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INTRACELLULAR COLONIZATION OF SEAGRASS ROOTS BY ACETOGENIC AND SULFIDOGENIC BACTERIA
Citation:
Devereux, R D., H. C. Pinkart, K. Kusel, AND H. L. Drake. INTRACELLULAR COLONIZATION OF SEAGRASS ROOTS BY ACETOGENIC AND SULFIDOGENIC BACTERIA. Presented at 1999 Annual Meeting of the American Society for Microbiology, Chicago, IL, May 30-June 3, 1999.
Description:
The contribution of seagrasses to the stability and fertility of estuarine ecosystems is well established. Loss of seagrasses in recent years to disease and coastal development underscores the importance of understanding the microbial ecology of seagrasses, and the possible roles of plant-microbe interactions in sustaining these plants. Seagrasses are rooted in highly reduced, anoxic sediments dominated by anaerobic processes. Lipid biomarkers and most-probable-numbers (MPN) of two anaerobic bacterial groups, acetogens and sulfate reducers, were enriched in a seagrass bed sediment relative to unvegetated sediment. The seagrass rhizosphere is a habitat that is oxygenated during the day, becoming anaerobic at night. Despite such in situ oxygenation, microcosm experiments demonstrated a tight association of anoxic sulfidogenic and O-demethylating acetogenic activities with excised, washed, sediment-free Halodule wrightii roots. However, little is known about the colonization of roots by anaerobic bacteria. To locate the root-associated anaerobes, cross sections of H. wrightii roots were hybridized with specific 33P-labelled rRNA probes and examined by microautoradiography. Most bacteria resided in the rhizoplane. However, hybridization with newly designed probes for (i)cluster I of the clostridia, (ii)Acetobacterium and (iii)Desulfovibrio revealed considerable colonization of the intracellular spaces deeper in the root cortex. In fact, 60% of root cells nine layers beneath the surface were colonized by Desulfovibrio. These direct enumerations, and MPN enumerations with washed roots, indicated that the seagrass root/rhizosphere could be a preferred habitat of acetogens and sulfate reducers. Hybridization of the cross sections with an Archaea Domain probe revealed possible colonization of the rhizoplane by methanogenic bacteria. The distribution of anaerobic bacteria suggests that a sharp oxygen gradient might emanate from the root stele. These results demonstrate that o