Screening of corrosive bacteria found in the zone of mixing of fresh and salt water in the Yangtze Estuary, and the impact of these bacteria on cultural relics

Recent studies of bacterial corrosion of cultural relics in seawater have mainly focused on deep 一 sea salty cement and light cement. However, there are few reports on corrosive bacteria in zones of mixing of fresh and salt water. Estuarine and coastal waters are rich in underwater cultural relics and the interaction of salt and fresh water affects the burial environment and cultural relics. In our study, muddy marine samples were collected at different levels of the sea bed near the No. II shipwreck at the North Channel of the Yangtze Estuary, and 18 strains of bacteria were isolated using non - directional enrichment medium. Four corrosive bacteria were identified by 16s rDNA sequence analysis ： Shewanella hafniensis, Bacillus wiedmannii, Exiguobacterium aquaticum and Exiguobacterium indicum. First, the isolated bacteria were cultured, until they reached their logarithmic growth periods, in a30 °C shaker at a rotational speed of 220 r/min, and then they were allowed to act upon porcelain, pottery, wood and other common cultural relic materials. After a 7 - day incubation, those cultural relic materials were sprayed with gold, dried and then observed using scanning electron microscopy (SEM). Biofilms are closely related to the occurrence of microbial corrosion. In addition to the reported corrosive strains, we also found two previously - unreported bacteria—Shewanella xiamenensis and Aeromonas hydrophila— that also produce biofilms on the surface of materials. The experimental results provide scientific reference for explaining the burial environment of cultural relics in the mixture area of fresh water and salt water at estuarine and coastal areas and better understanding of cultural relic conservation. Compared with previous literature, our study expands in the following three aspects ： 1) the corrosion phenomena of corrosive bacteria on porcelain, pottery and wood cultural relic materials were studied, and the formation of biofilms closely related to microbial corrosion was observed, so as to demonstrate the corrosive effects of Shewanella hafniensis, Bacillus wiedmannii, Exiguobacterium aquaticum and Exiguobacterium indicum on porcelain, pottery and wood materials; 2) two previously - unreported corrosive bacteria - Shewanella xiamenensis and Aeromonas hydrophila, which can also form biofilms on the surface of materials - were found for the first time, preliminarily indicating that these two bacteria also have potential corrosive effects; 3) Bacillus was explored as the dominant strain in the zone of mixing of fresh water and salt water in the Yangtze Estuary, further revealing the richness of the marine bacterial library because more corrosive bacteria (different from sulfate - reducing bacteria) were found in our study. This study focuses on the relationship between corrosive bacteria abounding in the zone of mixing of fresh water and salt water in the Yangtze Estuary and the corrosion of porcelain, pottery and wood cultural relics, which, to some extent, lays a foundation for the study of corrosive bacteria in the waters. It is conducive to the following application to cultural relic conservation and other related aspects.