Microbial trophic levels within the carbon cycle in oligotrophic and nutrient
impacted regions of Water Conservation Area 2-A
Andrew Ogram, Ashvini Chauhan, Hector Castro, Ilker Uz and K. R. Reddy
University of Florida, Gainesville, FL
We are interested in the impact of nutrient enrichments on the pathways by which
carbon is mineralized, and linkages between microbial species and carbon
mineralization in Water Conservation Area 2-A (WCA-2A) of the northern
Everglades. Information on the specific pathways followed by the carbon cycle
will provide important insight into the impacts of nutrient enrichment on
biogeochemical cycling and may be used in development of sensitive indicators
of nutrient impact and ecosystem restoration.
Carbon is transferred through a series of trophic levels composed of different
species of microorganisms in anaerobic systems, such as those found in wetlands
soils. Cellulolytic microorganisms initially attack plant material, and primary
fermentative organisms metabolize smaller molecules. A variety of fermentation
products, potentially including propionate, butyrate, H2, CO2, acetate, and lactate,
are subsequently used by a lower trophic groups, including methanogens,
homoacetogens, and sulfate reducing bacteria. The specific pathway followed and
the species composition of individual trophic levels depends to a large extent on a
variety of environmental factors, including the nutrient status of the environment.
We have been studying relationships between lower trophic groups and position
along phosphate gradients in WCA-2A, and similar studies on cellulolytic and
fermentative bacteria were recently initiated. Included in the lower trophic groups
we are currently investigating are syntrophic fatty acid oxidizing bacteria
(“syntrophs”), methanogens, and sulfate reducing bacteria (SRB). The
compositions of lower trophic levels are less likely to be significantly impacted by
daily fluctuations in environmental conditions than are higher trophic groups
because they are generally deeper in soil profiles and have very narrow substrate
utilization ranges. In addition, lower trophic groups will likely provide indicators
of structures and compositions of higher trophic groups because the quality and
quantity of carbon and electron donors produced by fermentors will control the
composition and structures of the lower groups.
SRB have largely been ignored in the Everglades and other fresh water systems,
but are sensitive to eutrophication due to increased concentrations of sulfate (their
preferred terminal electron acceptor) and potential changes in electron donors
resulting from changes in fermentation patterns. SRB can be divided into two
broad subdivisions that belie physiological and ecological roles of the two groups:
complete and incomplete oxidizers. Complete oxidizers typically utilize a broader
range of substrates than do incomplete oxidizers, and may be considered as
generalists compared with the more specialist incomplete oxidizers. A dominance
of complete oxidizers may suggest the availability of a broader range of substrates
compared with those environments where incomplete oxidizers dominate. We
have found that, for at least one genus of SRB (Desulfotomaculum), complete
oxidizers dominate incomplete oxidizers in eutrophic regions of WCA-2A, and
that incomplete oxidizers dominate in oligotrophic regions of the marsh.
We have studied the distribution of Desulfotomaculum species associated with
complete and incomplete oxidization at two sampling times (Spring and Summer,
2001) in oligotrophic (U3) and eutrophic (F1) regions of WCA-2A. A clear
relationship was observed between the distribution of complete and incomplete
oxidizers and the region of the marsh. Complete oxidizers are almost exclusively
found in U3, whereas incomplete oxidizers clearly dominate F1. Laboratory
enrichments on various substrates support the observation that SRB in F1 utilize a
greater diversity of fermentation products than do those in U3.
Studies on methanogens and syntrophs reflect a similar dependence of
composition and physiological group on nutrient status, although somewhat less
clear cut than for SRB. Like SRB, methanogens may be divided into two
functional groups that represent different physiologies, and hence, different
ecologies: the acetotrophs and the hydrogenotrophs. Most acetotrophs are limited
to use of acetate for electrons and carbon, whereas the hydrogenotrophs exhibit a
broader carbon substrate range, and can use H2 as an electron donor.
Hydrogenotrophic methanogens are the most numerous methanogens in F1, F4
(an area of transition between the eutrophic F1 and oligotrophic U3), and U3.
Significantly, most probable numbers of hydrogenotrophs in F1 outnumber
acetotrophs by a factor of 100,000, but only by a factor of 100 in F4 and U3.
This may be due to higher concentrations of H2 in F1 resulting from greater
activities of fermentative bacteria. In addition, most probable numbers of
syntrophic bacteria capable of oxidizing propionate and butyrate are ten to one
hundred times higher in F1 than U3, respectively. Syntrophs work together with
hydrogenotrophic methanogens to oxidize propionate and butyrate, such that
hydrogenotrophic methanogens are required to remove H2 in order to make the
oxidation thermodynamically feasible. The higher numbers of syntrophs in U3
likely led to higher numbers of hydrogenotrophic methanogens. These trends are
supported by molecular characterization of DNA from enrichments, and
significantly more propionate and butyrate oxidation are observed in microcosms
constructed from F1 than either F4 or U3. Finally, acetotrophs are also regarded
as specialists, approximately analogous to the SRB complete oxidizers, and
hydrogenotrophs are regarded as generalists, like the SRB incomplete oxidizers.
It is likely that the composition of lower trophic levels of prokaryotes and the
specific pathways by which carbon is cycled in wetlands is controlled largely by
the diversity and quantity of fermentation products, and hence by the activities of
primary fermenters in these environments. Our data on SRB, syntrophs, and
methanogens suggest that a broader range of fermentation substrates is produced
in nutrient impacted areas than in oligotrophic areas of the marsh. A greater
diversity of fermentation products may be due to the presence of different species
of fermenters in the different regions, or that similar fermenters are present and
respond to different environmental conditions that may affect the fermentation
pathways of these strains. We are currently investigating the activities of
fermentative bacteria in F1, F4 and U3 to answer these questions.
Andrew Ogram, University of Florida, 2169 McCarty Hall, Gainesville, FL
32611-0290,
Phone: 352-392-5790, Fax: 352-392-3902, avo@mail.ifas.ufl.edu