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Cyanobacteria associated with the phyllosphere of the seagrass Cymodocea rotundata: Diversity, diel nifH expression and nitrogenase activity: Diversity, nifH expression and activity in seagrass
Stockholm University, Faculty of Science, Department of Botany. (Plant Physiology, Birgitta Bergman's group)
Institut de Ciéncies del Mar (ICM), CMIMA-CSIC, Barcelona.
University of Dar es Salaam, Tanzania.
Stockholm University, Faculty of Science, Department of Botany. (Plant Physiology)
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Tropical seagrass ecosystems are highly productive and extremely important for sustaining marine life. As seagrasses are associated with complex assemblages of poorly examined epiphytic microbes, we proposed that nitrogen-fixing microorganisms may contribute to the productivity. The morphological and genetic diversity (based on the 16S rRNA and nifH genes) of cyanobacteria and diel variations in nifH gene expression, NifH protein levels and nitrogenase (nitrogen-fixing) activity were examined in the phyllosphere of Cymodocea rotundata of coastal areas of the western Indian Ocean (Tanzania). The 16S rRNA and nifH gene analyses during two consecutive years (October-November, 2007 and 2008) revealed the dominance of a mixed cyanobacterial community. Most sequences represented free-living uncultured cyanobacteria previously reported as benthic in the region, clearly separated from marine planktonic phylotypes, while a few sequences clustered with cyanobacterial symbionts of diatoms. Appreciable, but varying nitrogenase activities were found on a diel as well as monthly basis, with the highest activity encountered, 358 ± 232 and 258 ± 139 nmol C2H4 g-1 h-1, in November. On a diel basis, nifH gene expression coincided with the NifH protein level (Oct 2008) and nitrogenase activity. At day time, nifH gene expression primarily originated from heterocystous phylotypes, while from non-heterocystous filamentous phylotypes (mainly Oscillatoriales) at night. The data suggest that a variety of diazotrophic cyanobacteria are common among the epiphytes on Cymodocea and we propose that these may represent a valuable source of ‘new’ nitrogen in the often oligotrophic, but ecologically important seagrass ecosystems.

Keyword [en]
Cymodocea rotundata, diazotrophs, diversity, nifH, nitrogenase activity, seagras phyllosphere
National Category
Cell and Molecular Biology Botany Ecology
Research subject
Marine Ecology; Plant Physiology; Molecular Biology
URN: urn:nbn:se:su:diva-38267OAI: diva2:308491
Sida/SAREC Bilateral Marine Science Programme
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2010-04-07Bibliographically approved
In thesis
1. Genetic variability and nitrogenase activity of cyanobacterial communities associated with tropical seagrass meadows (western Indian Ocean)
Open this publication in new window or tab >>Genetic variability and nitrogenase activity of cyanobacterial communities associated with tropical seagrass meadows (western Indian Ocean)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tropical seagrass ecosystems are highly productive and important for sustaining marine life and associated coastal societies. In this study, the diversity and role of nitrogen-fixing cyanobacteria associated with five common seagrass genera in coastal regions of the western Indian Ocean (WIO; Tanzania) were examined, as well as the impact of anthropogenic activities. Cyanobacteria were characterized morphologically and genetically (16S rRNA and nifH gene phylogeny), as were diel variations in nifH gene expression, NifH protein levels and nitrogenase activity. The results revealed that WIO seagrass beds supported rich cyanobacterial diversity and that these represented approx. 83% of total clones obtained (DNA and RNA nifH clone libraries). Non-heterocystous genera, such as Oscillatoria, Lyngbya, Leptolyngbya, Phormidium and Microcoleus dominated, while heterocystous morphotypes such as Calothrix were less frequent and unicellular morphotypes (e.g. Gloeocapsa, Chroococcus and Chroococcidiopsis) were few. Additionally, the phylogenetic analysis revealed several novel uncharacterized cyanobacterial clades. Cyanobacterial composition and nitrogenase activity varied over seasons and between the seagrass species. Day time nitrogenase activity originated primarily from heterocystous phylotypes, while non-heterocystous filamentous phylotypes fixed nitrogen at night. The highest activity in the diel cycle was 358 ± 232 nmol C2H4 g-1 h-1at 09.00 associated with epiphytes of the seagrass Cymodocea. Nitrogenase activity was consistently lower in anthropogenically disturbed (eutrophication) seagrass sites. Such data suggest that diazotrophic cyanobacteria may be a significant source of ‘new’ nitrogen in the often oligotrophic coastal regions of tropical oceans. It is also proposed that the rapid shifts in the cyanobacterial population and function found may also be used as early disturbance indicator in coastal management practices.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University, 2010. 57 p.
cyanobacteria, nitrogenase activity, diazotrophs, seagrass, western Indian ocean
National Category
Research subject
Plant Physiology
urn:nbn:se:su:diva-38278 (URN)978-91-7447-001-7 (ISBN)
Public defence
2010-05-06, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00 (English)
SIDA SAREC Bilateral Marine Sciences ProjectThe Swedish Foundation for International Cooperation in Research and Higher Education
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Available from: 2010-04-14 Created: 2010-04-07 Last updated: 2010-04-14Bibliographically approved

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