ALBERT

Description: The effects of ocean acidification on ecosystems remain poorly understood, because it is difficult to simulate the effects of elevated CO2 on entire marine communities. Natural systems enriched in CO2 are being used to help understand the long-term effects of ocean acidification in situ. Here, we compared biofilm bacterial communities on intertidal cobbles/boulders and bedrock along a seawater CO2 gradient off Japan. Samples sequenced for 16S rRNA showed differences in bacterial communities with different pCO2 and between habitat types. In both habitats, bacterial diversity increased in the acidified conditions. Differences in pCO2 were associated with differences in the relative abundance of the dominant phyla. However, despite the differences in community composition, there was no indication that these changes would be significant for nutrient cycling and ecosystem function. As well as direct effects of seawater chemistry on the biofilm, increased microalgal growth and decreased grazing may contribute to the shift in bacterial composition at high CO2, as documented by other studies. Thus, the effects of changes in bacterial community composition due to globally increasing pCO2 levels require further investigation to assess the implications for marine ecosystem function. However, the apparent lack of functional shifts in biofilms along the pCO2 gradient is a reassuring indicator of stability of their ecosystem functions in shallow ocean margins.

Description: The effects of increasing atmospheric CO(2) on ocean ecosystems are a major environmental concern, as rapid shoaling of the carbonate saturation horizon is exposing vast areas of marine sediments to corrosive waters worldwide. Natural CO(2) gradients off Vulcano, Italy, have revealed profound ecosystem changes along rocky shore habitats as carbonate saturation levels decrease, but no investigations have yet been made of the sedimentary habitat. Here, we sampled the upper 2 cm of volcanic sand in three zones, ambient (median pCO(2) 419 µatm, minimum Omega (arag) 3.77), moderately CO(2)-enriched (median pCO(2) 592 µatm, minimum Omega (arag) 2.96), and highly CO(2)-enriched (median pCO(2) 1611 µatm, minimum Omega (arag) 0.35). We tested the hypothesis that increasing levels of seawater pCO(2) would cause significant shifts in sediment bacterial community composition, as shown recently in epilithic biofilms at the study site. In this study, 454 pyrosequencing of the V1 to V3 region of the 16S rRNA gene revealed a shift in community composition with increasing pCO(2). The relative abundances of most of the dominant genera were unaffected by the pCO(2) gradient, although there were significant differences for some 5 % of the genera present (viz. Georgenia, Lutibacter, Photobacterium, Acinetobacter, and Paenibacillus), and Shannon Diversity was greatest in sediments subject to long-term acidification (〉100 years). Overall, this supports the view that globally increased ocean pCO(2) will be associated with changes in sediment bacterial community composition but that most of these organisms are resilient. However, further work is required to assess whether these results apply to other types of coastal sediments and whether the changes in relative abundance of bacterial taxa that we observed can significantly alter the biogeochemical functions of marine sediments.