Parsing the relative importance of environmental (recent disturbances) and spatial factors (historical processes) in determining community structure is a core issue in ecology. The Bohai Bay is a typical semi‐enclosed bay located in the north of China, surrounding by the metropolitan area with anthropogenic disturbances made it a complex marine coastal system with pollution gradients, where the distributions and determinants of bacterioplankton communities remain unclear. In this study, we collected surface water samples from 19 sites across Bohai Bay at about 100 km scale to investigate the relative roles of local environments and regional spatial factors in shaping bacterioplankton community composition (BCC). The environmental parameters in the sampling region showed gradient change according to the geographic variation. Several abundant OTUs were significantly correlated with the pollution parameters in the studied area, and 16 OTUs of them showed distinct distribution pattern in different polluted regions with obvious geographic segmentation, which indicated the effects of pollution gradient and dispersal limitation on specific taxon. The BCCs did not show obviously clustering effect between different polluted regions, which indicated the complexity for explaining the BCC variation in the studied region. The partial Mantel test revealed stronger spatial effects on beta diversity than those of local environmental factors, which indicated that dispersal limitation accounted more for the beta diversity than environmental heterogeneity. Furthermore, variation partitioning analysis (VPA) conducted by combining the environmental variables, linear trends, and principal coordinates of the variables from neighbor matrices (PCNM) showed that it was the joint effects of environmental and spatial factors contributed to the explained variation of BCC in the studied area. Considering the special human geography characteristics of Bohai Bay, the unmeasured biotic/abiotic factors, stochastic factors, and anthropogenic disturbances may be responsible for the unexplained variation of the BCC.

Mass mortality of farmed small abalone Haliotis diversicolor occurred in Fujian, China, from 2009 to 2011. Among isolates obtained from moribund abalones, the dominant species AP37 exhibited the strongest virulence. After immersion challenge with 10 6 CFU ml −1 of AP37, abalone mortalities of 0, 53 and 67% were induced at water temperatures of 20°C, 24°C, and 28°C, respectively. Following intramuscular injection, AP37 showed a low LD 50 (median lethal concentration) value of 2.9 × 10 2 CFU g −1 (colony forming units per gram abalone wet body weight). The LT 50 (median lethal time) values were 5.2 h for 1 × 10 6 CFU abalone . For further analysis of virulence, AP37 was screened for the production of extracellular factors. The results showed that various factors including presence of flagella and production of extracellular enzymes, such as lipase, phospholipase and haemolysin, could be responsible for pathogenesis. Based on its 16S rRNA gene sequence, strain AP37 showed > 98.8% similarity to Vibrio harveyi, V. campbellii, V. parahaemolyticus, V. algi nolyticus, V. na trie gens and V. rotiferianus, so it could not be identified by this method. However, multi-locus sequence analysis (MLSA) of concatenated sequences, including the rpoD, rctB, gyrB, toxR and pyrH genes, identified strain AP37 as V. harveyi. Phenotypic characters of AP37 were identified by API 20E. In antibiotic susceptibility tests, strain AP37 exhibited susceptibility to 7 antibiotics and resistance to 13. This is the first report of a V. harveyi-related species being linked with the mass mortality of adult abalone H. diversicolor in southern China.

A mass mortality of farmed juvenile abalones (Haliotis discus hannai) occurred in South China in December of 2012. Combined traditional culturing with terminal restriction fragment length polymorphism (T-RFLP) methods, microbial communities associated with juvenile abalones were investigated under different survival conditions, including healthy (HA) and moribund (HM) abalones in the healthy pond with low mortalities, and healthy (DA) and moribund (DM) abalones in the diseased pond with a high mortality rate. The results indicated that both HA and DM exhibited greater microbial diversity and evenness. Moreover, the number of bacterial colony forming units (CFUs) from HA and DM were higher than other samples from the same pond. HA and DA samples harboured absolutely dominant T-RFs with 504 bp (76.2% of total T-RFs) and 436 bp (63.2%) respectively. Nesterenkonia sandarakina, Staphylococcus haemolyticus and Agarivorans albus were the dominate isolates in HA samples, but no Vibrio genus was isolated even on vibrio-specific medium. Reversely, in the DM samples, up to 94% of the total bacterial CFUs were composed of the family Vibrionaceae, represented by Photobacterium swingsii (44%) and Vibrio shilonii (45%). Also, Vibrio was the only cultivated genus from the HM group. Of these Vibrio species, V. atlanticus were detected in HM, DA and DM samples and could generate the T-RF 504 bp. The microbial composition and diversity of juvenile abalones in different surviving states provided insights into the stability and dynamics of microbial communities during disease outbreak which could be helpful to predict and to control disease in the future.

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