Bipolar Division Soil Rhizosphere Oxygen supply limited by

Bipolar Division

Soil & Rhizosphere • Oxygen supply limited by mineral particle size, organic matter content and water content. • Plant roots may also add oxygen to deeper soils or anaerobic soils. • Water saturation leads to anaerobic conditions and increased denitrification. • Distribution of microbes depends on organic matter supply and source (humus and root exudates)

Oxygen Gradients Within soil aggregates oxygen decreases toward the interior. Creates microniches for fermentative and anaerobic respiring bacteria. Organic matter and availability of alternate electron acceptors (e. g. nitrate, sulfate, carbon dioxide) will partly determine which anaerobic respiring bacteria thrive where.

Soil Close-Up Geosmin = “earthy smell” = Cyanobacteria = Actinomycetes

Bulk Soil vs Rhizosphere • Bulk soil is that away from the influence of plant roots (i. e. water, oxygen, exudates). • Bulk soil community is subject to greater environmental variability. • Rhizosphere community has greater microbial biomass and activity compared with that in bulk soil.

Nitrogen Fixing Actinomycetes Frankia spp. Nodulate woody non-leguminous plants, such as Alnus spp (alders). Association is called actinorhizae.

Crown Gall Disease caused by Agrobacterium spp.

T-plasmid Function

Mycorrhizae Fungi Hoch et al. , 1998 Ectomycorrhizal association between Douglas Fir (Pseudotsuga menziesii) and a boletus-like mushroom (Suillus caerulescens). Hoch et al. , 1999

Linking sporocarp to ectomycorrhizae • Internal Transcribed Spacer (ISR) – PCR • Amplifies the region between 18 S and 28 S r. RNA genes in fungi. • RFLP analysis to fingerprint and determine a match (sporocarp to “tip”). Kbp Alu I S 1 T 1 a Hinf I T 1 b S 1 T 1 a . T 1 b 650 492 369 246 123