2.1 Abundance of Vibrio
Vibrioare widely distributed in estuaries and marine environments, and mainly in nearshore areas. Vibriogenerally exhibit two different growth strategies, either as a free-living form or attached to biological or non-biological surfaces, where they can co-exist with the host or cause host disease . For example, some Vibrioliving in squid or other organisms can be used as the source of luminescence of light-emitting organs and also an important part of the combination of biofilm and macroalgae .
Vibrioeasily grow on conventional medium (such as seawater 2216E agar medium) and selective medium (such as thiosulfate citrate bile salt sucrose agar medium, TCBS) and can carry out a variety of metabolic activities . In some studies based on culture, Vibriocan account for 10% of culturable marine bacteria , and the average abundance in estuaries and nearshore waters is 103 ~ 106 CFU L−1. However, in studies using non-culture methods, Vibriopopulation only accounts for about 1% of the total plankton bacteria in nearshore waters, and the average abundance in estuaries and nearshore waters is 104 ~ 108 16 s rRNA copies L−1 . Their Vibrioabundance was found to be between 15 and 2395 CFU mL−1 in a study of tropical estuaries and coastal water in Malaysia . In addition, studies have shown a high density of Vibrioon the surface and in the body of marine animals such as fish, shrimp, mollusks, corals, sponges, zooplankton, algae and seaweed . For example, in a study examining the effects of aquaculture on Vibriocommunities, the relative abundance of the 16SrRNA gene sequence reads 16 from seaweed samples were the highest by sequencing water, sediment, seaweed and tissue samples obtained in the aquaculture area of Hainan . This is also consistent with studies describing Vibriocommunities as important components of seagrass bacterial communities. These bacteria account for 25% of the culturable bacteria in seaweed off the coast of Hainan province .
2.2 Diversity of Vibrio
At least 110 Vibrioshave been found and reported, and more may be found in the future. Among the Vibriosthat have been described, several are commonly associated with human diseases, among which V. cholerae, V. parahaemolyticus, and V. vulnificusare recognized as human pathogenic bacteria, while Vibrio alginolyticus, V. anguillarum, V. harveyi, Vibriofluvialis, Vibriofurniss, Vibriometschnikovii, and Vibriomimicus are primarily marine animal pathogenic bacteria but occasionally associated with human infections [32, 33, 34, 35, 36].
Vibriousually has species-specific salinity and temperature preferences, and different kinds of Vibriomay exist in different environments. They exit from deep-sea hydrothermal vents and sediment are more than 6, 000 meters deep to seawater 10, 500 meters deep in the mariana trench [37, 38]. For example, the optimal growth temperature and salinity for Vibrio devil, first isolated from deep-sea hydrothermal vents, is 30 ~ 45 °C and 20 ~ 50 ppt, respectively . The salinity-dependent Vibriocarinii is mainly present in seawater in the range of salinity from the Baltic Sea to the Mediterranean Sea . Vibrio pacinii, Vibrio cyclotrophicus, Vibrio lentus, and some unnamed Vibriohave also been found at low temperatures .
At present, studies on the diversity of Vibriocommunities in marine environments are mainly based on Vibrioisolated and cultured . However, due to the low interspecific resolution of the 16S rRNA gene, the use of 16S rRNA gene similarity as a major interspecific marker for the phylogenetic relationships of Vibriosappears to have lost its effect. Multiple-locus sequence analysis (MLSA) and other novel phylogenetic markers such as the iron absorption regulatory gene furhave been used as alternative approaches [42, 43]. In order to study the diversity of environmental Vibrio, Siboni et al. first extracted DNA from seawater, and then used 16S rRNA gene primers specific to Vibrioto conduct high-throughput sequencing, thus making it possible to more intuitively and effectively explore the diversity of Vibriocommunities . In another study by Bei et al., the abundance and community structure of Vibriospecies at different depths was studied using Vibrio-specific16S rRNA gene high-throughput sequencing and quantitative PCR (qPCR) techniques as well as traditional culture methods .
2.3 The influence of environmental factors on Vibriocommunity
In a marine environment, abundance and community composition of Vibriois affected by many factors, including temperature, salinity, pH, water depth, dissolved oxygen and transparency [45, 46]. Chemical factors are mainly the concentrations of inorganic and organic nutrients. In addition, biological factors such as protozoa, viruses, marine animals and algae also affect the change of Vibriocommunity. Therefore, under the interaction between biological and non-biological factors, the Vibriocommunity in the environment shows complex dynamic changes.
The abundance and community structure of Vibrioin seawater is generally considered to be related to temperature and salinity. Temperature is the most important factor affecting the change of Vibriocommunity. Under general conditions, the relationship between Vibrioand water temperature shows a positive correlation. Growth of Vibriopopulation can be observed in short-term temperature rise and long-term temperature change related to climate change [47, 48, 49]. At present, many coastal areas around the world have been reported an increase in the number of Vibrio. For example, some researchers have used continuous plankton recording equipment to show that the increase in sea temperature has caused an increase in the number of Vibrioin parts of the North Atlantic and North Sea . In Peru, Alaska and the gulf of Mexico and other regions also reported that due to the increase in water temperature, some pathogenic Vibriospecies began to increase . Moreover, some cases of infection caused by Vibriohave also been reported to be associated with abnormally high water temperatures .
