Acetogenic and sulfate-reducing bacteria inhabiting the rhizoplane and deep cortex cells of the sea grass Halodule wrightii

Recent declines in sea grass distribution underscore the importance of unde rstanding microbial community structure-function relationships in sea grass rhizospheres that might affect the viability of these plants. Phospholipid fatty acid analyses showed that sulfate-reducing:bacteria and clostridia w ere enriched in sediments colonized by the sea grasses Halodule wrightii an d Thalassia testudinum compared to an adjacent unvegetated sediment. Most-p robable-number analyses found that in contrast to butyrate-producing clostr idia, acetogens and acetate-utilizing sulfate reducers were enriched by an order of magnitude in rhizosphere sediments. Although sea grass roots are o xygenated in the daytime, colorimetric root incubation studies demonstrated that acetogenic O-demethylation and sulfidogenic iron precipitation activi ties were tightly associated with washed, sediment-free H. wrightii roots. This suggests that the associated anaerobes are able to tolerate exposure t o oxygen. To localize and quantify the anaerobic microbial colonization, ro ot thin sections were hybridized with newly developed P-33-labeled probes t hat targeted (i) low-G+C-content gram-positive bacteria, (ii) cluster I spe cies of clostridia, (iiii) species of Acetobacterium, and (iv) species of D esulfovibrio. Microautoradiography revealed intercellular colonization of t he roots by Acetobacterium and Desulfovibrio species. Acetogenic bacteria o ccurred mostly in the rhizoplane and outermost cortex cell layers, and high numbers of sulfate reducers were detected on all epidermal cells and inwar d, colonizing some 60% of the deepest cortex cells. Approximately 30% of ep idermal cells were colonized by bacteria that hybridized with an archaeal p robe, strongly suggesting the presence of methanogens. Obligate anaerobes w ithin the roots might contribute to the vitality of sea grasses and other a quatic plants and to the biogeochemistry of the: surrounding sediment.