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Background Coastal zones are transitional areas between land and sea where large amounts of organic and inorganic carbon compounds are recycled by microbes. Especially shallow zones near land have been shown to be the main source for oceanic methane (CH4) emissions. Water depth has been predicted as the best explanatory variable, which is related to CH4 ebullition, but exactly how sediment methanotrophic bacteria mediates these emissions along water depth is unknown. Here, we investigated the activity of methanotrophs in the sediment of shallow coastal zones with high CH4 emissions within a depth gradient from 10–45 m. Field sampling consisted of collecting sediment slices from eight stations along a coastal gradient (0–4 km from land) in the coastal Baltic Sea. We combined real-time measurements of surface water CH4 concentrations, acoustic detection of CH4 seeps in the bottom water, and sediment DNA plus RNA sequencing.
Results The relative abundance and CH4 oxidizing activity (pMMO; particulate methane monooxygenase) of the dominant methanotroph Methylococcales was significantly higher in deeper costal offshore areas (36–45 m water depth) compared to adjacent shallow zones (10–28 m). This was in accordance with the shallow zones having CH4 concentrations in the surface water, as well as more CH4 seeps from the sediment. Furthermore, our findings indicate that the low prevalence of Methylococcales and their activity was restrained to the euphotic zone (indicated by PAR data, photosynthesis proteins, and 18S rRNA data of benthic diatoms). This was also indicated by a positive relationship between water depth and the relative abundance of Methylococcales and pMMO.
Conclusions We detected a low relative abundance of methanotrophs and CH4 oxidizing activity in shallow coastal areas, this can partly explain the difference in CH4 emissions between shallow and deep coastal areas (and the relationship between CH4 emission and water depth). Potentially a reduced activity of methanotrophs also facilities the build-up of CH4 bubbles in the sediment. CH4 emissions from the study area has previously been calculated to be comparable to that of subarctic lakes, and it is suggested that shallow coastal waters, similarly to inland waters, are hotspots for CH4 emissions.
Keywords
oceanic methane emissions, coastal zone, sediment methanotrophs
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This is a preprint, a preliminary version of a manuscript that has not completed peer review at a journal. Research Square does not conduct peer review prior to posting preprints. The posting of a preprint on this server should not be interpreted as an endorsement of its validity or suitability for dissemination as established information or for guiding clinical practice.
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History
CURRENT STATUS: Posted
View the published article at Frontiers in Microbiology.
Version 2
Posted 13 Feb, 2020
No community comments so far
No comments provided
Metrics
Comments: 0
PDF Downloads: ...
HTML Views: ...
Subject Areas
General Microbiology
License:
This work is licensed under a CC BY 4.0 License. Read Full License
View the published article at Frontiers in Microbiology.
Background Coastal zones are transitional areas between land and sea where large amounts of organic and inorganic carbon compounds are recycled by microbes. Especially shallow zones near land have been shown to be the main source for oceanic methane (CH4) emissions. Water depth has been predicted as the best explanatory variable, which is related to CH4 ebullition, but exactly how sediment methanotrophic bacteria mediates these emissions along water depth is unknown. Here, we investigated the activity of methanotrophs in the sediment of shallow coastal zones with high CH4 emissions within a depth gradient from 10–45 m. Field sampling consisted of collecting sediment slices from eight stations along a coastal gradient (0–4 km from land) in the coastal Baltic Sea. We combined real-time measurements of surface water CH4 concentrations, acoustic detection of CH4 seeps in the bottom water, and sediment DNA plus RNA sequencing.
Results The relative abundance and CH4 oxidizing activity (pMMO; particulate methane monooxygenase) of the dominant methanotroph Methylococcales was significantly higher in deeper costal offshore areas (36–45 m water depth) compared to adjacent shallow zones (10–28 m). This was in accordance with the shallow zones having CH4 concentrations in the surface water, as well as more CH4 seeps from the sediment. Furthermore, our findings indicate that the low prevalence of Methylococcales and their activity was restrained to the euphotic zone (indicated by PAR data, photosynthesis proteins, and 18S rRNA data of benthic diatoms). This was also indicated by a positive relationship between water depth and the relative abundance of Methylococcales and pMMO.
Conclusions We detected a low relative abundance of methanotrophs and CH4 oxidizing activity in shallow coastal areas, this can partly explain the difference in CH4 emissions between shallow and deep coastal areas (and the relationship between CH4 emission and water depth). Potentially a reduced activity of methanotrophs also facilities the build-up of CH4 bubbles in the sediment. CH4 emissions from the study area has previously been calculated to be comparable to that of subarctic lakes, and it is suggested that shallow coastal waters, similarly to inland waters, are hotspots for CH4 emissions.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
This is a list of supplementary files associated with this preprint. Click to download.
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