Decomposition: Particulate Organic MaHer
283
The following exercises examine the rates of
decomposition and mineralization of phytoplankton, macrophytes, and leaf fall. Since rates of
decomposition are slow (~1 to 3%/day), it is
EXERCISES
MINERALIZATION OF PHYTOPLANKTON
necessary to use long-term experiments, or to use
radioactively labeled organic matter to measure
the rates of production of metabolic end products
in short-term experiments.
The method discussed here measures the mineralization rates of phytoplanktonic carbon (both
particulate and dissolved fractions) in lakes. The rates of decomposition are determined by
monitoring 14COz evolved from the microbial respiration of naturally occurring particulate and
dissolved phytoplankton detritus labeled with 14C (Cole and Likens, 1979; Cole et ai., 1984). The
designs of the incubation bottle and of the 14COz trapping system provide a method for
measuring mineralization rates in intact microbial communities under in situ temperatures and
dissolved gas concentrations without extreme manipulations.
Algal DOC can be mineralized rapidly by bacteria. For example, Cole (1985) found that only
about 12% of the net algal primary productivity in Mirror Lake, a softwater lake in New
Hampshire, accumulated in the sediments each year. Many studies of this subject have shown
that from 75 to 95% of the phytoplanktonic organic carbon is decomposed before reaching the
sediments [cf., Wetzel (1983)].
Procedures
1. Obtain large (ca. 31) volumes of water with natural phytoplankton populations from two or
more depths within the epi- and metalimnion of a lake.
2. Place the samples into large, ground-glass-stoppered bottles (2.51) and add approximately
40 /.lCi NaH 14 C0 3 to each. Mix well by gentle inversion several times. Caution: Strictly
observe all precautionary measures for handling radioactive materials (see Exercise 14). Clip
the bottles to a suspension line and return the bottles to the depth from which they were
collected. Incubate the samples in situ for 3 days. This time of incubation should exceed the
average turnover time of the algae and should be adequate to obtain reasonably uniform
labeling of the algal cells.
3. After the incubation, divide the well-mixed sample into two parts. From one part, harvest
cells by filtration from a known volume onto precombusted (500°C) glass fiber filters. Use a
2-1 filtration flask and save a large portion ofthe filtrate. Remove the filters, subject them to
fumes (not liquid) of Hel for 30 min. in a hood, and dry under desiccation (see the simple
methods for this operation discussed in Exercise 14). The treatment with fumes kills the algae
and removes residual, adherent H 14 C03 -·
4. From the second part of the sample, remove several50-ml aliquots of the labeled algae, filter
onto membrane filters (0.45-/.lm pore size), and treat the filters identically to the glass fiber
filters (Step 3). Determine the amount of 14C assimilated (primary productivity) by assaying
the incorporated 14C; use the methods discussed in Exercise 14 (liquid scintillation
radioassay).
5. The filtrate from the labeled algae and detritus (Step 3) presumably contains phytoplankton
exudates as well as solubilized phytoplankton components in various stages of decomposition. Bubble this filtrate vigorously in a hood with CO2 or CO 2 -enriched air for 12 h to
remove residual 14C02. Filter the sparged filtrate through 0.45-/.lm pore size membrane
filters. Slowly evaporate the filtrate to dryness at 50°C or lyophilize to dryness. Reconstitute
283
The following exercises examine the rates of
decomposition and mineralization of phytoplankton, macrophytes, and leaf fall. Since rates of
decomposition are slow (~1 to 3%/day), it is
EXERCISES
MINERALIZATION OF PHYTOPLANKTON
necessary to use long-term experiments, or to use
radioactively labeled organic matter to measure
the rates of production of metabolic end products
in short-term experiments.
The method discussed here measures the mineralization rates of phytoplanktonic carbon (both
particulate and dissolved fractions) in lakes. The rates of decomposition are determined by
monitoring 14COz evolved from the microbial respiration of naturally occurring particulate and
dissolved phytoplankton detritus labeled with 14C (Cole and Likens, 1979; Cole et ai., 1984). The
designs of the incubation bottle and of the 14COz trapping system provide a method for
measuring mineralization rates in intact microbial communities under in situ temperatures and
dissolved gas concentrations without extreme manipulations.
Algal DOC can be mineralized rapidly by bacteria. For example, Cole (1985) found that only
about 12% of the net algal primary productivity in Mirror Lake, a softwater lake in New
Hampshire, accumulated in the sediments each year. Many studies of this subject have shown
that from 75 to 95% of the phytoplanktonic organic carbon is decomposed before reaching the
sediments [cf., Wetzel (1983)].
Procedures
1. Obtain large (ca. 31) volumes of water with natural phytoplankton populations from two or
more depths within the epi- and metalimnion of a lake.
2. Place the samples into large, ground-glass-stoppered bottles (2.51) and add approximately
40 /.lCi NaH 14 C0 3 to each. Mix well by gentle inversion several times. Caution: Strictly
observe all precautionary measures for handling radioactive materials (see Exercise 14). Clip
the bottles to a suspension line and return the bottles to the depth from which they were
collected. Incubate the samples in situ for 3 days. This time of incubation should exceed the
average turnover time of the algae and should be adequate to obtain reasonably uniform
labeling of the algal cells.
3. After the incubation, divide the well-mixed sample into two parts. From one part, harvest
cells by filtration from a known volume onto precombusted (500°C) glass fiber filters. Use a
2-1 filtration flask and save a large portion ofthe filtrate. Remove the filters, subject them to
fumes (not liquid) of Hel for 30 min. in a hood, and dry under desiccation (see the simple
methods for this operation discussed in Exercise 14). The treatment with fumes kills the algae
and removes residual, adherent H 14 C03 -·
4. From the second part of the sample, remove several50-ml aliquots of the labeled algae, filter
onto membrane filters (0.45-/.lm pore size), and treat the filters identically to the glass fiber
filters (Step 3). Determine the amount of 14C assimilated (primary productivity) by assaying
the incorporated 14C; use the methods discussed in Exercise 14 (liquid scintillation
radioassay).
5. The filtrate from the labeled algae and detritus (Step 3) presumably contains phytoplankton
exudates as well as solubilized phytoplankton components in various stages of decomposition. Bubble this filtrate vigorously in a hood with CO2 or CO 2 -enriched air for 12 h to
remove residual 14C02. Filter the sparged filtrate through 0.45-/.lm pore size membrane
filters. Slowly evaporate the filtrate to dryness at 50°C or lyophilize to dryness. Reconstitute