Insights into microbially mediated cementation in modern beachrock in the Xisha Islands, South China Sea

The present paper supplies biological and lithological evidence for the mechanisms of biochemical cementation in modern beachrocks on the tropical Yongle Atoll, Xiasha Islands in the South China Sea. High-throughput sequencing was used to precisely determine the epilithic microbial community in beachrocks. Microscopic mineralogical examination of sedimentary grains further showed complex processes from rapid carbonate precipitation. The results from 18/16 s rRNA diversity examination indicated that green algae and cyanobacteria only occupied a small proportion of the community, albeit they had considerable amounts. The occurrences of Lyngbya (Family Oscillatoriaceae), Calothrix and Dichothrix (Family Rivulariaceae) with light colour may be related to calcified sheaths coated on the living cells. The short rod-like Mg calcite in Lyngbya sheaths and anhedral to euhedral Mg calcite in Calothrix or Dichothrix sheaths were organominerals mediated most likely by photosynthesis of microorganisms with chloroplasts. Different initial crystal patterns were closely related to different cyanobacterial extracellular polymeric substance (EPS) structures. After cell death, these microcrystals were released into the voids and removed by interstitial water. The carbonate seeds scattered all around the particle surface and then began to enlarge the grain size with the degradation of organic matter. Different paths of crystal growth eventually cemented the intertidal grains abiotically or biotically. This case sheds light on cryptic microbially mediated mineralization in beachrock. Even though chemical processes were considerable in the formation of the beachrocks on the modern reef flat, the pervasive microbial geological processes occurred quietly but effectively.