Climate change and Vibrios: a retrospective study

It is predicted that, as a result of climate change, the sea surface temperature (SST) will increase by a few degrees during this century. A number of studies have investigated the long-term effects of this on eukaryotic populations, but little experimental evidence has been gathered for the long-term effects on the abundance and diversity of marine prokaryotes, because it is widely believed that lower trophic levels are less sensitive to environmental change.

However, there is evidence that incidences of Vibrio-related diseases, such as cholera and seafood-borne illnesses, have increased with climate warming. Bathing infections associated with Vibrio species have also increased in Northwest European countries. It is not yet clear whether vibrios are increasing within bacterial communities in coastal marine waters, and this paper aims to assess the link between the occurrence of Vibrio and SST, by applying retrospective molecular and pyrosequencing analysis to formalin-fixed samples from the archive of the Continuous Plankton Recorder (CPR) survey. The CPR is a plankton sampler towed from ships over long distances; plankton is collected on a band of silk with a mesh size of 270µm, and smaller plankton components such as mucilage and detrital particles often become trapped in the silk fibrils. It is likely that the CPR system also captures a significant proportion of Vibrio species which are present in the nutrient-rich plankton at high densities during the warmer months, and so provides a long-term record for these species.

This study analysed a set of 55 samples collected from two areas located off the Rhine and Humber estuaries in the North Sea, in August, from 1961 to 2005. The samples had been stored in formalin, which can cause DNA degradation and crosslinking between adjacent DNA molecules, as well as interfere with the enzyme activity of the PCR, making reliable molecular quantification difficult. Therefore a PCR technique was developed by the authors to overcome these problems, in order to assess the presence and abundance of Vibrio species.

A significant correlation was found between a long-term increase in relative Vibrio abundance and an increase in SST off the Rhine estuary, but not off the Humber estuary, which the authors attribute to generally higher SST values in the Rhine area. Temperatures generally exceeded 18°C in the Rhine area yet never exceeded 18°C in the Humber area. Data from the two sites was pooled together, and 45% of the variance in Vibrio data was explained by an increase in SST.

The late 1980s saw an ecological regime shift in the North Sea associated with an increased incursion of warm water from the Atlantic. Since then, SST of UK waters has risen by approximately 0.6 – 0.8°C per decade. This study found that there was a marked increase in abundance of Vibrios off the Rhine estuary after the late 1980s, which appears to correlate with the regime shift. A shift in community composition was also observed, with bacteria belonging to the genus Vibrio becoming dominant within the plankton-associated bacterial community.

The authors point out that the number of free-living vibrios may be even higher than the number associated with plankton, and this is something that requires further investigation. The results of the study provide a long-term pattern of bacterial abundance in response to rising SST, and this is something that deserves further attention in light of the effects that many Vibrio species have upon human and animal health.

A review of: Vezzulli L. et al. (2012) Long-term effects of ocean warming on the prokaryotic community: evidence from the vibrios. The ISME Journal, 6: 21-30.