EXERCISE 20
Decomposition: Relative
Bacterial Heterotrophic Activity
on Soluble Organic Matter
The decomposition of organic matter in aquatic
ecosystems by microorganisms generally involves two processes: (1) the hydrolytic degradation of high molecular weight organic polymers
into compounds of low molecular weight, such
as glucose, cellobiose, and amino acids; and (2)
the non hydrolytic oxidative mineralization of
low molecular weight organic compounds to
inorganic compounds, especially CO 2 , H 2 S,
NH4 +, and P0 4 -3. Measurement of the rates of
decomposition and of mineralization of organic
matter in natural waters is difficult and has been
approached in a number of ways.
To analyze the rates of reactions involved in
in situ decomposition and mineralization of
organic matter, both biochemical and geochemical approaches have been taken. Geochemical
methods involve the chemical analysis of organic
and inorganic compounds in water and sediments. Changes in the chemical composition
often reflect the biochemical events that have
occurred in the environment as a consequence of
microbial activities.
Biochemical methods address more directly
fundamental questions of the chemical nature,
concentrations, and the rates at which substrates
are utilized by bacteria for energy. Certain techniques have been used to measure a community
response by following the rates of respiratory
activity, of utilization of specifically labeled
organic substrates, or of polymer degradation by
enzymatic activity [e.g., Cunningham and Wetzel
(1989)]. Heterotrophic bacterial communities
consist of a heterogeneous composite of populations in various physiological states. Moreover,
the dissolved organic substrates comprise a spectrum from very labile compounds that are readily
reactive with the enzymes of the microflora, to
highly refractory substrates that are utilizable
only slowly by highly specialized organisms.
Estimates of in situ rates of planktonic community respiration have been made by separating
the larger photosynthetic organisms from the
bacteria by filtration and then analyzing either
the oxygen consumption or CO 2 production [see
Sorokin and Kadota (1972)]. However, separation of the organisms by filtration does not give
consistent results. Furthermore, under anaerobic
conditions, decomposition occurs by multifaceted fermentation processes. Fermentation uses
alternate electron acceptors to produce a variety
of reduced metabolic end products, such as
methane, volatile fatty acids, and alcohols, in
addition to CO 2 ,
This exercise offers a simple approach to
measure the relative heterotrophic activity of
microbes in natural waters. In situ bacterial
communities are analyzed by the uptake and
mineralization of a 14C-Iabeled organic substrate
with Michaelis-Menten enzyme kinetics. This
271
Decomposition: Relative
Bacterial Heterotrophic Activity
on Soluble Organic Matter
The decomposition of organic matter in aquatic
ecosystems by microorganisms generally involves two processes: (1) the hydrolytic degradation of high molecular weight organic polymers
into compounds of low molecular weight, such
as glucose, cellobiose, and amino acids; and (2)
the non hydrolytic oxidative mineralization of
low molecular weight organic compounds to
inorganic compounds, especially CO 2 , H 2 S,
NH4 +, and P0 4 -3. Measurement of the rates of
decomposition and of mineralization of organic
matter in natural waters is difficult and has been
approached in a number of ways.
To analyze the rates of reactions involved in
in situ decomposition and mineralization of
organic matter, both biochemical and geochemical approaches have been taken. Geochemical
methods involve the chemical analysis of organic
and inorganic compounds in water and sediments. Changes in the chemical composition
often reflect the biochemical events that have
occurred in the environment as a consequence of
microbial activities.
Biochemical methods address more directly
fundamental questions of the chemical nature,
concentrations, and the rates at which substrates
are utilized by bacteria for energy. Certain techniques have been used to measure a community
response by following the rates of respiratory
activity, of utilization of specifically labeled
organic substrates, or of polymer degradation by
enzymatic activity [e.g., Cunningham and Wetzel
(1989)]. Heterotrophic bacterial communities
consist of a heterogeneous composite of populations in various physiological states. Moreover,
the dissolved organic substrates comprise a spectrum from very labile compounds that are readily
reactive with the enzymes of the microflora, to
highly refractory substrates that are utilizable
only slowly by highly specialized organisms.
Estimates of in situ rates of planktonic community respiration have been made by separating
the larger photosynthetic organisms from the
bacteria by filtration and then analyzing either
the oxygen consumption or CO 2 production [see
Sorokin and Kadota (1972)]. However, separation of the organisms by filtration does not give
consistent results. Furthermore, under anaerobic
conditions, decomposition occurs by multifaceted fermentation processes. Fermentation uses
alternate electron acceptors to produce a variety
of reduced metabolic end products, such as
methane, volatile fatty acids, and alcohols, in
addition to CO 2 ,
This exercise offers a simple approach to
measure the relative heterotrophic activity of
microbes in natural waters. In situ bacterial
communities are analyzed by the uptake and
mineralization of a 14C-Iabeled organic substrate
with Michaelis-Menten enzyme kinetics. This
271