Decomposers
The cycling of materials such as carbon, water, and other nutrients
is mainly dependent upon soil-dwelling decomposer organisms such as bacteria
fungi, earthworms, and insects. Bacteria and fungi are the most abundant
of the microbial decomposers, numbering in the billions in only one handful
of soil!
As essential components of
the environment, fungal and bacterial microbes break down dead and discarded
organic materials, supplying a continuous source of nutrients for the
plants in surrounding soil. In general, decomposers break down the proteins,
starches, and other complex organic molecules that were all once the components
of living organisms, and “as products of their own metabolism, [they]
convert elements such as nitrogen, phosphorous, calcium, and sulfur into
forms that can be utilized by plants” (Dirty, 2). According to several
researchers at the University of Jyväskylä, "Reduction in the
species diversity of the lowest levels (decomposer fungi) of the food
web [become] particularly well manifested as reduced decomposition rate
and stagnated nutrient dynamics." They also emphasized that certain variations
of microorganisms can play a critical role in controlling nitrogen cycles
and plant growth in general.
In addition, it is true that
decomposer species tend to be rather resistent to changes in the availablitiy
of organic material or changes to the environment in general, but they
are still affected by several key factors. For instance, increases in
temperature will cause more rapid decomposition reactions--just as would
occur for most chemical reactions. However, too high of an increase in
temperature affects the microbes adversely. Moisture is also usually favorable
unless there is so much moisture that the living environment is waterlogged.
When waterlogging occurs, some microbes will die while others thrive on
the excess moisture. On the other hand, light tends to be bad for
most decomposer microbes.
Concerning the issue of oil
drilling on the North Slope of ANWR, one would expect an increase in the
amount of organic as well as inorganic waste material in the local environment.
This is important, because although fungi and microorganisms thrive off
of organic materials such as animal flesh, fecies, dead plant matter,
nut shells, etc., it is much harder and time consuming for them
to decompose human-made materials that are either high in cellulose--i.e.
cotton and paper-cardboard--or metallic or plastic. In fact, metals and
plastics are almost impossible to decompose by the microorganisms and
fungi--they break down primarily, and over a long period of time, due
to weathering processes. Currently advances are being made in the development
of biodegradable plastics, but there remains the majority of non-biodegradable
plastics. Furthermore, if the diversity of decomposer species decreases
in ANWR due to changes in their environment--whether the changes are related
to climate, the introduction of oil drilling, or increases/decreases in
populations of consumers--it can be assumed that all species that depend
upon the decomposers are either directly or indirectly affected by such
a flux.
Species:
Although it is true that “no one
has yet detailed all the species of fungi that live in Alaska” and “several
thousand species of fungi exist, but scientists have only described about
50,000,” much can be said about the characteristics and effectiveness of
these incredible decomposers. (http://www.gi.alaska.edu/ScienceForum/ASF15/1562.html)
According to the book Ecology of Arctic Environments, research has shown
that frost free days and varying levels of nitrogen in soil are not greatly important to the survival of fungi, while temperature,
moisture and pH are. In addition, it appears that the cold of the arctic
may “have a selective effect” on certain species such as Chrysosporium pannorum. As for arctic fungal
species in general, several studies have illustrated that “fungal h yphal length increased with
increasing soil moisture and temperature, and decreasing soil pH” (Woodin
and Marquiss).
Unfortunately for our purposes, the fungal species lists and biomass
values that are available are almost all composed from the research efforts
of the Tundra Biome group of the IBP, and are therefore from the long ago
period of 1964-1974. Species known to have been studied by the Tundra Biome
group include Eriophorum and Carex (Woodin and Marquiss).
General Characteristics:
Suffice to say, “fungi, the subjects in this kingdom, live in
the woods, in refrigerators, on rocks, and between our toes. Without them,
the natural world would cease to function.”
Fungi are part of a very interesting kingdom. In fact, it is so diverse
and complex that they are the only species that make up the kingdom! To
function on a regular basis, fungi digest their food through the use of
extracellular fluids. In other words, digestion occurs outside of their
bodies. The extracellular fluids that they excrete are acids and enzymes
that act to break down organic material. They then absorb the simple molecules
of food through their cell walls, because they do not have stomachs.
Furthermore, many fungal species exist in symbiotic relationships with
vegetation such as trees. An example of such a relationship occurs when
fungi beneath the soil cling on to tree roots, resulting in roots that are
coated with hairy fungi called mycorrhizae. The fungi help trees and other
plants absorb minerals (http://www.gi.alaska.edu/ScienceForum/ASF15/1562.html).
Furthermore,
“decomposer microorganisms require a balance of carbon and nitrogen that
requires a far greater turnover of nitrogen accompanying carbon dioxide
production…in order to simultaneously meet energetic and nutritional needs”
(Reynolds et. al, 311). Plants even compete both with each other and with
soil microbiota for the nitrogen and nutrients released through decomposition.
Therefore, when the availability of nitrogen is lower, it is harder for
a variety of plants to survive.
