Ocean acidification shows negligible impacts on high-latitude bacterial community structure in coastal pelagic mesocosms

A.-S Roy,J A Gilbert,J Laroche,U Riebesell,S Romac,J G Caporaso,J I Nissimov,H Schunck,S Owens,S M Gibbons,M Sperling,L Bittner,M Mühling

doi:10.5194/bg-10-555-2013

Abstract

Abstract. The impact of ocean acidification and carbonation on microbial community structure was assessed during a large-scale in situ costal pelagic mesocosm study, included as part of the EPOCA 2010 Arctic campaign. The mesocosm experiment included ambient conditions (fjord) and nine mesocosms with pCO2 levels ranging from ~145 to ~1420 μatm. Samples for the present study were collected at ten time points (t–1, t1, t5, t7, t12, t14, t18, t22, t26 to t28) in seven treatments (ambient fjord (~145), 2 × ~185, ~270, ~685, ~820, ~1050 μatm) and were analysed for "small" and "large" size fraction microbial community composition using 16S RNA (ribosomal ribonucleic acid) amplicon sequencing. This high-throughput sequencing analysis produced ~20 000 000 16S rRNA V4 reads, which comprised 7000 OTUs. The main variables structuring these communities were sample origins (fjord or mesocosms) and the community size fraction (small or large size fraction). The community was significantly different between the unenclosed fjord water and enclosed mesocosms (both control and elevated CO2 treatments) after nutrients were added to the mesocosms, suggesting that the addition of nutrients is the primary driver of the change in mesocosm community structure. The relative importance of each structuring variable depended greatly on the time at which the community was sampled in relation to the phytoplankton bloom. The sampling strategy of separating the small and large size fraction was the second most important factor for community structure. When the small and large size fraction bacteria were analysed separately at different time points, the only taxon pCO2 was found to significantly affect were the Gammaproteobacteria after nutrient addition. Finally, pCO2 treatment was found to be significantly correlated (non-linear) with 15 rare taxa, most of which increased in abundance with higher CO2.

Highlights

The acidification of our oceans, caused predominantly by dissolution of anthropogenic carbon dioxide (CO2) in seawater, has the potential to affect the physiology of marine microbes
The extensive database of 16S rRNA sequence obtained in this study provided the high resolution necessary to study subtle but significant changes in community structure hinted at in prior studies
The relative importance of sampling strategy, Si, PO4, primary production, temperature, and pH in structuring the community depended greatly on the time at which the community was sampled in relation to the phytoplankton blooms

Summary

Introduction

The acidification of our oceans, caused predominantly by dissolution of anthropogenic carbon dioxide (CO2) in seawater, has the potential to affect the physiology of marine microbes. Experimental manipulation of the partial pressure of carbon dioxide (pCO2) in marine mesocosms has demonstrated speciesspecific physiological responses to elevated dissolved CO2 concentrations. The response of bacterial communities to elevated pCO2 concentrations is less defined, with mixed reports of both significant increases in bacterial protein production (Grossart et al, 2006), and no significant changes in microbial community structure (Tanaka et al, 2008; Allgaier et al, 2008; Newbold et al, 2012). During the 2008 PeECE III mesocosms study, elevated pCO2 had no significant impact on bacterial abundance, diversity, or activity; the community structure of the small size fraction bacteria was significantly altered by the induced phytoplankton bloom (Allgaier et al, 2008; Arnosti et al, 2011; Riebesell et al, 2008)

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Conclusion