Analysis of deep-layer and bottom circulations in the South China Sea based on eight quasi-global ocean model outputs

This study is a preliminary analysis of the South China Sea(SCS)deep circulations using eight quasi-global high-resolution ocean model outputs.The goal is to assess models’ability to simulate these deep circulations.The analysis reveals that models’deep temperatures are colder than the observations in the World Ocean Atlas,while most models’deep salinity values are higher than the observations,indicating models’deep water is generally colder and saltier than the reality.Moreover,there are long-term trends in both temperature and salinity simulations.The Luzon Strait transport below 1500 m is 0.36 Sv when averaged for all models,smaller compared with the observation,which is about 2.5 Sv.Four assimilated models and one unassimilated(OCCAM)display that the Luzon deep-layer overflow reaches its minimum in spring and its maximum in winter.The vertically integrated streamfunctions below 2400 m from these models show a deep cyclonic circulation in the SCS on a large scale,but the pattern is different from the diagnostic streamfunction from the U.S Navy Generalized Digital Environment Model(GDEM-Version 3.0,GDEMv3).The meridional overturning structure above 1000 m is similar in all models,but the spatial distribution and intensity below 1500 m are quite different from model to model.Moreover,the meridional overturning below 2400 m in these models is weaker than that of the GDEMv3,which indicates a deep vertical mixing process in these models is biased weak.Based on the above evaluation,this paper discusses the impacts of T/S initial value,topography,and mixing scheme on the SCS deep circulations,which may provide a reference for future model improvement.

Draft genome of an Aerophobetes bacterium reveals a facultative lifestyle in deep-sea anaerobic sediments

Aerophobetes （or CD12） is a recently defined bacterial phylum, of which the metabolic processes and ecological importance remain unclear. In the present study, we obtained the draft genome of an Aerophobetes bac- terium TCSI from saline sediment near the Thuwal cold seep in the Red Sea using a genome binning method. Analysis of 16S rRNA genes of TCS1 and close relatives revealed wide distribution of Aerophobetes in deep-sea sediments. Phylogenetic relationships showed affinity between Aerophobetes TCS 1 and some thermophilic bac- terial phyla. The genome of TCS1 （at least 1.27 Mbp） contains a full set of genes encoding core metabolic path- ways, including glycolysis and pyruvate fermentation to produce acetyl-CoA and acetate. The identification of cross-membrane sugar transporter genes further indicates its potential ability to consume carbohydrates preserved inthe sediment under the microbial mat. Aerophobetes bac- terium TCS1 therefore probably carried out saccharolytic and fermentative metabolism. The genes responsible for autotrophic synthesis of acetyl-CoA via the Wood-Ljung- dahl pathway were also found in the genome. Phylogenetic study of the essential genes for the Wood-Ljungdahl pathway implied relative independence of Aerophobetes bacterium from the known acetogens and methanogens. Compared with genomes of acetogenic bacteria, Aero- phobetes bacterium TCS 1 genome lacks the genes involved in nitrogen metabolism, sulfur metabolism, signal trans- duction and cell motility. The metabolic activities of TCS 1 might depend on geochemical conditions such as supplies of CO2, hydrogen and sugars, and therefore the TCSI might be a facultative bacterium in anaerobic saline sedi- ments near cold seeps.