Although heterotrophic prokaryotes (HP) play a crucial role in biogeochemical carbon cycles, microbial oceanographic studies associated with heavy precipitation-induced large-scale freshwater runoff are understudied in the East China Sea (ECS), the largest continental shelf in the northwest Pacific. To elucidate the impact of Yangtze River diluted water (YRDW) on HP production (HPP) and growth-limiting resources, we conducted comprehensive microbial oceanographic measurements in combination with analysis of satellite images and optical property analyses of dissolved organic carbon (DOC) over three consecutive years in the northern ECS. Our results revealed that the HPP and chlorophyll a were consistently highest in summer due to the supply of excess DOC and nutrients via YRDW, which is intriguing considering the enhanced HPP coupled with spring phytoplankton bloom in middle latitudes in general. However, the exceptionally great YRDW runoff induced by heavy rainfall resulted in excessive supply of terrestrial-origin recalcitrant DOC and nutrients imbalance with high N : P ratio (34), which was responsible for the limited DOC bioavailability and phosphorus-limitation for the HPP. Accordingly, the enhanced HPP-to-primary production ratio (>0.5) in summer may suggest enhanced carbon flow via microbial food web, potentially altering food-web structure and energy transfer efficiency. Our results, demonstrating that YRDW can either stimulate or suppress HPP, provide new insights into microbial responses to large-scale freshwater discharge, which may be relevant to systems influenced by substantial freshwater inputs (e.g., Amazon River and Arctic Ocean).

We investigated the spatial distribution of prokaryotic heterotrophic production (PHP), its limiting resources, and the metabolic balance between prokaryotic carbon demand (PCD) and primary production (PP) in summer in the central Yellow Sea (YS). Due to the formation of strong stratification, the water column was divided vertically into two water masses: nutrient-poor and high-temperature Yellow Sea Surface Water (YSSW) and nutrient-rich and low-temperature Yellow Sea Cold Bottom Water, and the YSSW was further divided into northern and southern part. In the northern YSSW, where dissolved organic carbon (DOC) concentration was higher, relatively higher PHP and prokaryotic respiration (PR) were observed compared to the southern YSSW. However, despite the high DOC concentration and lability of dissolved organic matter, PHP and PR were relatively lower than those reported in coastal YS. Limiting resource incubation experiments demonstrated that PHP was stimulated in samples amended with P, which suggested that prokaryotic growth was restricted by P deficiency. The P-limited PHP in high DOC condition suggested that P deficiency for prokaryotic growth would inhibit prokaryotic organic carbon uptake, leading to DOC accumulation in the surface layer. The metabolic balance appeared to be autotrophy (PCD:PP < 1) in the mixed layer, and strong stratification might prevent the CO2 accumulated below the thermocline from moving to the surface. Overall results imply that when P deficiency due to the physical structure of the water column limits prokaryotic growth in the central YS in summer, net autotrophy may occur.

Sea ice melting driven by climate change-induced warming is continuously increasing in the Arctic Ocean. Although heterotrophic microbes play a pivotal role in biogeochemical processes, the impact of heterotrophic microbial respiration (HMR) on biogeochemical carbon cycles associated with the expansion of melt pond (MP) in the Arctic Ocean is understudied. We investigated variations of HMR across three distinct MP types (i.e., open MP, brackish MP, and closed MP) in the western Arctic Ocean. HMR was significantly higher in open MP (12.2 ± 5.64 mmol O2 m−3 day−1) compared to closed MP (2.25 ± 1.96 mmol O2 m−3 day−1), brackish MP (2.87 ± 2.28 mmol O2 m−3 day−1), and ambient seawater (4.88 ± 1.92 mmol O2 m−3 day−1). A strong correlation between dissolved organic carbon (DOC) and HMR suggests that sea-ice-derived DOC enhances heterotrophic microbial metabolism in open MPs. The prokaryotes carbon demand (27.8 ± 26.8 mmol C m−3 day−1) required to sustain prokaryotes metabolism was greater than the organic carbon produced through primary production (PP; 0.05 ± 0.02 mmol C m−3 day−1), suggesting that the remineralization of organic carbon supplied from the sea-ice communities to CO2 exceeds carbon fixation through PP. The high CO2 production in the open MPs (2.35 Tg C day−1), which is 20 times greater than the CO2 uptake by the MPs (0.12 Tg C day−1), suggests that warming-induced expansion of MPs stimulates microbial metabolisms, thereby serving as a positive CO2 feedback to the atmosphere. Our results provide new insights into carbon cycling driven by microbial responses to MP expansion under Arctic warming.

