Abstract
Polynyas in the polar seas are regarded as windows through which ecosystem responses associated with global climate changes are to be noticed. However, little information is available on benthic microbial communities in the Amundsen Sea polynya (ASP), where environmental changes due to global warming are occurring rapidly, from which future climate change-induced ecosystem responses could be assessed. We performed high-throughput sequencing of 16S rRNA genes and quantitative PCR in combination with biogeochemical analyses and metabolic rate measurements to determine the composition, diversity and controls of major microbial communities in sediments of the ASP. A large fraction of the sequenced benthic microbial community (40% on average) in the polynya was uniquely affiliated with the phylum Planctomycetes, whereas Thaumarchaeota (51% on average) predominated in non-polynya areas. The relative abundance of Planctomycetes correlated significantly with organic carbon (Corg) content in the polynya sediment underlying the Phaeocystis-dominated water column. These results suggest that Planctomycetes comprise a major bacterial group utilizing relatively recalcitrant Corg produced primarily by Phaeocystis blooms. In contrast, the predominance of chemolithoautotrohic Thaumarchaeota in the sea-ice zone was attributed to low Corg supply due to low primary productivity in the ice-covered water column. The Planctomycetes-dominated microbial communities in the ASP is in stark contrast to that Proteobacteria (Delta- and Gammaproteobacteria) occupy ecological niches as primary mineralizers of organic materials in most benthic systems in the Southern Ocean, where organic materials in the sediments mostly originate from diatom blooms. Given that microbial communities respond quickly to environmental changes, and that global climate change is proceeding rapidly in the ASP, our results suggest that any modifications in the Planctomycetes-dominated microbial communities will provide valuable insight into changes in organic matter transport to the seafloor that may result from shifts in planktonic communities from Phaeocystis to diatoms associated with climate change.
Highlights
In marine surface sediments, microorganisms occur in enormous numbers (Whitman et al, 1998), are extremely diverse (Huber et al, 2007), and are notably involved in global carbon, nitrogen, and sulfur cycles (Canfield et al, 2005; Orcutt et al, 2011)
Characterization of benthic microbial communities is underrepresented in the Southern Ocean, especially in the Amundsen Sea where environmental changes due to global warming are occurring rapidly
Planctomycetes predominated benthic bacterial communities in the polynya sediment underlying the Phaeocystisdominated water column, which suggests that Planctomycetes comprise a major bacterial group utilizing relatively recalcitrant Corg produced primarily by Phaeocystis blooms
Summary
Microorganisms occur in enormous numbers (approximately 1.7 × 1028 cells worldwide) (Whitman et al, 1998), are extremely diverse (Huber et al, 2007), and are notably involved in global carbon, nitrogen, and sulfur cycles (Canfield et al, 2005; Orcutt et al, 2011). Because of the combined effects of enhanced light conditions and iron supply resulting from melting sea ice, polynyas are one of the most productive marine ecosystems (Sedwick and DiTuillo, 1997; Arrigo and van Dijken, 2003; Montes-Hugo and Yuan, 2012), and are regarded as significant sinks for atmospheric CO2 (Miller and DiTuillo, 2007; Arrigo et al, 2008). Due to the significant gas exchange (CO2, DMS, etc.) and heat flux between the sea surface and atmosphere, polynyas are regarded as windows through which we can anticipate ecosystem responses associated with climate changes (Smith and Barber, 2007)
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