditated interactions that will evolve in microorganisms.