Masahiro Suzumura

Control of Phosphate Concentration through Adsorption and Desorption Processes in Groundwater and Seawater Mixing at Sandy Beaches in Tokyo Bay, Japan

Field observation was conducted to monitor phosphate concentrations in groundwater and seawater mixing at two sandy beaches in Futtsu and Miura in Tokyo Bay, Japan. Dissolved phosphate concentrations were measured along transects from fresh groundwater aquifer to seawater adjacent the beaches. The concentrations were often high (up to 46 µM) in fresh groundwater samples (Cl− < 0.2‰). Coastal seawater, on the other hand, exhibited low phosphate concentrations (1.5 µM or less). Along the transects, phosphate generally displayed non-conservative behavior during mixing of fresh and saline waters in the aquifer; concentrations as high as 100 µM were found around the upper limit of seawater intrusion (Cl− = ∼2‰). Laboratory experiments were executed to identify the processes that control the phosphate behavior in the mixing processes. The results revealed that adsorption-desorption processes by the aquifer sand particles could significantly control the phosphate concentrations in the groundwater. Furthermore, the adsorption and/or desorption was found to be a function of salinity; the equilibrium concentration of dissolved phosphate in slurry of sand and water was the highest in freshwater and decreased considerably in saline water. The extreme concentration of phosphate may be caused by release from sand particles coinciding with the rapid change in salinity with tide.

Concentrations of lipid phosphorus and its abundance in dissolved and particulate organic phosphorus in coastal seawater

Abstract Analytical procedures to determine lipid phosphorus (lipid P) in seawater were developed and tested using a combination of solvent and solid-phase extraction. Lipid P was extracted from particulate and dissolved matter in seawater with a solvent mixture of chloroform and methanol. Silica gel solid-phase extraction (SPE) was evaluated using phospholipid and hydrophilic compounds. SPE exhibited high recoveries of phospholipids from the solvent mixture and was also effective in removing contamination from hydrophilic compounds. The method was applied to coastal seawater samples to investigate the quantitative significance of lipid P in marine organic P. In samples collected from Tokyo Bay, Japan, and Corpus Christi Bay, USA, concentrations of particulate lipid P ranged from 31 to 300 nM, which corresponded to 3–14% of the total particulate P. Concentrations of dissolved lipid P were low ranging from 0.7 to 6 nM, which were less than 1% of the total dissolved organic P concentrations. Thus, lipid P is a significant component of particulate P, but a rather minor component of dissolved organic P.

Dissolved Phosphorus Pools and Alkaline Phosphatase Activity in the Euphotic Zone of the Western North Pacific Ocean

We measured pools of dissolved phosphorus (P), including dissolved inorganic P (DIP), dissolved organic P (DOP) and alkaline phosphatase (AP)-hydrolyzable labile DOP (L-DOP), and kinetic parameters of AP activity (APA) in the euphotic zone in the western North Pacific Ocean. Samples were collected from one coastal station in Sagami Bay, Japan, and three offshore stations between the North Pacific subtropical gyre (NPSG) and the Kuroshio region. Although DIP concentrations in the euphotic zone at all stations were equally low, around the nominal method detection limit of 20 nmol L-1, chlorophyll a (Chl a) concentrations were one order of magnitude greater at the coastal station. DOP was the dominant P pool, comprising 62–92% of total dissolved P at and above the Chl a maximum layer (CML). L-DOP represented 22–39% of the total DOP at the offshore stations, whereas it accounted for a much higher proportion (about 85%) in the coastal surface layers. Significant correlations between maximum potential AP hydrolysis rates and DIP concentrations or bacterial cell abundance in the offshore euphotic zone suggest that major APA in the oligotrophic surface ocean is from bacterial activity and regulated largely by DIP availability. Although the range of maximum potential APA was comparable among the environmental conditions, the in situ hydrolysis rate of L-DOP in the coastal station was 10 times those in the offshore stations. L-DOP turnover time at the CML ranged from 4.5 days at the coastal station to 84.4 days in the NPSG. The ratio of the APA half-saturation constant to the ambient L-DOP concentration decreased markedly from the NPSG to the coastal station. There were substantial differences in the rate and efficiency of DOP remineralization and its contribution as the potential P source between the low-phosphate/high-biomass coastal ecosystem and the low-phosphate/low biomass oligotrophic ocean.

Differences in elimination efficiencies of Escherichia coli in freshwater and seawater as a result of TiO2 photocatalysis

The effects of UV irradiation on the respiration and survival of Escherichia coli in various concentrations of aqueous NaCl were investigated in the presence of a photocatalyst. In this study, we anticipated that the photocatalysis of residual chlorine generated in a solution containing Cl(-) would result in bacterial elimination. Our results indicated a gradual reduction in the E. coli survival ratio in freshwater; however, no decrease in total abundance was observed during 8 h of photocatalysis with UV irradiation. Conversely, the survival ratio of E. coli in the artificial seawater decreased drastically as a consequence of photocatalysis, with a concomitant decrease in total abundance. These results revealed that the chlorinated active species that formed on the photocatalyst surface influenced the observed inactivation.

