Connecting Ocean Warming and Vibrios using the Continuous Plankton Recorder

Chitin-containing plankton, especially copepods form an important reservoir for Vibrio bacteria. Some Vibrio species are well known pathogens and cause disease, food poisoning, septicaemia and wound infections in humans. Rather worryingly, there is now greater evidence that the incidents of Vibrio-related illnesses are increasing globally with climate warming.  

Traditionally, microbial communities were assumed to be unaffected by increases in sea surface temperature (SST) but increasing evidence is challenging this view. Much of this was down to a lack of historical data, which has subsequently left a gap between the longer term effects of sea surface warming and the abundance of microbial communities. A study by Vezzulli et al., (2012) overcame this problem by using retrospective molecular analysis of the bacterial communities sampled with the Continuous Plankton Recorder (CPR) to examine the long term effects of ocean warming on the abundance of Vibrio bacteria in the North Sea.

Figure 1: A summary of the methods used in Vezzulli et al., (2012). The sample sites taken from between 1961-2005 in the Rhine and Humber estuaries and the Continuous Plankton Recorder (CPR) tow are shown in (a), with the silk cutting, sample processing, DNA extraction and analysis summarised in (b-e). This allows the calculation of the relative abundance of Vibrio spp. cells within the sample.

CPR samples provided historical data on planktonic and microbial communities, from the Rhine and the Humber estuaries in August between 1961 and 2005, coupled with SST time-series data for these locations. DNA was extracted from the filtering silks of the CPR and analysed using molecular and pyrosequencing analysis to assess the presence and relative abundance of Vibrio species. The methods are summarised in figure 1. Despite the potential pitfalls of recovering and analysing genomic DNA from formalin-preserved samples, this study used a modified extraction and purification method to salvage the DNA from the CPR silks. Taxon data was normalised across the different years. This allowed for differences in absolute operational taxonomic units (OTU) read counts due to degradation from age and formalin preservation. Analysis focused on the Alphaproteobacteria and the Gammaproteobacteria, both of which are abundant in seawater and considered to be unlikely sources of contamination in the lab.

Statistical analysis of Vibrio and copepod abundance in CPR samples combined with SST data showed that SST accounted for 45% of the variance in Vibrio data, emphasising the strong link between surface warming and an increased abundance in Vibrio species in the North Sea since 1961. There was a significant positive correlation between the increase in abundance of Vibrio species and increased SST in the Rhine estuary but not the Humber. It was believed that the SST in the Humber did not exceed 18 °C during the sampling period, which would explain this result, as Vibrio spp. normally thrive above this temperature. Additionally, the analysis showed that there had been a distinct ecological regime shift in the late 1980s, whereby Vibrio species became both more abundant and increasingly dominant within plankton communities in the North Sea. This shift was associated with increased penetration of warmer Atlantic water into the North Sea.

This study has helped highlight the impact of SST on the diversity of bacterial communities associated with plankton on a longer time scale via a retrospective sampling method. This is crucial for understanding the risk to human health with future warming. One issue I do have is that the authors selected to display the pyrosequencing data for the years 1961, 1972, 1976, 1998 and 2004, leaving a big gap between 1976 and 1998. The ecological regime shift happened between those years so one or two additional year samples may have helped clarify exactly how this shift in bacterial diversity occurred.

Reference: Vezzulli, L., Brettar, I., Pezzati, E., Reid, P.C., Höfle, M.G. and Pruzzo, C. (2012) Long-term effects of ocean warming on the prokaryotic community: evidence from the vibrios, The ISME Journal, 6, 21-30.

Found at: http://www.nature.com/ismej/journal/v6/n1/abs/ismej201189a.html