Microbial-Activated Sediment Traps Associated with Oncolite Formation Along a Peritidal Beach, Northern Arabian (Persian) Gulf, Kuwait

Abstract

Identification of microbial communities within shoreline sediments and sediment precipitates from the Tigris-Euphrates delta (northern Kuwait) were determined by microscopic/nanoscopic studies, and by molecular analysis. Oncolites are syn-diagenetic carbonate precipitates that are surviving in a shallow subtidal to intertidal siliciclastic environment with periodically excessive hydraulic energy, extreme salinity (up to 47 per mil), and high concentrations of organic matter. X-ray diffraction techniques reveal that oncolite cortices are predominantly composed of calcite, quartz, halite and dolomite, associated with minor fractions of clay minerals. Quantitative analysis of the Corey Shape Factor reveals distinct morphological populations but with local overlap. A plot of the Equivalent Diameter vs. Corey Shape Factor provided the best indicator of the morphological relationships within the total oncolite population, indicating a hydrodynamically controlled morphological distribution defining intertidal and subtidal oncolite classes. Direct microscopic examination of the samples indicates that diatoms are the most abundant eukaryotic algae in subtidal sediments and within actively precipitating carbonate cements, especially the genus Navicula. In contrast, filamentous cyanobacteria from the genus Anabaena are most abundant in the intertidal zone sediments. The PCR-DGGE of the 16SrRNA gene of the cyanobacteria shows a higher diversity for this genus of bacteria in all sediment samples and that the cyanobacterial population in the diagenetically precipitating oncolites are closely related to the population found in the subtidal sediments. Dunaliella viridis dominates the culturable algae obtained from the four tidal zones. Our results indicate that a range of microbial populations are actively contributing to the formation of microbially-induced sedimentary structures in the extreme conditions of the southern Tigris-Euphrates delta.

Keywords:

• oncolites
• oncoids
• Arabian (Persian) Gulf
• siliciclastics
• mats
• sand bars
• cements
• Corey Shape Factor
• Equivalent Diameter
• microbial communities
• 16SrRNA gene-PCR-DGGE

Acknowledgments

We wish to thank our colleagues in the Electron Microscopy Unit (EMU) of Kuwait University for their help in obtaining the images. This project was partly supported under SAF Grant Number GS03/01. Thanks to Linny Merlin Jose, Susan Eapen and Ahmad Al-Amer for the technical assistance. The remarks by two reviewers greatly improved the manuscript.