TEMPORAL CHANGES IN THE BACTERIAL COMMUNITIES OF SOIL, RHIZOSPHERE, AND ENDORHIZA ASSOCIATED WITH FIELD-GROWN CUCUMBER (CUCUMIS-SATIVUS L.)

In order to understand how bacterial communities develop in natural ec osystems, the culturable aerobic-heterotroph bacterial communities of three habitats (rhizosphere, endorhiza, and soil) associated with fiel d-grown cucumber roots were examined for quantitative and qualitative changes in structure, over time. Soil and root samples were taken 0, 7 , 14, 21, 35, and 70 days after planting (DAP) in 1994, and 0, 7, 14, 28, 42, and 70 DAP in 1995. Individual bacterial colonies of aerobic-h eterotroph bacteria were sampled randomly and characterized using the Sherlock System (Microbial ID, Inc., Newark, Del), for fatty acid meth yl ester analysis. More than 6,000 bacterial isolates were identified to the genus level and used in calculating richness, diversity, and si milarity indices for the three habitats. Diversity was assessed using Hill's modifications of Shannon's index and Simpson's index (N1 and N2 , respectively), while similarity was determined using the coefficient of biotic similarity. The same general trends were observed for the c ommunity structures of each habitat in both years. The soil was the le ast diverse and the rhizosphere was most diverse throughout both growi ng seasons. Bacillus and Arthrobacter spp. were dominant in soil, whil e Gram-negative bacteria tended to be more abundant in the rhizosphere and endorhiza. The initial membership of the endorhiza (7-14 DAP) app eared to be a function of the rhizosphere community, since both habita ts were composed of similar genera. However, the endorhiza was less di verse than the rhizosphere because one or two genera tended to be domi nant in each sample, particularly Agrobacterium and Enterobacter spp. Analysis of community similarity indicated that the soil was a distinc t habitat and remained relatively unchanged throughout the study. The rhizosphere and endorhiza were dissimilar from the soil and from each other at particular sampling times. These results indicate that the so il, rhizosphere, and endorhiza communities are structurally distinct ( the relative abundance of a particular bacterial genera differs among habitats) but share a common membership. Thus, when examining bacteria l communities, both the qualitative (i.e., membership) and quantitativ e (i.e., abundance) aspects must be examined in order to study shifts in community structure.