Investigations on decomposition in the arctic have been conducted primarily
by Flanagan and Scarborough as part of the Tundra Biome project that occurred
from 1964-74. They stated that the best soil pH for cellulose decomposition
by about 60% of Alaskan strains was between 4.5 and 5 (acidic), while a
slightly more basic pH of 6 was best for the decomposition of pectin and
starches (Woodin and Marquiss).
It is observed that the “fungal species composition in arctic environments
is very similar to those in temperate latitudes, although the isolates are
often adapted to low temperatures” (Woodin and Marquiss). Nevertheless,
in the extreme soils that are more characteristic of the North Slope (aka
the 1002 region—that which is in debate for oil drilling), fungal biomass
is clearly seasonal and may overall be lower than that which is found in temperate
ecosystems.
At the University of Alaska Fairbanks, the Institute of Arctic Biology
scientists are attempting to determine the “characteristics of tundra and
forest fungi that allow them to live in cold soils and to release nitrogen
and phosphorus as they decompose organic remains.” The resistance
of fungi to oil spills or other environmental disturbances must be investigated
and known, for “if the stability of the fungi is undermined, an entire forest
or tundra grazing land can be destroyed (http://www.gi.alaska.edu/ScienceForum/ASF1/183.html)”
Most Vulnerable Characteristics:
Although the relationships
between nutrient mineralization, microbial immobilization, and plant uptake
still need to be better documented and understood, enough is known about
Alaska’s tundra to understand conditions that affect the resident decomposer
species (Reynolds, et. al).
Basically, low temperatures,
the existence of permafrost, low nutrient input and frequent waterlogged
conditions result in a reduced rate of organic matter turnover and cycling
of organically bound nutrients. In addition, the accumulation of dead organic
matter enhances these conditions which inhibit decomposition. Meanwhile,
soil temperature and water regimes also affect anaerobic respiration by
decomposers in the tundra soil. Normally, warmer temperatures will increase
respiration rates and increased levels of moisture will as well, but if
an environment is overly saturated, decomposer activity is inhibited. In
such saturated sites, there are larger accumulations of organic matter due
to the limit on decay and peat tends to form. For example, the Imnavait Creek
watershed in northern AK normally has a layer of organic matter overlying
mineral soil that ranges from 10-40 cm in thickness, due to its rather low
rate of decay (Reynolds et. al).
Ice microbial communities, although for the most part poorly understood,
are challenged physiologically and ecologically by meltwater fluxes. Basically,
in the summer low-salinity meltwater promotes the flushing of a substantial
fraction of the sea-ice microbial habitat. Due to steadily increasing temperatures
in the arctic, such freshwater immersion may be increasing in duration and
extent as precipitation and snow melt amounts increase (http://siempre.arcus.org/4DACTION/wi_pos_displayAbstract/6/440).
Habitat
A pleasant reflection from a woman
who once visited the arctic:
“I was in for a surprise when we looked at the bottom of the ice core
sample. I grew up in Michigan and spent my winters boring fishing holes
into Lake Huron, so I was familiar with freshwater ice. But unlike that
ice, the bottom surface of sea ice is not smooth. It has a very rough surface
and is distinctly greenish-brown in color. The color is caused by a large
increase in biological material --mostly algae such as diatoms .
The color also comes from dissolved organic material that supports the
growth of bacteria. There is a surprisingly high diversity of viruses and
fungi as well. Crustaceans feed on the several hundred different species
of algae that live in this bottom-most layer of ice, and fish feed on the
crustaceans. It's a complex food web. Yet standing here on the icy surface,
you'd never know this ecosystem was there. You have to penetrate down through
the ice to have any chance of discovering it (http://www.astrobio.net/news/print.php?sid=467).”
Plant growth can be limited by a lack of organic compounds in the soil,
an effect that occurs because decomposing matter takes longer as it becomes
integrated into the soil. Due to the even slower function of decomposition
in cool climates, soil is slow to recover from any disturbances caused by
the force of erosion, animals, or humans.
Even so, the freezing and thawing that takes place over the course of
a year in the arctic actually speeds the access for bacteria and fungi
to the insides of cells of dead plants and animals. This can happen because
when a cell freezes, the water in its tissue expands, therefore breaking
cell walls and making it possible for materials to diffuse in and out of
the cells once the tissue thaws. Despite this activity, the extreme cold
of the arctic has the net effect of slowing decomposition (http://www.blm.gov/education/00_resources/articles/alaskas_cold_desert/classroom.html).
Ocean currents and the proximity to land are both factors upon which
nutrient distribution in water depends. The weathering of rocks on land
causes many of these nutrients to be carried into the ocean by rivers, which
is why many of the most nutrient rich oceanic areas are near river mouths.
In addition, the decay of plant and animal material by bacterial processes
recycles the nutrients. Therefore, the areas on the Coastal Plain at 1002
where the rivers empty into the ocean are highly concentrated with nutrients
and in turn attractive habitat for birds and other wildlife to find nutrition
(oceanexplorer.noaa.gov/explorations/
02arctic/welcome.html).