To better understand the tidal effects on dynamics of inorganic nutrients and its implications for outwelling processes, we conducted time-series observations on the fluctuations in concentrations and benthic flux of inorganic nutrients with tidal cycle in macrotidal Garolim Bay (GB), Yellow Sea. The strong tidal current and rapid mixing stimulated resuspension of suspended particulate matter (SPM), particulate organic carbon (POC), and chlorophyll-a (Chl-a), which were predominant during the spring tide. Concentrations of ammonium (NH4+) and NOx (sum of nitrite and nitrate) were not correlated with tidal variation, while concentrations of phosphate (PO43−) and silicate (SiO2) were inversely correlated with tidal levels, except for PO43− at surface layer during neap tide. In addition, analyses of the benthic nutrient fluxes (0.04–3.48 mmol m−2 d−1 for NH4+, 0.18–0.39 mmol m−2 d−1 for NOx, 0.03–0.12 mmol m−2 d−1 for PO43−, and 1.62–5.47 mmol m−2 d−1 for SiO2) and nutrient ratio revealed that there was a relatively excess efflux of PO43− and SiO2. The inverse fluctuations of P and Si with tidal level and their excess benthic release indicated that GB sediment acts as a net source of PO43− and SiO2 to the water column and to offshore water, which further suggested that the benthic nutrient flux in the GB may have a substantial effect on phytoplankton production in the adjacent coastal water column.

We investigated changes in heterotrophic bacterial metabolic activities and associated carbon cycles in response to a change in dominant phytoplankton communities during two contrasting environmental conditions in austral summer in the Amundsen Sea polynya (ASP), Antarctica: the closed polynya condition in 2014 (ANA04) and the open polynya condition in 2016 (ANA06). In ANA04, Phaeocystis antarctica predominated phytoplankton biomass, comprising 78% of total phytoplankton carbon biomass, whereas diatoms and Dictyocha speculum accounted for 45% and 48% of total phytoplankton carbon biomass, respectively, in ANA06. Bacterial production (BP) showed a significant positive correlation with only chlorophyll-a (Chl-a, rho = 0.66, p < 0.001) in P. antarctica -dominated ANA04, whereas there were significant positive relationships of BP with various organic carbon pools, such as chromophoric dissolved organic matter (CDOM, rho = 0.84, p < 0.001), Chl-a (rho = 0.59, p < 0.001), and dissolved organic carbon (DOC, rho = 0.51, p = 0.001), in ANA06 when diatoms and D. speculum co-dominated. These results indicate that BP depended more on DOC directly released from P. antarctica in ANA04, but was supported by DOC derived from various food web processes in the diatom-dominated system in ANA06. The BP to primary production (BP : PP) ratio was three-fold higher in P. antarctica -dominated ANA04 (BP: PP = 0.09), than in diatom- and D. speculum -co-dominated ANA06 (BP : PP = 0.03). These results suggested that the microbial loop is more significant in Phaeocystis -dominated conditions than in diatom-dominated conditions. In addition, the decreases in BP : PP ratio and bacterial respiration with increasing diatom proportion in the surface mixed layer indicated that the change from P. antarctica to diatom predominance enhanced biological carbon pump function by increasing particulate organic carbon export efficiency. Consequently, our results suggest that bacterial metabolic response to shifts in phytoplankton communities could ultimately affect larger-scale ecological and biogeochemical processes in the water column of the ASP.

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOM H ) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m -3 d -1 ) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOM H , which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m -3 d -1 ) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m -3 d -1 ). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOM H in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

Multiyear monitoring of the marine environment of the Japan/East Sea was continued by the joint Korean–Russian expedition of the R/V Akademik Oparin (cruise 58) in October–November 2019, including changes in the sea circulation and ventilation and biogeochemical processes as a result of current climate changes and growing anthropogenic impacts. Studies of mesoscale eddies associated with the branches of the Tsushima Warm Current have been continued. Methane flare has been discovered for the first time at the eastern Primorye slope.

