Vertical variation in Vibrio community composition in Sansha Yongle Blue Hole and its ability to degrade macromolecules

With the advantages of wide distribution,fast growth,and broad metabolic spectrum to organic carbon compounds,Vibrio may play an important role in organic carbon cycling. However,the ecological roles of Vibrio in many marine environments have not been explored. Here,the world's deepest 'blue hole',the Sansha Yongle Blue Hole (SYBH) in the South China Sea,which is a geographically semi-enclosed environment featuring unique chemical characters,was investigated. The abundance,diversity and carbon source utilization capability of Vibrio were studied by quantifcation and high-throughput sequencing of Vibrio specifc 16S rRNA genes and cultivation methods. The abundance of Vibrio in water column of the SYBH ranged from 3.78 ×10⁴ to 7.35 ×10⁶ 16S rRNA gene copies L^-1. Free-living Vibrio was more abundant than particle-associated Vibrio (~1.20×10⁶ versus~2.68×10⁵ gene copies L^-1),indicating that Vibrio prefers a free-living life style. The Vibrio assemblages showed clear vertical stratifcation and could be divided into three groups: aerobic-transition,middle anaerobic and bottom anaerobic zones. Dissolved oxygen (DO),temperature,pH and salinity were the main environmental factors afecting the abundance and community composition. Cultivated Vibrio demonstrated a degrading capability to various macromolecular substrates,including starch,Tween 20/40/80,DNA,gelatin,alginate,casein,chitin,lecithin,κ-carrageenan,mannan,xylan and hyaluronic acid. This suggests that Vibrio could produce a variety of highly active extracellular enzymes. Our study provides new insights into the distribution pattern and possible role in carbon cycle of Vibrio in the unique environment of a 'blue hole'.