Date Published: April 23, 2019
Publisher: Public Library of Science
Author(s): Brett Froelich, Raul Gonzalez, Denene Blackwood, Kellen Lauer, Rachel Noble, Iddya Karunasagar.
A decade long study was conducted to investigate the ecological, biological, and temporal conditions that affect concentrations of Vibrio spp. bacteria in a well-studied lagoonal estuary. Water samples collected from the Neuse River Estuary in eastern North Carolina from 2004–2014 (with additional follow-up samples from Fall of 2018) were analyzed to determine Vibrio spp. concentrations, as well as the concentrations of inorganic and organic nutrients, fecal indicator bacteria, phytoplankton biomass, and a wide range of other physio-chemical estuarine parameters. A significant increase in Vibrio spp. was observed to occur in the estuary over the examined period. Strikingly, over this long duration study period, this statistically significant increase in total culturable Vibrio spp. concentrations does not appear to be correlated with changes in salinity, temperature, or dissolved oxygen, the three most commonly cited influential factors that predict estuarine Vibrio spp. abundance. Furthermore, shorter term (~3 years) data on specific Vibrio species (V. vulnificus and V. parahaemolyticus)show that while Vibrio spp. are increasing overall as a genus, the numbers of some key potentially pathogenic species are decreasing as a part of the total population, further supporting the concept that quantification of the entire genus is not a worthwhile use of resources toward predicting levels of specific potentially pathogenic species of public health concern. The significant increase in this concentration of Vibrio spp. in the studied estuary appears to be related to nitrogen and carbon in the system, indicating a continued need for further research.
Bacteria in the genus Vibrio exhibit a great deal of variation, both phenotypically and genotypically. Most bacteria of the Vibrio genus are important aquatic ecosystem members that can be found in fresh, brackish, and marine waters, often with strong, species-specific salinity preferences [1–3]. Vibrio are fast-growing, with some species capable of doubling in less than ten minutes, and are therefore able to take rapid advantage of pulses of nutrient or shifts in meteorological conditions [4,5]. Vibrio spp. are ubiquitous across aquatic environments and over longer time scales they are biogeochemically important members of mesohaline estuarine environments. While most Vibrio spp. are not pathogenic, there exist several species that are pathogenic to humans, fish, eels, shellfish, or other species [6–10]. The increasing number of infections caused by Vibrio spp., especially Vibrio vulnificus, has generated great a deal of attention and research. Furthermore, Vibrio spp. play important roles in ecosystem function and organismal population dynamics, participating in nitrogen fixation, chitin degradation, and metabolism of algal polysaccharides [11–13]. Other well-studied species serve as symbionts, living inside squid or other organisms and functioning as the source of luminescence in light organs and can be important members of biofilms and macroalgal associations, while still other Vibrio sp. are capable of degrading petroleum [14,15]. While often monitored for short term changes, there is far less information on the long-term shifts in Vibrio spp. populations.
This is the longest Vibrio spp. monitoring program that has taken place in the State of North Carolina, and perhaps along the east coast of the United States. Vibrio spp. were routinely monitored for over ten years. The NRE, the site of the monitoring program, is part of the Albemarle-Pamlico Estuarine System (Fig 1). The NRE experiences large seasonal variability in nutrient concentrations  and is affected by anthropogenic inputs, both urban and agricultural. The estuary is also heavily used, both commercially and recreationally, and is part of the Intracoastal Waterway. The NRE is undergoing eutrophication, driven in part by urban expansion, agricultural runoff, and high degree of livestock operations occurring in the watershed [41,49]. Thus, factors driving the health of this important estuarine ecosystem are changing making an understanding of the dynamics of bacterial populations important to study. This continuous monitoring effort revealed that Vibrio spp. concentrations appear to be increasing in the NRE in eastern NC (Fig 3). Yearly averages show that Vibrio spp. means tended to increase one year, lower the next, and then exhibit an even larger increase the following year (Fig 2). A seasonal ARIMA model shows that the increase was heaviest starting in 2011, and in 2012 throughout the rest of the study, Vibrio spp. exhibited continuous detection, even in the winter months (Fig 4). The increase of winter Vibrio spp. is especially noticeable when monthly averages from the beginning, middle, and ending of the monitoring program are compared (Fig 5). The largest increases, based on monthly averages, were in the cold winter months. This indicates that the seasonal reduction in either live or culturable Vibrio spp., that had been considered normal, is no longer as pronounced. This has important ramifications for species-specific shifts in the total Vibrio spp. population, and especially important ramifications for winter-dominant commercial shellfish harvest. Vibrio spp. have been thought to either enter the viable-but-non-culturable state or overwinter in the sediments, but they are detectable year-round now [35,50–52]. Interestingly, not all Vibrio species are behaving in the same fashion. Individual species data were only collected the last 4 years of the study including V. vulnificus and V. parahaemolyticus. V. vulnificus showed a significant decrease during that period while the total Vibrio spp. and V. parahaemolyticus populations did not (Fig 7). This seems to indicate that some species are becoming more abundant, and that the species makeup of the Vibrio spp. population may be in flux. Shifts in water column conditions have been shown in the NRE previously to affect certain Vibrio species in differing manners, such as was seen after the prolonged drought around 2007 (Fig 13) caused V. vulnificus to nearly disappear from the estuary while more salt tolerant species were thriving . Similar effects have been reported in the Gulf Coast