The expansion of the aquaculture industry has resulted in accumulation of phosphorus (P)-rich organic matter via uneaten fish feed. To elucidate the impact of fish farming on P dynamics, P speciation, and benthic P release along with partitioning of organic carbon (C org ) mineralization coupled to sulfate reduction (SR) and iron reduction (FeR) were investigated in the sediments from Jinju Bay, off the southern coast of South Korea, in July 2013. SR in the farm sediment was 6.9-fold higher than the control sediment, and depth-integrated (0–10 cm) concentrations of NH 4 + , PO 4 3– , and H 2 S in pore water of the farm sediment were 2.2-, 3.3-, and 7.4-fold higher than that in control sediment, respectively. High biogenic-P that comprised 28% of total P directly reflected the impact of P-rich fish feed, which ultimately enhanced the bioavailability (58% of total P) of P in the surface sediment of the farm site. In the farm sediment where SR dominated C org mineralization, H 2 S oxidation coupled to the reduction of FeOOH stimulated release of P bound to iron oxide, which resulted in high regeneration efficiency (85%) of P in farm sediments. Enhanced P desorption from FeOOH was responsible for the increase in authigenic-P and benthic P flux. Authigenic-P comprised 33% of total P, and benthic P flux to the overlying water column accounted for approximately 800% of the P required for primary production. Consequently, excessive benthic P release resulting directly from oversupply of P-rich fish feed was a significant internal source of P for the water column, and may induce undesirable eutrophication and harmful algal blooms in shallow coastal ecosystems.

As sediments play an important role as either a sink or a source of phosphorus (P) for water column, it is important to elucidate the major P fractions and behaviors (i.e., mobilization and immobilization) in the sediments to better understand P cycles in local and global scale. We investigated major P speciation associated with the partitioning of organic carbon (Corg) oxidation in the sediments to elucidate the P dynamics at two contrasting sediments in the continental shelf (EB1) and rise (EC1) in the Ulleung Basin (UB), East Sea. Sulfate reduction (SR) pre-dominated Corg oxidation at shelf site (EB 1), comprising % of Corg oxidation, whereas Mn- and Fe-reduction combined accounted for >80% of Corg oxidation in Mn-oxide and Fe-oxide-rich basin site (EC 1). Under SR-dominated condition (EB 1), H2S oxidation coupled to reductive dissolution of FeOOH to form precipitation of FeS induced the accumulation of dissolved iron and phosphate in the pore water. On the other hand, phosphate in the Mn- and Fe-oxide-rich basin sediments (EC 1) was depleted because the P released through organic matter decomposition or reductive dissolution of Fe oxide/Mn oxide was effectively adsorbed to the metal-oxides in the surface sediments. Sequential extraction of P phases revealed that Fe bound P (52-65% of total P) was the major phase in the surface sediments of both sites. Interestingly, the organic P (Porg) fraction was 2.4-times higher at the basin site (12 μmol g-1) than at the shelf site (5 μmol g-1). Corg : Porg ratios presented as redox proxies in sediments were 644 and 191 for EB1 and EC1, respectively,. The results indicate that Porg has an effective preservation relative to Corg under sub-oxic conditions (EC1), whereas Porg was preferentially regenerated under anoxic conditions (EB1). Overall, the dynamics of P in the UB sediments were largely regulated by the partitioning of Corg oxidation pathways (i.e., sulfate reduction vs. metal reduction) and resultant interaction between Fe/Mn-S-P.

We investigated bacterial production (BP) and respiration (BR), in combination with phytoplankton and environmental parameters, to elucidate major carbon sources regulating heterotrophic bacterial metabolic activity and to evaluate variations of trophic balance associated with seawater warming in Gyeonggi Bay (GB). BP was not significantly correlated with primary production (PP, p > 0.05), but was significantly correlated with dissolved organic carbon (DOC, p < 0.01). Bacterial growth efficiency (BGE) was generally low (average 0.06) and decreased with increasing C/N ratio of DOM and concentration of humic-like fluorescent DOM (FDOM H ). This uncoupling between bacteria and phytoplankton and low BGE suggests that bacterial growth largely relied on allochthonous input of DOC, but metabolic activities of the bacteria were suppressed by the low nutritional quality of the FDOM H . High BP/PP and BR/PP ratios (generally >1) implied that trophic balance of GB represented heterotrophic conditions, and ratios showed an inverse relationship with Chl- a concentrations. In comparison to the early 1990s, increasing water temperatures were associated with significant decreases in PP and Chl- a (by a factor of 9 and 2, respectively), whereas changes in BP were insignificant; these changes resulted in a 14-fold increase in the BP/PP ratio. Therefore, our results strongly imply that warming in GB intensified the carbon flux through the microbial loop, which may reduce the efficiency of energy transfer to higher trophic levels.

