Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 385

FIGURE 18.15 Some aquatic organisms involved in mutualistic interactions; The pouch of
the water fern Azolla (A) that contains endosymbiotic Nostoc (B), (C) Nostoc parmeliodies
containing the midge Cricotopus nostocicola, (D) Hydra with endosymbiotic Chlorella,
(E) the diatom Epithemia with cyanobacterial endosymbionts (courtesy of Rex Lowe), and
(F) bacteria attached to a heterocyst of Anabaena flos-aquae (courtesy of Hans Paerl).
carbon source that would become toxic and limit the other. These interac-
tions are well described by Fenchel and Finlay (1995). A specific example
of syntrophy is interspecies H 2 transfer in which hydrogen-generating mi-
crobes facilitate activity of methanogens. The removal of H 2 is beneficial
to the microbes that produce it because their activity is inhibited by high
concentrations of H 2 . Understanding this and other syntrophic interactions
is central to describing anaerobic sewage digestion.
An interesting mutualism occurs between the chironomid midge larva
Cricotopus nostocicola and the cyanobacterium Nostoc parmelioides
(18.15C). The midge receives sustenance from the Nostoc and lives inside it
until pupation and emergence as an adult (Brock, 1960). In turn, the midge
increases the photosynthetic rate of the Nostoc (Ward et al., 1985) by alter-
ing its morphology and by attaching it more firmly onto rocks so it can ex-
tend into flow and have a smaller diffusive boundary layer (Dodds, 1989).
A general type of mutualistic interaction that is moderately common
in aquatic microbial communities involves animals that ingest algal cells
and obtain fixed carbon from them. Presumably the alga receives protec-
tion from predation and inorganic nutrients from the animal host. Organ-
isms with this type of interaction include Hydra (Fig. 18.15D) and Para-
mecium bursuri with the green alga Chlorella . Dinoflagellates have ingested
several different types of algae in this fashion, including cyanobacteria and
green algae.
Similarly, chemoautotrophic bacteria are ingested by protozoa. The
most studied of these interactions is the sequestering of methanogenic bac-
teria by protozoa (Fenchel and Finlay, 1995). This is important because the