Basic Population Dynamics:
An interesting piece of trivia:
“According to Alaska Science Nuggets, every acre of tundra contains
more than 2 tons of live fungi! The result of all of this organic matter
decomposing (since the rollback of the North Slope glaciers about 12,000
years ago) is that there is a layer of 3 to 6 feet of peat that overlies
the tundra! (http://www.aksta.org/trivia_dec00.html)"
It has been found that populations
of soil bacteria in tundra areas are very close to the same as those found
in other regions “and no types unique to tundra regions have been recognized”
(Woodin and Marquiss). However, estimates of bacterial biomass are greatly
influenced by the unreliability of plate and direct microscopic counts as
well as the difficulty in figuring out “whether the cells are alive, dead,
or in a non-culturable but viable state” (Woodin and Marquiss). Efforts
to make direct counts of bacterial biomass have demonstrated that biomass
increases with decreasing latitude, as is evidenced by the counts made from
oven-dried soil which range from 2.26 μg g‾1 at Stordalen to 8600 in a horizon
at Moor House (Woodin and Marquiss).
Current Status:
According to information compiled
by Reynolds etc. al, “current levels of soil moisture appear close to
optimum for decomposition…any net changes in soil moisture may decrease
carbon mineralization.” They also reported on recent simulations performed
under various climate change scenarios. The simulations suggest that there
is a “large potential variability compared to current carbon and nitrogen
dynamics, depending on the rates and directions of changes in soil moisture
and temperature regimes.”
What has been learned about decomposers from other
oil drilling sites:
We have learned from
drilling sites worldwide, in general, that there are certain common contaminants
associated with oil development. Such contaminants primarily include diesel
fuel, crude oil, drilling waste and seawater and brine; some other less
common pollutants are glycol, fire-fighting agents and methanol. One statement
by Jorgenson observes that “past spills on the North Slope [indicate] that
ecological damage is minor and that ecosystems exhibit good potential for
recovery from levels of damage associated with oiling and cleanup. Although
the potential damage from a major spill can be high, the cumulative effect
of damage from oil is much less than for other impacts such as gravel placement
and drilling waste management. While the overall risk of major spills is
low, prevention efforts must be vigorous and special care must be taken
near rivers, streams, and connected waterbodies to minimize the movement
of spilled oil over larger areas.” Of course, one must also take into account
that oil spills are much less common than diesel spill which have historically
occurred most often on gravel pads...(Jorgenson). In addition, it is important
to note that “the cumulative effects of development at Prudhoe Bay progressively
affected larger areas as road networks increasingly modified water flows”
(Reynolds etc. al).
What one can gather from the above information in relation to decomposer
species is that the introduction of oil drilling almost inevitably brings
with it contaminants, and contaminants such as waste and diesel fuel and
crude oil, etc. most often soak into the ground and have the potential to
affect soil organisms. Basically, the pollutants previously mentioned are
capable of greatly increasing decomposition rates, which means the proliferation
of certain microorganisms that may not necessarily be healthy to the environment.
This is a personal speculation based on various sources.
Likely changes to come due to natural effects:
Model simulations conducted recently
predict that over the course of the next 50 years, summer precipitation
may increase by as much as 20-30%, while summer temperatures may increase
by 3˚-6˚C. Such drastic changes will in turn modify the decay of litter,
and therefore nutrient cycling processes as a whole (Reynolds etc. al).
The above predictions are closely related to variations in atmospheric
levels of carbon dioxide. It is likely that due to natural processes as
well as current industrial practices that the levels of carbon dioxide in
the atmosphere will continue to rise and affect the earth’s climate in a
number of ways. In relation to decomposer species, several impacts of rises
in carbon dioxide levels are: 1) change in the population of rhizophere bacteria;
2) greater soil respiration; 3) higher enzymatic activities in root regions;
4) modified mycorrhizal activities, stimulating the uptake of phosphorous
by plants in the system (Reynolds etc. al). A 3-year study involving 680
ppm carbon dioxide exposure to plants examined the enzymatic characteristics
of roots, associated mycorrhizae and surrounding soils in tussock tundra.
The study showed that exocellulase and endocellulase activities were higher
in the mycorrhizal rhizomorphs, and lower in Oe and Oi horizons at greater
carbon dioxide areas. This means that the increase in the amount of carbon
dioxide oozing from plant roots could be inhibiting cellulose activities
in these soils (Reynolds etc. al).
It is believed that because the biodiversity of microbial species in the
Arctic “is inadequately characterized, it is only possible to guess at the
community’s response to environmental perturbations” (Woodin and Marquiss).
There are molecular methods of simulation that are currently being developed,
and hopefully they will soon lead to a broader understanding of the functional
roles of fungi and microbes.
Reference:
1. Sarah J. Woodin & Mick Marquiss. (1997).
Ecology of Arctic Environment