Types of Aquatic Organisms - Freshwater Ecology: Concepts and Environmental Applications - page 119

biologists accept this as a theory for the origin of chloroplasts and mito-
chondria. Mitochondria and chloroplasts found in eukaryotes were initially
cells of purple photosynthetic bacteria and cyanobacteria (blue-green algae),
respectively. This evolutionary innovation occurred several billion years ago
and vastly increased the complexity of organisms. Molecular evidence strongly
supports independent origins of mitochondria and chloroplasts. The numer-
ous less tightly integrated intracellular associations that exist in aquatic
ecosystems provide partial support for the serial endosymbiosis theory: For
example, inclusion of photosynthetic algae in protozoa and small animals is
functionally similar to chloroplasts in plants. Such associations are found in
freshwaters, including Chlorella in Hydra, Paramecium bursari, and Spongilla .
There are six or more kingdoms of organisms if one accepts molecular
taxonomy that indicates the three major domains of organisms. The king-
doms include the Plantae, Animalia, Fungi, and Bacteria. The group that has
been classified as Protista likely will be divided into several kingdoms given
the broad spread in molecular trees (Fig. 7.1) and to maintain a classification
scheme consistent with evolutionary origins (Hickman and Roberts, 1995),
but the classification of this group is currently in flux (Patterson, 1999). The
Archaea probably contains two kingdoms as well (Woese et al., 1990).
One point that becomes clear when analyzing the molecular evidence
is that the divergence within the Bacteria and Archaea exceeds that of the
Eukarya. Thus, the diversity of morphology and behavior that is so evident
in the plants and animals has arisen recently. Molecular diversification is
the forte of the Bacteria and Archaea.
CLASSIFICATION OF ORGANISMS BY FUNCTIONAL SIGNIFICANCE
Taxonomy of organisms is based not only on phylogenetic relation-
ships but also on their functional roles in communities and ecosystems.
Such classifications include how the organisms acquire carbon, what habi-
tat they occupy, and how they interact with other organisms. These con-
cepts are discussed here because they are used in the following chapters to
characterize organisms.
Organisms can be autotrophic (“self-feeding”) and rely on CO 2 as the
primary source of carbon to build cells. The other option is to be het-
erotrophic (“other feeding”) and acquire carbon for cells from organic car-
bon (Table 7.2). Some organisms are able to use both autotrophic and het-
erotrophic processes to obtain carbon.
Most autotrophs use light as an energy source to reduce CO 2 to or-
ganic carbon and are classified as photoautotrophic (i.e., photosynthetic
organisms). Some microbes are able to use chemical energy instead of light
as a source of energy to allow use of CO 2 ; these organisms are chemoau-
totrophic . The chemoautotrophs are less important to most carbon bud-
gets than photoautotrophs, but they dominate some unusual environments
and play a key part in several nutrient cycles (see Chapter 13).
Organisms that acquire carbon from living organisms (predation, her-
bivory, parasitism, etc.), dissolved or particulate organic compounds, or
dead organisms (decomposers and carrion eaters) are heterotrophic. Het-
erotrophs that decompose organic carbon are sometimes called sapro-
phytes or detritivores .