Salinity was the second largest factor affecting the abundance of Vibrio, and Vibriohad a positive correlation with salinity, but the relationship might also be covered by increases in temperature and nutrient concentration [53, 54]. Not only that, some studies found that short-term salinity changes do increase the concentration of Vibrio, but long-term salinity changes have no significant effect on the overall trend of Vibrio, for example, there are studies found that abundance of Vibriois affected by salinity and chlorophyll A concentration, but only when the salinity is less than 20 ppt, the effect of salinity is significant . In another study on the abundance of microbial communities in Guanabara Bay, the researchers constructed an artificial neural network that could simulate the response of environmental microbial communities to environmental parameters. The results showed that temperature had a positive correlation with the abundance of Vibrio, and salinity had a negative correlation with the abundance of Vibrio. Transparency had a positive correlation with chlorophyll concentration but had little to do with the number of Vibrio. Moreover, these physical parameters were more related to the abundance of Vibriothan in total phosphorus and total nitrogen . The authors deduced that due to the high degree of eutrophication in the bay, the microbial community had reached its maximum capacity to absorb and utilize nutrients, and the growth of the microflora was no longer restricted by nutrients. On the contrary, salinity, temperature and transparency jointly determined the number of Vibrio.
Although the composition and abundance of Vibriocommunities are closely related to temperature and salinity, in temperate regions, concentrations of organic and inorganic nutrients and phytoplankton communities appear to be more important drivers of seasonal changes in Vibriocommunities because annual changes in temperature are not significant.
In a study of wetlands in Macchiatonda Regional Nature Reserve, it was found that the CFU abundance of TCBS depended on temperature and salinity, and the effect of temperature was greater than that of salinity (27% and 20%, respectively), but since temperature and salinity accounted for only 40% of the total CFU abundance, other environmental and biological factors had to play a role in driving Vibrioabundance in the system of the region . In another ten-year study of the mouth of the Newz River in North Carolina, the United States, it seems that similar views have been confirmed. During the study, the temperature of the estuary did not change significantly, but the number of some Vibriosclosely related to the temperature increased. The salinity of the estuary showed a trend of increasing to the highest and decreasing during the study. The increase of the number of Vibrioin the estuary had to be in conformity with the decrease in salinity. When the salinity increased, the number of Vibriosin the mouth of the river increased. Some specific Vibrio, such as V. vulnificus, had almost declined to undetectable levels, and the final conclusion was that the concentration of Vibrioin the area appeared to be independent of changes in the three factors commonly used to predict Vibrioabundance, including salinity, temperature, and dissolved oxygen. Although the overall abundance of Vibriowas on the rise, the number of some potential pathogenic species was decreasing, and the concentration of Vibrioin the estuaries was predicted to be related to nitrogen and carbon in the environment . In addition, studies have shown that ammonium radical promotes the growth of Vibrio, while silicic acid and phosphate have opposite effects on Vibriopopulation . Dissolved organic carbon (DOC) has a strong impact on the ecology of Vibrio.DOC provides a large amount of nutrients needed for Vibrioliving in estuarine and marine habitats. Vibriocan absorb, metabolize and produce organic matter, thus changing its chemical properties and bioavailability . Therefore, in temperate regions where the temperature is relatively stable, factors other than physical parameters such as temperature and salinity may play a more important role. However, when the degree of nutrition is high and the microbial community has reached the maximum capacity to absorb and utilize nutrition, physical factors are more relevant to the abundance of Vibrio.
Dissolved oxygen is an important hydrological parameter, which affects the number of Vibriobacteria by affecting their metabolism. Due to hypoxia, the Vibriopopulation will switch from breathing mode to fermentation mode . The abundance of free-living and particulate-related fractions of Vibriowas negatively correlated with dissolved oxygen (48.7% ~ 105.8% saturation) in the coastal area of georgia, USA . A negative correlation between Vibrioabundance and dissolved oxygen (5 ~ 11 mg L−1) was found in the North Carolina estuary . In addition, a study on Yongle Blue Cave in Sansha, Hainan, China found that in the deepest Blue Cave in the world, due to the strong stratification limiting the vertical exchange of oxygen, the water body was divided into an upper aerobic zone and a lower anoxic zone. The strong DO gradient resulted in no significant correlation between Vibrioabundance and temperature, but Vibrioabundance was very high at a depth of 100 m (the interface between aerobic and anoxic) .
In addition to physical factors, biological factors also play an important role in affecting changes in the Vibriocommunity. The virus has a strong lethal effect on Vibrioand can greatly affect the change of Vibriocommunity. Some researchers have identified a virus with a wide host range from infected Vibrio, which can kill 34 Vibriostrains of four species . For some special species, changes in biological factors may have a stronger effect on their abundance than non-biological parameters [62, 63]. Recently, it has been proved that there is a significant correlation between the abundance of particle-associated Vibrioand the community composition of phytoplankton, and it is speculated that this may be related to the bioavailability of dissolved organic matter released from phytoplankton .
Finally, Vibriocan enter a viable but non-culturable state (VBNC) under adverse environmental conditions (such as oligotrophic, excessively high or low temperature, high salt, extreme pH, and sunlight radiation). This physiological state is reversible, and when the conditions become favorable again, the pathogen will recover . The cells could still survive in this dormant state, but it could not be detected by the traditional culture method, which might show a higher resistance to exogenous stress and maintain the active virulence factors . However, conditions for Vibrioto enter “recovery” from “dormancy” are not completely clear.