**2.3 The influence of environmental factors on** *Vibrio* **community**

In a marine environment, abundance and community composition of *Vibrio* is 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 *Vibrio* community. Therefore, under the interaction between biological and non-biological factors, the *Vibrio* community in the environment shows complex dynamic changes.

The abundance and community structure of *Vibrio* in seawater is generally considered to be related to temperature and salinity. Temperature is the most important factor affecting the change of *Vibrio* community. Under general conditions, the relationship between *Vibrio* and water temperature shows a positive correlation. Growth of *Vibrio* population can be observed in short-term temperature rise and long-term temperature change related to climate change [47–49]. 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 *Vibrio* in parts of the North Atlantic and North Sea [50]. In Peru, Alaska and the gulf of Mexico and other regions also reported that due to the increase in water temperature, some pathogenic *Vibrio* species began to increase [51]. Moreover, some cases of infection caused by *Vibrio* have also been reported to be associated with abnormally high water temperatures [52].

Salinity was the second largest factor affecting the abundance of *Vibrio*, and *Vibrio* had 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 *Vibrio* is affected by salinity and chlorophyll A concentration, but only when the salinity is less than 20 ppt, the effect of salinity is significant [29]. 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 *Vibrio* than in total phosphorus and total nitrogen [55]. 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 *Vibrio* communities are closely related to temperature and salinity, in temperate regions, concentrations of organic and inorganic nutrients and phytoplankton communities appear to be

#### *Community Change and Pathogenicity of* Vibrio *DOI: http://dx.doi.org/10.5772/intechopen.96515*

more important drivers of seasonal changes in *Vibrio* communities 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 *Vibrio* abundance in the system of the region [32]. 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 *Vibrios* closely 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 *Vibrio* in the estuary had to be in conformity with the decrease in salinity. When the salinity increased, the number of *Vibrios* in 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 *Vibrio* in the area appeared to be independent of changes in the three factors commonly used to predict *Vibrio* abundance, including salinity, temperature, and dissolved oxygen. Although the overall abundance of *Vibrio* was on the rise, the number of some potential pathogenic species was decreasing, and the concentration of *Vibrio* in the estuaries was predicted to be related to nitrogen and carbon in the environment [2]. In addition, studies have shown that ammonium radical promotes the growth of *Vibrio*, while silicic acid and phosphate have opposite effects on *Vibrio* population [56]. Dissolved organic carbon (DOC) has a strong impact on the ecology of *Vibrio.* DOC provides a large amount of nutrients needed for *Vibrio* living in estuarine and marine habitats. *Vibrio* can absorb, metabolize and produce organic matter, thus changing its chemical properties and bioavailability [57]. 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 *Vibrio* bacteria by affecting their metabolism. Due to hypoxia, the *Vibrio* population will switch from breathing mode to fermentation mode [58]. The abundance of free-living and particulate-related fractions of *Vibrio* was negatively correlated with dissolved oxygen (48.7% ~ 105.8% saturation) in the coastal area of georgia, USA [59]. A negative correlation between *Vibrio* abundance and dissolved oxygen (5 ~ 11 mg L−1) was found in the North Carolina estuary [60]. 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 *Vibrio* abundance and temperature, but *Vibrio* abundance was very high at a depth of 100 m (the interface between aerobic and anoxic) [5].

In addition to physical factors, biological factors also play an important role in affecting changes in the *Vibrio* community. The virus has a strong lethal effect on *Vibrio* and can greatly affect the change of *Vibrio* community. Some researchers have identified a virus with a wide host range from infected *Vibrio*, which can kill 34 *Vibrio* strains of four species [61]. 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 *Vibrio* and the community composition of phytoplankton, and it is speculated that this may be related to the bioavailability of dissolved organic matter released from phytoplankton [64].

Finally, *Vibrio* can 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 [65]. 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 [66]. However, conditions for *Vibrio* to enter "recovery" from "dormancy" are not completely clear.
