Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 384

Sloughing, grazing

Time since disturbance

Long filamentous,
with epiphytes

Increasing light and
nutrient competition

Long filamentous

Short filaments,
stalks

Dense
mucilage

Crustose, prostate,
or gelatinous

Substratum

Scraping and rasping

Collecting and scraping Gathering, shredding

and piercing

Scraping
epiphytes

Grazing pressures

FIGURE 18.14 Conceptual diagram of successional sequence of an algal biofilm and the as-
sociated increases in nutrient and light competition. The type of grazer is indicated at each
stage (after Steinman, 1996).

found in guts of aquatic invertebrates include Vibrio, Pseudomonas,
Flavobacterium, Micrococcus, and Aeromonas (Harris, 1993). These mi-
crobes can benefit organisms by decomposing food that would otherwise
be unavailable and by outcompeting pathogenic organisms. Interactions
with gut microorganisms have not been well studied, but there are docu-
mented cases of aquatic invertebrate digestion being aided by gut microbes
and increases in resistance to toxins associated with gut fauna (Harris,
1993).
Association of nitrogen-fixing bacteria or cyanobacteria with plants is
probably the most likely interaction involving nutrient cycling to be ac-
cepted as mutualistic. Examples from aquatic habitats include the interac-
tion between the water fern Azolla (18.15A) and the nitrogen-fixing hete-
rocystous cyanobacterium Anabaena azollae (Fig. 18.15B). Many wetland
or riparian plants may also associate with nitrogen-fixing microbes, in-
cluding the flowering plant Gunnera and the cyanobacterium Nostoc
(Meeks, 1998). Alder is a common tree in boreal riparian zones that has
nitrogen-fixing bacteria associated with its roots and can be a significant
source of fixed N to nutrient-limited systems (Rytter et al., 1991). The di-
atom Epithemia contains nitrogen-fixing cyanobacteria (De Yoe et al.,
1992), and both organisms may benefit from the interaction (18.15E). My-
ccorhizal interactions that can be mutualistic occur in some wetland plants
(Søndergaard and Laegaard, 1977; Rickerl et al., 1994). Such relationships
have been demonstrated to increase competitive ability for nutrients in
some lake macrophytes (Wigand et al., 1998).
Syntrophy is complementary metabolism. Anaerobic microorganisms
in anoxic habitats commonly exhibit syntrophy. In these interactions, each
organism provides the other with an organic carbon source or utilizes a