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Changes in the epiphytic cyanobacterial community and diazotrophic activity on the tropical seagrasses induced by varying nutrient regimes
Stockholm University, Faculty of Science, Department of Botany. (Plant Physiology, Birgitta Bergman's group)
University of Dar es Salaam.
Institut de Ciéncies del Mar (ICM), CMIMA-CSIC, Barcelona.
Stockholm University, Faculty of Science, Department of Botany. (Mats Björk)
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Seagrasses were subject to different nitrogen (N) and phosphorus (P) regimes in a flow trough system (four weeks) to study the influence of moderate nutrient enrichments on the associated cyanobacterial diversity and diazotrophic activity. The seagrass Cymodocea serrulata (R. Brown) were collected outside an urbanized area, west of the Unguja Island, Zanzibar in the Indian Ocean. The epiphytic cyanobacterial community was characterized morphologically (light microscopy) and phylogenetically by DGGE fingerprinting using the 16S rRNA and nifH genes. A lower diversity of both heterocystous and non-heterocystous cyanobacteria was apparent in the natural seagrass stands, when compare to the pulsed nutrient additions that stimulated the growth of the cyanobacterial epiphytes. Non-heterocystous morpho- and genotypes (e.g. Lyngbya, Oscillatoria and Leptolyngbya) dominated and were intermixed with a few representatives of the heterocystous genus Calothrix. Many of the cyanobacterial sequences retrieved represented uncultured and potentially novel diazotrophic phylotypes. Diel nifH gene transcription levels and patterns, and the diel nitrogenase activity (acetylene reduction; max levels of 70.1 ±.19 nmol C2H4 g-1 h-1) verified the presence of a distinct proportion of diazotrophs, which however was negatively affected even by moderate nutrient additions. Although the seagrass host remained unaffected, the increased nutrients levels, mimicking anthropogenic eutrophication in these coastal regions, promoted a rapid change in the epiphytic community composition

Keyword [en]
nifH gene, DGGE, epiphytes, diazotrophy, seagrass
National Category
Ecology Botany
Research subject
Marine Ecology; Plant Physiology
URN: urn:nbn:se:su:diva-38271OAI: diva2:308498
Sida/SAREC Bilateral Marine Science ProgrammeThe Swedish Foundation for International Cooperation in Research and Higher Education
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2011-04-14Bibliographically 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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