Utilizing environmental DNA and imaging to study the deep-sea fish community of Takuyo-Daigo Seamount.

The increase in interest of mining at seamounts means there is a critical need to establish baseline inventories through environmental survey, with the aim of promoting the conservation and stewardship of these remote habitats. To efficiently evaluate fish biodiversity around a seamount, we compared environmental DNA (eDNA) methods using seawater and sponge samples against methods using imagery obtained with a remotely operated vehicle (ROV) and a free-fall deep-sea camera lander called the Edokko Mark I on the Takuyo-Daigo Seamount (153.0°E, 23.5°N) in the northwestern Pacific Ocean. We detected a total of 18 fish families by these methods. The fish fauna detected on the seamount included many families commonly found in deep-sea areas and were similar to the fish fauna of other seamounts located at similar latitudes in the northwestern Pacific. Significant differences in the patterns of detection of fish families between the eDNA and imaging methods is attributed to the differing powers of detection of some fish groups between methods (related to primer compatibility and fish size). For deep-sea fish, the difference in fish composition at the family level between seawater and sponge eDNA methods was not significant, but the difference between Edokko Mark I and ROV methods was significant; the latter difference is likely due to whether or not bait is used to attract fish. Although the eDNA workflow implemented here requires improvements, the use of eDNA and imaging methods in combination provided better insight into the biodiversity of deep-sea fishes in the deep-sea around a seamount, where our knowledge of the fish fauna has been extremely limited. Our recovery of eDNA from seawater and sponge samples around the seamount demonstrates the potential of these methods for facilitating environmental baseline surveys and impact assessments of mining activities to obtain results not previously possible with the use of visual methods only.

Impact of Concurrent aerobic-anaerobic Methanotrophy on Methane Emission from Marine Sediments in Gas Hydrate Area.

Microbial methane oxidation has a significant impact on the methane flux from marine gas hydrate areas. However, the environmental fate of methane remains poorly constrained. We quantified the relative contributions of aerobic and anaerobic methanotrophs to methane consumption in sediments of the gas hydrate-bearing Sakata Knoll, Japan, by in situ geochemical and microbiological analyses coupled with 13C-tracer incubation experiments. The anaerobic ANME-1 and ANME-2 species contributed to the oxidation of 33.2 and 1.4% methane fluxes at 0-10 and 10-22 cm below the seafloor (bsf), respectively. Although the aerobic Methylococcaceae species consumed only 0.9% methane flux in the oxygen depleted 0.0-0.5 cmbsf zone, their metabolic activity was sustained down to 6 cmbsf (based on rRNA and lipid biosyntheses), increasing their contribution to 10.3%. Our study emphasizes that the co-occurrence of aerobic and anaerobic methanotrophy at the redox transition zone is an important determinant of methane flux.

Low blank sampling method for measurement of the nitrogen isotopic composition of atmospheric NOx.

The nitrogen isotopic composition of nitrogen oxide (NOx) is useful for estimating its sources and sinks. Several methods have been developed to convert atmospheric nitric oxide (NO) and/or nitrogen dioxide (NO2) to nitrites and/or nitrates for collection. However, the collection efficiency and blanks are poorly evaluated for many collection methods. Here, we present a method for collecting ambient NOx (NO and NO2 simultaneously) with over 90% efficiency collection of NOx and low blank (approximately 0.5 µM) using a 3 wt% hydrogen peroxide (H2O2) and 0.5 M sodium hydride (NaOH) solution. The 1σ uncertainty of the nitrogen isotopic composition was ± 1.2 ‰. The advantages of this method include its portability, simplicity, and the ability to collect the required amount of sample to analyze the nitrogen isotopic composition of ambient NOx in a short period of time. Using this method, we observed the nitrogen isotopic compositions of NOx at the Tsukuba and Yoyogi sites in Japan. The averaged δ15N(NOx) value and standard deviation (1σ) in the Yoyogi site was (-2.7 ± 1.8) ‰ and in the Tsukuba site was (-1.7 ± 0.9) ‰ during the sampling period. The main NOx source appears to be the vehicle exhaust in the two sites.

Methane diffusion affects characteristics of benthic communities in and around microbial mat-covered sediments in the northeastern Japan sea.

We investigated relationships between features of benthic macrofaunal communities and geochemical parameters in and around microbial mat-covered sediments associated with a methane seepage on Sakata Knoll in the eastern Japan Sea. A depression on top of the knoll corresponds to a gas-hydrate-bearing area with seepage of methane-rich fluid, and microbial mats cover the seafloor sediments. Sediment cores were collected at three sites for this study: one within a microbial mat, a second a few meters outside of the microbial mat, and a third from a reference site outside the gas-hydrate-bearing areas. Morphological analysis showed that the site inside the microbial mat had higher macrofaunal density and biomass compared with the other sites. 18S rRNA gene analysis showed that annelids were dominant in the surface sediment inside the microbial mat with the possible occurrence of microbial anaerobic oxidation of methane (AOM), whereas in the surface sediments outside the microbial mat and at the reference site the predominant species belonged to phylum Cercozoa. Morphological analysis also showed that the surface sediment inside the microbial mat noticeably favored annelids, with dorvilleid Ophryotrocha sp. and ampharetid Neosabellides sp. identified as major constituents. Statistical analysis showed that sulfidic sediment conditions with concentrations of H2S up to 121 µM resulting from AOM likely resulted in the predominance of annelids with tolerance to sulfide. Both the 18S rRNA genes and macrofaunal characteristics showed that benthic biodiversity among the three sites was greatest outside the microbial mat. The site outside the microbial mat may represent geochemical transition conditions, including a lower rate of upward methane gas-flow compared with the site inside the microbial mat. The high biodiversity there might result from the presence of species specifically suited to the transition zone as well as species also found in photosynthesis-based communities of the background environment.