When microbes are subjected to temporal changes in nutrient availability, growth rate and substrate affinity can contribute to competitive fitness and thereby affect microbial community structure. This hypothesis was tested using planktonic bacterial communities exposed to nutrient additions at 1-, 3-, 7-, or 14-day intervals. Growth rates after nutrient addition were inversely proportional to the pulse interval and declined from 0.5 h ؊1 to 0.15 h ؊1 as the pulse interval increased from 1 to 14 days. The dynamics of community structure were monitored by 16S rRNA gene PCR-denaturing gradient gel electrophoresis. At pulse intervals of more than 1 day, the community composition continued to change over 130 days. Although replicate systems exposed to the same pulse interval were physiologically similar, their community compositions could exhibit as much dissimilarity (Dice similarity coefficients of <0.5) as did systems operated at different intervals. Bacteria were cultivated from the systems to determine if the physiological characteristics of individual members were consistent with the measured performance of the systems. The isolates fell into three bacterial divisions, Bacteroidetes, Proteobacteria, and Actinobacteria. In agreement with community results, bacteria isolated from systems pulsed every day with nutrients had higher growth rates and ectoaminopeptidase specific activities than isolates from systems pulsed every 14 days. However, the latter isolates did not survive starvation longer than those provided with nutrients every day. The present study demonstrates the dynamic nature of microbial communities exposed to even simple and regular environmental discontinuities when a substantial pool of species that can catabolize the limiting substrate is present.In most natural ecosystems microorganisms are likely to experience alternating periods of unrestricted growth with surplus nutrients, nutrient-limited growth, and starvation (6, 35). For example, phototrophs are exposed to light-dark cycles (23), with resulting effects on substrate supply to heterotrophs (1). These nutrient pulses can take place at all scales in nature (36)-over kilometers in coastal marine upwellings (19) and millimeters in laminated microbial mats. For heterotrophic microorganisms, the availability of organic substrates as the carbon and energy source is probably the most significant environmental restriction on growth (24). This is particularly true in unsaturated soil habitats, where nutrient mobility is restricted and surface-attached bacteria may remain dormant for extended periods (6).The interval between nutrient pulses could have substantial effects on microbial community composition and perhaps on microbial community function. When a nutrient pulse recurs, the physiological traits that are selected may be in opposition to those required for the microbes to survive an extended period of starvation. The exposure of a starved community of bacteria to an energy source results in interspecies competition (9) and was predicted to s...

Soybean [Glycine max (L.) Merr.] is one of the most important sources of plant‐based oil worldwide. Linolenic acid is associated with undesirable flavors and poor stability in soybean oil. Partial hydrogenation is typically used to reduce the levels of linolenic acid in soybean oil; however, this process results in the production of trans fatty acids. Studies have shown a link between increased risk of coronary heart disease and trans fatty acids present in hydrogenated oils. Thus, it is crucial to seek out genetic strategies to develop soybean lines with lower linolenic acid. To identify novel and useful alleles that could help reduce levels of linolenic acid in soybean, we screened a chemically mutagenized population. Three lines with reduced levels of linolenic acid in seed were identified. Mutant lines contained 3.8 to 4.5% of the total fatty acids as linolenic acid and carried independent mutations in the FAD3A gene encoding a desaturase responsible for the conversion of linoleic acid to linolenic acid in soybean seeds. To track the mutant alleles in a segregating population, allele specific markers were developed and the genetic association was confirmed with the reduced level of linolenic acid. These soybean lines will serve as a source of reduced linolenic acid to meet the breeding objectives of providing improved soybean lines.

Soybean oil has a wide variety of uses, and stearic acid, which is a relatively minor component of soybean oil is increasingly desired for both industrial and food applications. New soybean mutants containing high levels of the saturated fatty acid stearate in seeds were recently identified from a chemically mutagenized population. Six mutants ranged in stearate content from 6–14% stearic acid, which is 1.5 to 3 times the levels contained in wild-type seed of the Williams 82 cultivar. Candidate gene sequencing revealed that all of these lines carried amino acid substitutions in the gene encoding the delta-9-stearoyl-acyl-carrier protein desaturase enzyme (SACPD-C) required for the conversion of stearic acid to oleic acid. Five of these missense mutations were in highly conserved residues clustered around the predicted di-iron center of the SACPD-C enzyme. Co-segregation analysis demonstrated a positive association of the elevated stearate trait with the SACPD-C mutation for three populations. These missense mutations may provide additional alleles that may be used in the development of new soybean cultivars with increased levels of stearic acid.

Sixteen replicate microcosms were inoculated with a mixed assemblage of heterotrophic bacteria and provided with discrete pulses of protein as carbon and energy source. The dynamics of community structure were monitored by 16S rRNA gene polymerase chain reaction denaturant gradient gel electrophoresis (PCR-DGGE). The results were consistent with a strong role for biological interactions in maintaining diversity. Replicate microcosms developed different microbial communities. For systems exposed to nutrient pulses every 7 days, the number of DGGE bands averaged 13 +/- 4 (mean +/- SD) and the Dice similarity coefficient between pairs ranged from 0.08 to 0.67. In each of 16 systems provided protein once each day, there were dynamic changes over the first 30 days but community composition was stable over the next 20 days. However, most systems differed from each other; two-thirds of the pairwise comparisons had similarity coefficients in the range of 0.35-0.63. These 16 systems contained 10 +/- 2 phylotypes (mean +/- SD) and in aggregate 34 phylotypes were found in the 16 systems. Most phylotypes were found in < 25% of the systems, and there were not strong networks of association among phylotypes.