Microbes and Plants - Freshwater Ecology: Concepts and Environmental Applications - page 133

of cyanobacteria to inhabit very deep waters and still remain photosyn-
thetically active. The ecological consequences of phycobilins were dis-
cussed in Chapter 3 (Fig. 3.10).
Cyanobacteria are often difficult for herbivores to consume, partly be-
cause of their gelatinous coatings and their toxin production (Sidebar 8.2).
These toxins have likely evolved to limit grazing but are very broad spec-
trum and may also have adverse effects on fish and humans.
Benthic species of cyanobacteria can be found in diverse habitats, in-
cluding wetlands, streams, and temporary waters. Benthic forms are found
more commonly in oligotrophic waters. I discuss benthic cyanobacteria in
Chapter 15 on hot springs and hypersaline habitats and in the context of
motility and behavior (Chapter 18).
PROTOCTISTA
The protists include a wide variety of organisms from single celled to
multicellular, including algae (other than cyanobacteria) and the protozoa.
Together these groups are responsible for much of the primary production
and nutrient recycling that occurs in aquatic habitats. These diverse and el-
egant organisms are often the first microbes students see and have
doubtlessly inspired numerous careers in microbiology and aquatic ecol-
ogy. Those whose imaginations have not yet been captured may want to
examine the excellent photomicrographs of algae by Canter-Lund and
Lund (1995).
Eukaryotic Algae
A wide variety of species of algae are found in freshwaters, but only
the most common will be discussed here. The algae are defined as nonvas-
cular eukaryotic organisms that are capable of oxygenic photosynthesis
and contain chlorophyll a . Some algae may be considered protozoa because
they have colorless forms that survive by ingesting other organisms. Table
8.3 and the following text summarize the characteristics of selected groups
of algae.
Rhodophyceae, the Red Algae
Red algae are rare in freshwaters and are restricted mainly to running
water. For example, Batrachospermum (Fig. 8.5) is a red alga found in
streams and springs throughout the world. The algae are red because of
their pigments. They contain phycobilins similar to those found in the
cyanobacteria but in different proportions, leading to a red hue.
Chrysophyceae
The Chrysophyceae are common in oligotrophic lakes as planktonic
species. They have flagella, and interestingly some of them are able to in-
gest particles as a food source. A common genus is Dinobryon (Fig. 8.5).
The large size of Dinobryon probably makes them difficult for herbivorous
zooplankton to consume.