LONG-TERM EFFECTS OF OCEAN WARMING ON THE THE PROKARYOTIC COMMUNITY : EVIDENCE FROM THE Vibrio ASSEMBLAGE

Vibrio are a genus of gram-negative, rod-shaped bacteria belonging to the class Gammaproteobacteria. This abundant taxa is responsible for causing some well-known diseases within the human population.  These include Vibrio cholera; responsible for cholera epidemics, Vibrio vulnificus; known to be the origin of septicaemia and shellfish associated death, and Vibrio parahaemolyticus; the cause of seafood gastroenteritis. However, it should be noted that there are a multitude of other Vibrio taxa that do not have a negative effect upon the human population, such as Vibrio fischeri and Vibrio harveyi.

It is well-known that Vibrio are thermo-dependant organisms and are more common in warmer climes, but there has been evidence to suggest that the reservoir of this genus is increasing due to a rise in global sea surface temperatures (SST). A paper by Pascual et al. (2000) documented that El Niño events, leading to a warmer climate, were correlated with abundant cholera outbreaks in Asia and South America. It has also been found that during the recent spate of warmer weather in Europe, an increase in reported wound infections {possibly from Vibrio vulnificus} has occurred in the North and Baltic seas. Other papers have also linked the mass mortality of marine organisms to an increase in abundance of Vibrio-based infections.

The aim of this investigation was to provide evidence that Vibrio has increased in dominance within the plankton-associated bacterial community of the North Sea over the last 44 years, and that this is significantly correlated with increasing sea surface temperature.

The samples used were preserved in formalin and collected from the Rhine and Humber estuaries; dating from August 1961-2005. These samples were collected via the Continuous Plankton Reader Survey. Small flecks were cut from these samples and were centrifuged before DNA extraction was conducted. The amount of DNA extracted was determined by using Pico Green fluorescence and real time PCR was used to amplify the DNA. Primers were then added to locate the V6 (hyper-variable) region, specific to the genus Vibrio. This DNA was then pyrosequenced and BLASTed against a reference data base to assess taxonomic diversity. The average sea surface temperature, phytoplankton colour index and total copepod abundance were taken by the Continuous Plankton Recorder at the time of acquiring the samples.

A positive, significant correlation was found between SST and an increase in Vibrio abundance in the Rhine estuary but not the Humber. This is thought to be due to higher SST in the Rhine estuary. However, other variables such as salinity, nitrate and phosphorus content of the estuaries were not measured but may have also caused the recorded increase in the genus. The variability in Vibrio abundance was calculated to be 45% due to SST, 50% including the total copepod abundance and phytoplankton colour, but there was no explanation for the other 50% of variability found. Pyrosequencing has suggested that Vibrio have not only increased in the abundance but also become dominant in the plankton-associated bacterial community, implying that a major shift has occurred. This statement seems particularly strong as other bacterial communities not associated with Gammaproteobacteria were not extensively discussed.

An ecological regime shift in the late 1980's was considered as being partially responsible for the bacterial community shift as it caused an increased incursion of warm oceanic water in the North Sea. This shift was known to affect all marine life.

We found this research crucial as it has implications to human health, crashes in marine mammal populations and also a threat to oceanic aquaculture. However it should be noted that only estuaries were tested and there could be many other factors affecting the increase in Vibrio abundance. These include human waste and pollution; and high nutrient levels. Also the Vibrio discussed are associated with brackish and estuarine waters and therefore we agree that they may not greatly affect the ocean as suggested. Lastly it has been suggested by other papers that the V3 region on the gene was a lot more reliable than the V6 region at detecting Vibrio species.   

Vezzulli L, Brettar I, Pezzati E, Reid P.C, Colwell R.R, Hofle M.G and Pruzzo C. 2012. Long-term effects of ocean warming on the prokaryote community: evidence from the Vibrios. ISME Journal. 6 : 21-30.

Written by Georgia Hall and Rachel Bransgrove