The effects of soil moisture on microbial reproductive patterns
patterns are essential to understanding the overall fitness of an
K-selected reproductive patterns often occur when conditions are
favorable. r-Selected reproductive
patterns often occur
when conditions are unfavorable. Higher
moisture in the soil is most favorable for soil microbes. This study
determine if r-Selected and K-selected patterns are indicative of the
moisture in the soil. Soil samples from
wetland and grassland from
Reproductive patterns are an important variable in population dynamics. Knowledge of how an organism reproduces gives insight into their competitive ability which gives light to their overall fitness. The environment an organism inhabits plays a significant role in the manner in which the organism reproduces due to biotic and abiotic limitations.
Organisms reproduce with a K-selected or an r-selected reproductive pattern. K-selected reproductive patterns tend to occur in environments that are most stable for an organism, providing a sufficient amount of food, resources, and habitat for the more competitive individuals in the species. Individuals that reproduce with K-selected reproductive patterns tend to be better competitors, as they have to survive in dense populations and compete for limited resources. K-selected populations remain at carrying capacity (K) and grow at a much slower rate. Individuals with r-selected reproductive patterns are not as competitive, and compensate with high rates of reproduction. r-Selection most often occurs when conditions are unstable and there are various environmental limitations on a population. These particular strategists reach K very infrequently, or never (Luckenbill, 1979).
Research has shown that moisture in the soil is one of the limiting factors for microbes (Tate & Terry, 1980). Wetter soil essentially is a more favorable environment for microbial communities. Soils high in moisture allow bacteria to obtain an intracellular solute concentration higher than the solute concentration of the soil, preventing plasmolysis. Soils low in moisture are less favorable since microbes then have to exert more energy into increasing intracellular concentrations (Stark & Firestone, 1994). This excessive expenditure of energy reduces the amount of energy exerted toward microbial fitness and competitive ability.
In this experiment, I observed the reproductive patterns of microbes in wetland and grassland soils. I hypothesized the microbes from samples of wetland would reproduce with a K-selected reproductive pattern, due to the more favorable, wetter conditions, and the microbes in the drier soil would reproduce with an r-selected reproductive pattern due to constant exposure to less favorable conditions causing these individuals to have a lower competitive ability. Analysis of reproductive patterns of microbes in the soil gives more insight into conditions that allow for favorable reproductive growth. This knowledge can in turn lead to a better understanding of the efficiency of soil microbial activity, such as nitrification and break down of organic material.
MATERIALS AND METHODS
sampling technique was used to obtain random samples from both the
Media were prepared and cultivation techniques were performed as described in Janssen et al., 2002 with slight modifications as follows:
Dilute Nutrient Broth (DNB):
0.04 g of Difco
broth (BD Diagnostic Systems,
g of dehydrated agar powder (Cat. No. CD
900B) and .066 g of Anhydrous, flaked CaCl2 (B-mesh) was
added to 1
L of sterile distilled water in a 2 L flask (Pyrex, No. 4980). Solution was mixed with a Teflon magnetic bar
on a hot plate stirrer (
samples were taken to the lab and processed within 3 hours. All handling of soil samples was conducted in
a Laminar Hood Class II Biohazard Cabinet (Labconco, Cat. No. 36213,
04). Soil Samples were placed in a 37
Celsius incubator for 20 minutes to partially dry out.
Rocks, grass, and sticks were removed and soil
was sieved through a brass sieve (10). 1
g of soil was added to 100 mL of sterile distilled water in 150 mL
flask to obtain dilution of 1:100. Soil
suspensions were dispersed by mixing with a vortex mixer (Vortex-Genie,
No. 12-812) on speed 8 for five minutes.
1 mL of the soil suspensions were then added to 9 mL of DNB to
dilution of 1:1000. Test tubes were
capped with polypropylene closures and mixed with a vortex mixer on
speed 8 for
10 seconds. 1 mL of the soil suspensions
were, again, added to 9 mL of DNB immediately, to obtain the dilution
1:10,000. Suspensions were immediately
plated after the last dilution to ensure the microbes would not settle
bottom of the test tubes. 200 microliters of soil suspensions were
agar plates. Inoculants were spread over
the agar with a glass triangle spreading rod.