We conducted experiments to investigate the effects of finfish aquaculture and to propose appropriate proxies for assessing their environmental impact. Due to enhanced fish feed input, sulfate reduction (SR) and the resulting metabolic products (H2S, NH4+, PO43−) were significantly greater at the farm than at the control site. Benthic release of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from farm sediment accounted for 52–837% and 926–1048%, respectively, of the potential DIN and DIP demand for phytoplankton production. The results suggest that excess organic loading in fish farms induces deleterious eutrophication and algal blooms in coastal ecosystems via benthic-pelagic coupling. Direct SR measurement provided the most useful information of all the parameters on organic contamination in fish farms. However, given its abundance, relatively lower chemical reactivity and relative ease of analysis, elemental sulfur was regarded as the most appropriate proxy for assessing the environmental impacts of finfish aquaculture.

Molecular analyses and biogeochemical measurements were combined to investigate the microbial communities associated with major terminal electron accepting processes and acetate oxidation at 2 contrasting sediments on the continental shelf (EB1) and basin (EB6) of the Ulleung Basin, East Sea. At EB1, sulfate reduction (SR) and iron reduction (FeR) dominated organic carbon (C-org) oxidation, accounting for 65 and 25% of anaerobic C-org oxidation, respectively. In contrast, manganese reduction (MnR) was responsible for >50% of anaerobic C-org mineralization at manganese oxide-rich EB6. Members of Desulfobacteraceae, known as putative sulfate-reducing bacteria (SRB), constituted a major C-org-oxidizing clade (22% of Bacteria) at EB1. Meanwhile, putatively Mn-reducing bacteria affiliated with Colwelliaceae, Shewanellaceae and Oceanospirillaceae were abundant in EB6 (8% of Bacteria). RNA-stable isotope probing (RNA-SIP) further identified Arcobacter as acetate-oxidizers associated with FeR, while no SRB were labeled at EB1. At EB6, microorganisms affiliated with Colwelliaceae and Oceanospirillaceae were identified as putative Mn-reducing acetate-oxidizers. Interestingly, at both sites, Thaumarchaeota were labeled with C-13 derived from acetate during the anoxic incubations. The results from RNA-SIP give new insights into the biogeochemical and ecological role of Arcobacter in FeR, and the metabolic activity of Thaumarchaeota under anoxia. As the upwelling intensity in the UB declines due to the rapid warming of surface waters, our results are relevant for evaluation of future changes in benthic biogeochemical processes and microbial communities in response to the variations of water-column productivity.

Biogeochemical process studies and molecular microbiological analyses were applied to assess the effect of invasive Spartina anglica (SA) on organic carbon (C-org) oxidation pathways and microbial community structures in intertidal sediments vegetated by the indigenous marsh plant Suaeda japonica (SJ) and unvegetated mud flats (UMF). Invasive S. anglica possessed 10 times the below-ground biomass of native S. japonica, which was responsible for releasing a substantial amount of labile dissolved organic matter and creating relatively oxidized conditions at the SA site. As a result, microbial metabolic activities measured by rates of anaerobic C-org oxidation, iron reduction (FeR) and sulfate reduction (SR) appeared to be greater at SA site compared with the SJ and UMF sites. SR was the dominant anaerobic respiration pathway at a depth of 0-10 cm for vegetated sediments, but the contribution of FeR to C-org oxidation was exceptionally high in the rhizosphere of the vegetated sites, comprising 60% and 70% of anaerobic C-org oxidation of SA and SJ, respectively. The iron turnover rate at the rhizosphere was 3 times higher at SA site (0.063 d(-1)) compared with the SJ site (0.023 d(-1)), indicating that the denser root system of invasive S. anglica greatly accelerates iron cycling. Bacterial communities based on 16S rRNA genes analysis revealed that members in Desulfuromonadaceae related to the reduction of FeOOH and S-0 were highly abundant at the relatively oxidized SA site, whereas Desulfobulbaceae, which are known as sulfate reducers, were more dominant at the relatively reduced SJ site. Similarly, two sulfur-oxidizing bacteria groups with different eco-physiological strategies thrived in each of the two vegetated sites. Thioprofundaceae in the Gammaproteobacteria were the predominant S-oxidizers at the less-reduced SA site, whereas Sulfurovum in the Epsilonproteobacteria dominated at the relatively reduced SJ site. Our results suggest that an invasion of tall S. anglica and its subsequent displacement of native S. japonica would greatly alter the biogeochemical C-Fe-S cycles and associated microbial communities, which ultimately generate multidirectional variations in ecological and biogeochemical processes in coastal ecosystems.

