Spatial structure of the microbial community in sandy carbonate sediment

The environmental conditions in permeable carbonate sands are in many ways different from those in fine-grained sediments, but little is known about how this affects the structure and composition of the microbial community. We studied the microbial community structure in relation to the geochemical zonation within a 4 m2 patch of a sandy carbonate sediment in Kane’ohe Bay, O’ahu, Hawaii. Porewater concentrations of oxygen and sulfide at depths between 15 and 52 cm were measured in situ using voltammetry, and sediment samples from 0 to 5 cm, 25 to 35 cm, and 70 to 80 cm depth were used to evaluate vertical and horizontal variation in the microbial community. Anoxic, suboxic (sulfide as well as oxygen below detection limit), or oxic conditions were measured in porewater from between 15 and 32 cm below the sediment/water interface, with no consistent depth pattern. All measurements below 32 cm indicated anoxic conditions, with sulfide concentrations ranging from <10 to about 200 μM. The microbial community revealed by denaturing gradient gel electrophoresis (DGGE) was homogeneous in the surface sediment, while large variations were observed at 25 to 35 cm depth, and smaller variations occurred at 70 to 80 cm depth. Richness analysis of clone libraries from each of the 3 depths indicated that the phylotype diversity was highest in the library from 25 to 35 cm depth. We suggest that the microbial community structure is correlated to local physico-chemical conditions in the sediment and that spatio-temporally heterogeneous redox conditions result in an increased microbial diversity. A monophyletic group of phylotypes that were highly divergent from any other sequences deposited in the GENBANK database was detected in the sediment. These phylotypes may represent chloroplasts or plastids associated with eukaryotes or a new lineage of Bacteria related to the Cyanobacteria. Using specific primers, members of the group were detected in sands from several coastal sites on the island of O’ahu.