All preparations were labeled by sample number and day,
incubated at 25 degrees Celsius in the dark.
10 agar plates were not inoculated and incubated as controls for
wetland and grassland, totaling 20 control plates.
Colonies were observed
every three to four days. Colonies were
counted on the same day, resulting in the grassland samples to be
day less of incubation than the wetland samples. Colonies
were observed using Darkfield Quebec
Colony Counter (Model: 3325) and tallied using Multiple-Tally
1-tailed, Independent T-Test was conducted on the results using Excel
compare growth between grassland and wetland cultures every three to
Counts of both grassland and wetland cultures were done on the same day. The number of days of incubation for
grassland cultures and wetland cultures were combined into two day
for analysis since grassland samples were plated the day after the
samples were plated. The higher number
in the interval represented the wetland samples and the lower number in
interval represented the grassland samples (Fig 1).
All soil samples were totally saturated, having a Soil Moisture Meter reading of 10. No colonies were formed on the twenty control plates eliminating the possibility of contamination. Colonies began to grow after 2 days of incubation. The average colony counts for the wetland and grassland are represented below (Fig. 1). The colony counts for the wetland on day 5 had an average of 63.1 with a standard deviation of 40.69; Day 8 had an average of 147.4 with a standard deviation of 81.69, and day 12 had an average of 172.4 with a standard deviation of 78.19. The colony counts for the grassland on day 4 had an average of 203.4 with a standard deviation of 92.53, day 7 had and average of 275.6 with a standard deviation of 115.15, and day 11-12 had an average of 287.4 with a standard deviation of 96.75 (Note: samples 11-15 were counted on day 11 and 16-20 were counted on day 12). The microbes from the samples collected in the grassland grew significantly faster than the microbes obtained from samples collected in the wetland (Fig 1, p<1.2 x 10-6). Colony growth on both plates appeared to slow down after the 7-8 day interval.
Fig. 1. Comparison of the
number of colonies grown per day
between wetland and grassland samples.
The significant difference between the growth of microbes from grassland and microbes in wetland may very well be indicative of environmentally induced growth patterns. In order to confirm this inference, more information needs to be obtained regarding the types of bacterial colonies that were formed. In this study, there was no deciphering between colonies, which becomes a limitation. There is no way of indicating whether or not some of the colonies from the grassland samples may have demonstrated k-selected growth patterns or if colonies from the wetland samples demonstrated r-selected growth patterns since colony counting was done under the assumption that all colonies were of the same species. Klappenblach et al. (2000) observed the time in which bacteria respond to variables in their environment is correlated with the number of rRNA operon genes in their DNA. More analysis of this aspect will give more insight into whether or not the displayed reproductive patterns in this study were solely environmentally induced or if there may have been some other biological factor involved. Furthermore, soil moisture may not have been the only limiting factor. Researchers have found that low nutrients in the soil cause microbes to grow slower due to lack of sufficient resources for metabolic activity (Klappenblach et al., 2000). Soils in the wetland may have been leached of the necessary nutrients, causing the microbes to adapt to growing at a slower rate. In addition, microbes in saturated soils like wetlands tend to be anaerobic (Tate & Terry, 1980). More techniques can be used to isolate the aerobic and anaerobic microbes to analyze the differences in their reproductive patterns.
When cultivating the microbes, the wet weight of the soil was obtained rather than the dry weight since the samples were not completely dry. This may have lessened the amount of microbes in the dilutions which may have been the reason for no visible colonies formed during the 1-2 Day interval. The dry weight should be obtained to observe whether or not it will result in faster colony formation. In addition, the colonies were incubated for three weeks. Literature supports an incubation of 10 weeks for maximum colony growth (Janssen et al., 2002). Consideration of these methods should be given when replicating this study.
both days of collecting samples, it rained.
This is the cause for the total saturation of the soil in both
grassland and wetland. The amount of
rain to each site is variable. Furthermore, because bacterial
very rapidly causing fast adaptations to their environment, the
moisture for two days may have altered the reproductive patterns of the
microbes in the grassland. Further research should be conducted on days
has not rained to observe the reproductive patterns under normal soil
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