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Applied and Environmental Microbiology, November 1999, p. 5117-5123, Vol. 65, No. 11
Gulf Ecology Division, U.S. EPA/National
Health and Environmental Effects Research Laboratory, Gulf Breeze,
Florida 32561,1 and Department of
Ecological Microbiology, BITOEK, University of Bayreuth, 95440 Bayreuth, Germany2
Received 12 April 1999/Accepted 18 August 1999
Recent declines in sea grass distribution underscore the importance
of understanding 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 were enriched in sediments
colonized by the sea grasses Halodule wrightii and
Thalassia testudinum compared to an adjacent unvegetated
sediment. Most-probable-number analyses found that in contrast to
butyrate-producing clostridia, acetogens and acetate-utilizing sulfate
reducers were enriched by an order of magnitude in rhizosphere
sediments. Although sea grass roots are oxygenated in the
daytime, colorimetric root incubation studies demonstrated that
acetogenic O-demethylation and sulfidogenic iron precipitation
activities were tightly associated with washed, sediment-free H. wrightii roots. This suggests that the associated anaerobes are
able to tolerate exposure to oxygen. To localize and quantify the
anaerobic microbial colonization, root thin sections were hybridized
with newly developed 33P-labeled probes that targeted (i)
low-G+C-content gram-positive bacteria, (ii) cluster I species of
clostridia, (iii) species of Acetobacterium, and (iv)
species of Desulfovibrio. Microautoradiography revealed
intercellular colonization of the roots by Acetobacterium and Desulfovibrio species. Acetogenic bacteria occurred
mostly in the rhizoplane and outermost cortex cell layers, and high
numbers of sulfate reducers were detected on all epidermal cells and
inward, colonizing some 60% of the deepest cortex cells. Approximately 30% of epidermal cells were colonized by bacteria that hybridized with
an archaeal probe, strongly suggesting the presence of methanogens. Obligate anaerobes within the roots might contribute to the vitality of
sea grasses and other aquatic plants and to the biogeochemistry of the
surrounding sediment.
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Acetogenic and Sulfate-Reducing Bacteria Inhabiting the
Rhizoplane and Deep Cortex Cells of the Sea Grass Halodule
wrightii
*
Corresponding author. Mailing address: Gulf Ecology
Division, U.S. EPA/NHEERL, 1 Sabine Island Dr., Gulf Breeze, FL
32561. Phone: (850) 934-9346. Fax: (850) 934-9346. E-mail:
devereux.richard{at}epamail.epa.gov.
Contribution no. 1064 from the Gulf Ecology Division, U.S.
EPA/NHEERL.
Present address: Department of Biological Sciences, Central
Washington University, Ellensburg, WA 98926.
Applied and Environmental Microbiology, November 1999, p. 5117-5123, Vol. 65, No. 11
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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