Diazotrophic microbial community of coastal microbial matsof the southern North Sea

摘要

The diazotrophic community in microbial mats growing along the shore of the North Sea barrier island Schiermonnikoog(The Netherlands) was studied using microscopy, lipid biomarkers, stable carbon (delta 13CTOC) andnitrogen (delta 15N) isotopes as well as by constructing and analyzing 16S rRNA gene libraries. Depending on theirposition on the littoral gradient, two types of mats were identified, which showed distinct differences regardingthe structure, development and composition of the microbial community. Intertidal microbial mats showed a lowspecies diversity with filamentous non-heterocystous Cyanobacteria providing the main mat structure. In contrast,supratidal microbial mats showed a distinct vertical zonation and a high degree of species diversity.Morphotypesof non-heterocystous Cyanobacteria were recognized as the main structural component in these mats. Inaddition, unicellular Cyanobacteria were frequently observed, whereas filamentous heterocystous Cyanobacteriaoccurred only in low numbers. Besides the apparent visual dominance of cyanobacterial morphotpyes, 16S rRNAgene libraries indicated that both microbial mat types also included members of the Proteobacteria and theCytophaga–Flavobacterium–Bacteroides group as well as diatoms. Bulk delta 15N isotopes of the microbial matsranged from +6.1 per thousand in the lower intertidal to )1.2 per thousand in the supratidal zone, indicating a shift from predominantlynitrate utilization to nitrogen fixation along the littoral gradient. This conclusion was supported by the presence ofheterocyst glycolipids, representing lipid biomarkers for nitrogen-fixing heterocystous Cyanobacteria, insupratidal but not in intertidal microbial mats. The availability of combined nitrogen species might thus be a keyfactor in controlling and regulating the distribution of the diazotrophic microbial community of Schiermonnikoog.