Occurrence of typhoons accompanied by heavy precipitation has increased for the past 40 years in northeast Asia. To elucidate the impact of three consecutive typhoon-induced heavy rainfall events and resultant freshwater runoff on the partitioning of organic carbon (Corg) oxidation and nutrient dynamics, we investigated the geochemical constituents, the rate of anaerobic Corg oxidation, sulfate reduction (SR), iron reduction (FeR) and P speciation in the intertidal mud flat of the Han River estuary, Yellow Sea. Corg oxidation by SR and FeR and their metabolic products (∑CO2, NH4+, H2S, Fe2+) decreased significantly (P < 0.05) during and immediately after the heavy rainfall. Additional mesocosm experiments demonstrated that potential N2 production rates increased up to 2.4 times with increased nitrate concentrations during freshwater runoff. The results suggest that denitrification becomes a significant Corg oxidation pathway substituting for SR during high-nitrate freshwater runoff, which may remove substantial portion of the N introduced into the estuary. P speciation analysis further revealed that the concentrations of iron bound P decreased by 2.2 fold during the heavy rainfall compared to that measured before the rainfall. The results suggest that an excess supply of riverine Si keeps P from binding to Fe, thereby stimulating P release. Taking projections of enhanced rainfall events in the future into account, our results suggest that the intensified storm events and resultant riverine runoff induces a shift of Corg oxidation pathways in the sediments, which ultimately alters C-N-P-S-Fe dynamics and may deepen N-limiting conditions in coastal ecosystems of the Yellow Sea.

A combination of biogeochemical analyses and molecular microbiological analyses were conducted to assess the environmental impact of finfish aquaculture and to elucidate the major microbial assemblages responsible for the production and removal of reduced sulfur compounds in fish-farm sediments. The average concentrations of H2S (123 µM) and NH4+ (1310 µM) and the dissimilatory sulfite reductase (dsr) gene copy number (1.9 × 109 copies cm-3) in the sediments at the farm site were 15-, 1.5- and 2-fold higher, respectively, than those measured at the less-impacted reference site. Accordingly, the sulfate reduction rate (SRR) at the farm site (118 mmol m-2 d-1) was 19-fold higher than that measured at the reference site (6.2 mmol m-2 d-1). Analyses of dsrA and 16S rRNA gene sequences revealed that the Syntrophobacteraceae and Desulfobulbaceae groups are the major sulfate-reducing bacteria around the fish-farm sediment. Interestingly, despite the high SRR (12.2-19.6 mmol m-2 d-1), the H2S concentration was low (<8 µM) in the top 0-2 cm of the fish-farm sediments. In this sulfide-mismatched zone, sulfur-oxidizing bacteria associated with Gamma- and Epsilonproteobacteria were abundant. Especially at the 1-2 cm depth, bacteria related to Sulfurovum in the Epsilonproteobacteria showed the highest relative abundance, comprising 62% of the 16S rDNA sequences. The results strongly suggest that Sulfurovum-like bacteria play a significant ecological and biogeochemical role in oxidation and reduction of reduced sulfur compounds from the organic-rich, highly sulfidic fish-farm sediments.

Experimental determinations of nitrogen cycling in deep‐sea sediments are strongly underrepresented in the databases. To investigate the total N 2 production rates and relative contribution of denitrification and anaerobic ammonium oxidation (anammox) to benthic fixed‐N removal processes, we conducted 15 N isotope‐labeling incubation experiments in whole cores and slurries at nine stations across the continental margin from the shelf (< 200 m) and into the deep (> 2000 m) Ulleung Basin (UB) in the East Sea. The total N 2 production rates (anammox plus denitrification) in the center of the UB (8.4 ± 0.2 μmol N m −2 h −1 ) were high compared to most other deep‐sea sediments at similar water depths. Denitrification rates decreased from the shelf (7.6 ± 0.6 μmol N m −2 h −1 ) to the basin (3.2 ± 0.4 μmol N m −2 h −1 ), in proportion to benthic oxygen consumption, whereas anammox rates remained relatively constant or even increased slightly (1.3–4.1 μmol N m −2 h −1 ). The contribution of anammox to the total N 2 production ( ra ) increased with increasing water depth from the shelf (ca. 17%) to the basin (ca. 56%). The enhanced ra in the center of the UB was associated with an increased availability of nitrite for anammox, which was likely a result of the competitive suppression of denitrification by manganese reduction under MnO 2 ‐rich conditions. Our results emphasize the importance of anammox as a sink for reactive nitrogen in deep‐sea sediments and contribute toward a mechanistic understanding of the factors controlling benthic reactive nitrogen loss in the ocean.