Modification of spatial distribution of 2,4-dichlorophenoxyacetic acid degrader microhabitats during growth in soil columns.

Bacterial processes in soil, including biodegradation, require contact between bacteria and substrates. Knowledge of the three-dimensional spatial distribution of bacteria at the microscale is necessary to understand and predict such processes. Using a soil microsampling strategy combined with a mathematical spatial analysis, we studied the spatial distribution of 2,4-dichlorophenoxyacetic acid (2,4-D) degrader microhabitats as a function of 2,4-D degrader abundance. Soil columns that allowed natural flow were percolated with 2,4-D to increase the 2,4-D degrader abundance. Hundreds of soil microsamples (minimum diameter, 125 microm) were collected and transferred to culture medium to check for the presence of 2,4-D degraders. Spatial distributions of bacterial microhabitats were characterized by determining the average size of colonized soil patches and the average number of patches per gram of soil. The spatial distribution of 2,4-D degrader microhabitats was not affected by water flow, but there was an overall increase in colonized patch sizes after 2,4-D amendment; colonized microsamples were dispersed in the soil at low 2,4-D degrader densities and clustered in patches that were more than 0.5 mm in diameter at higher densities. During growth, spreading of 2,4-D degraders within the soil and an increase in 2,4-D degradation were observed. We hypothesized that spreading of the bacteria increased the probability of encounters with 2,4-D and resulted in better interception of the degradable substrate. This work showed that characterization of bacterial microscale spatial distribution is relevant to microbial ecology studies. It improved quantitative bacterial microhabitat description and suggested that sporadic movement of cells occurs. Furthermore, it offered perspectives for linking microbial function to the soil physicochemical environment.

Geostatistical analysis of the distribution of NH(4)(+) and NO(2)(-)-oxidizing bacteria and serotypes at the millimeter scale along a soil transect.

Soil is known to be heterogeneous for different activities at several spatial scales. Most studies have focused on macro- and meso-scales but micro-scales are rarely addressed. Hence, the spatial structure of NH(4)(+)- and NO(2)(-)-oxidizers and of various serotypes of the latter was studied along two transects of approximately 10 cm, with two micro-samples taken from each millimeter. The presence of NH(4)(+)- and NO(2)(-)-oxidizers in a micro-sample was detected using colorimetric tests for the presence or absence of NO(2)(-) in cultures of the micro-samples. Geostatistics was used to determine the range of spatial influence of the bacterial types. For both types, semi-variograms indicated a non-random spatial pattern. The spatial dependence ranged from 2 to 4 mm for NO(2)(-)- and NH(4)(+)-oxidizers respectively, and the two bacterial types were not independently spatially located. Among the six serotypes of NO(2)(-)-oxidizers, only one exhibited a spatial dependence. The existence of a spatial structure at the millimeter scale suggests that micro-scale sampling should be employed for soil studies. Therby, data on bacterial populations and activities can be referred to a spatial scale which is meaningful to these organisms.

Clutch size manipulations in the chestnut weevil, Curculio elephas: fitness of oviposition strategies.

We test the adaptive value of clutch size observed in a natural population of the chestnut weevil Curculio elephas. Clutch size is defined as the number of immatures per infested chestnut. In natural conditions, clutch size averages 1.7 eggs. By manipulating clutch size in the field, we demonstrate that deviations from the theoretical "Lack clutch size", estimated as eight immatures, are mainly due to proximate and delayed effects of clutch size on offspring performance. We show the existence of a trade-off between clutch size and larval weight. The latter, a key life-history trait, is highly correlated with fitness because it is a strong determinant of larval survival and potential fecundity of offspring females. The fitness of different potential oviposition strategies characterized by their clutch sizes, ranging from one to nine immatures, was calculated from field- estimated parameters. Chestnut weevil females obtain an evolutionary advantage by laying their eggs singly, since, for instance, fitness of single-egg clutches exceeds fitness of two-egg clutches and four-egg clutches by 8.0% and 15.1% respectively.

Coin-flipping plasticity and prolonged diapause in insects: example of the chestnut weevil Curculio elephas (Coleoptera: Curculionidae).

Spreading of emergence over several years due to prolonged diapause in some larvae was shown in the chestnut weevil. Depending on the year the larvae buried themselves in the ground, 32-56% of live adults emerged after 2 or 3 years of underground life. Variability in the duration of diapause was assumed to correspond to tactics of adaptative "coin-flipping" plasticity. This plasticity must allow the chestnut weevil to respond to the unpredictability of its habitat as measured by the irregularity of chestnut production and summer drought. Indeed, fecundity and adult longevity did not lessen after 2 years of underground life. No drastic decrease in the population size of weevils occurs after bad years; for instance when the number of chestnuts on the study tree is less than 10 000, passers-by can collect all the fruit and about 95% of larvae developing in chestnuts are destroyed. Diapause nature (simple or prolonged) may be related to moisture and gas rates in the ground from October to December. These factors acting in autumn are not known to be involved in the physiological mechanisms that control the production of chestnuts.