Denitrification in a seashore sandy deposit influenced by groundwater discharge

The chemical compositions of ground water and organic matter in sediments were investigated at a sandy shore of Tokyo Bay, Japan to determine the fate of ground water NO3−. On the basis of Cl− distribution in ground water, the beach was classified into freshwater (FR)-, transition (TR)-, and seawater (SW)-zones from the land toward the shoreline. The NO3− and N2O did not behave conservatively with respect to Cl− during subsurface mixing of freshwater and seawater, suggesting NO3− consumption and N2O production in the TR-zone. Absence of beach vegetation indicated that NO3− assimilation by higher plants was not as important as NO3− sink. Low NH4+ concentrations in ground water revealed little reduction of NO3− to NH4+. These facts implied that microbial denitrification and assimilation were the likely sinks for ground water NO3−. The potential activity and number of denitrifiers in water-saturated sediment were highest in the low-chlorinity part of the TR-zone. The location of the highest potential denitrification activity (DN-zone) overlapped with that of the highest NO3− concentration. The C/N ratio and carbon isotope ratio (δ13C) of organic matter in sediment (< 100 -μm) varied from 12.0 to 22.5 and from −22.5 to −25.5‰, respectively. The δ13C value was inversely related to the C/N ratio (r2 = 0.968, n = 11), which was explained by the mixing of organic matters of terrestrial and marine origins. In the DN-zone, the fine sediments were rich in organic matters with high C/N ratios and low δ13C values, implying that dissolved organic matters of terrestrial origin might have been immobilized under slightly saline conditions. A concurrent supply of NO3− and organic matter to the TR-zone by ground water discharge probably generates favorable conditions for denitrifiers. Ground water NO3− discharged to the beach is thus partially denitrified and fixed as microbial biomass before it enters the sea. Further studies are necessary to determine the relative contribution of these processes for NO3− removal.

Blue carbon in human-dominated estuarine and shallow coastal systems

AbstractEstuarine and shallow coastal systems (ESCS) are recognized as not only significant organic carbon reservoirs but also emitters of CO2 to the atmosphere through air–sea CO2 gas exchange, thus posing a dilemma on ESCS’s role in climate change mitigation measures. However, some studies have shown that coastal waters take up atmospheric CO2 (Catm), although the magnitude and determinants remain unclear. We argue that the phenomenon of net uptake of Catm by ESCS is not unusual under a given set of terrestrial inputs and geophysical conditions. We assessed the key properties of systems that show the net Catm uptake and found that they are often characteristic of human-dominated systems: (1) input of high terrestrial nutrients, (2) input of treated wastewater in which labile carbon is highly removed, and (3) presence of hypoxia. We propose that human-dominated ESCS are worthy of investigation as a contributor to climate change mitigation.

Heterotrophic bacterial production and extracellular enzymatic activity in sinking particulate matter in the western North Pacific Ocean

Heterotrophic activities on sinking particulate matter (SPM) play an important role in SPM fluxes in the ocean. To demonstrate regional differences in heterotrophic activities on SPM, we measured heterotrophic bacterial production (HBP) in seawater (HBPSW) and SPM (HBPSPM) as well as potential extracellular enzyme activity (EEA) in SPM on a transect along 155°E in the western North Pacific Ocean in the subarctic (44°N), the Kuroshio Extension area (35°N), and the subtropical gyre (20°N). Depth-integrated HBPSW from the surface to 500 m was comparable between the locations, whereas HBPSPM at 44°N was substantially lower than at the other sites. We found the highest particulate organic carbon (POC) export flux and export efficiency to bathypelagic depths, and the lowest water temperatures, at 44°N. We found significant correlations between leucine aminopeptidase (LAPase) activity, β-glucosidase (BGase) activity, POC flux and particulate organic nitrogen flux. LAPase activity was two orders of magnitude higher than BGase activity, with a BGase:LAPase activity ratio of 0.027. There were no significant correlations between HBP and EEA in SPM except for lipase, and lipase activity was significantly correlated with temperature. We propose that hydrographic conditions are an important factor controlling heterotrophic bacterial activity and export efficiency of organic carbon to the deep ocean, as are the sources and abundance of SPM produced in the euphotic zone via primary production.

Impact on bacterial activities of ocean sequestration of carbon dioxide into bathypelagic layers

The ocean sequestration of carbon dioxide (CO2), direct injection of CO2 into bathypelagic layers, is one of the climate change mitigation options. It is essential to assess the potential environmental impacts on the marine ecosystem. In bathypelagic layers, bacteria are dominant organisms and play significant roles in oceanic carbon cycling through utilization and transformation of organic matter. We performed laboratory experiments by acidifying bathypelagic seawater with CO2 gas or buffer solutions to examine the impact on bacterial activities (abundance, production rate, and proportion of viable cells). In the laboratory experiments, we observed some potential effects by artificial changes in CO2 concentration, pH, or both, on bacterial activities. It was suggested that trophic conditions of bacterial assemblage strongly influence the magnitude of the impacts on bacterial activities and metabolisms by CO2 sequestration.

Methods for Determining Rates of Protein Synthesis via Dark CO2 Fixation by Marine Prokaryote

Methods were developed to determine rates of particulate and dissolved protein synthesis via dark CO2 fixation by marine prokaryotic assemblages. The methods are based on incorporation of [14C]-bicarbonate and separation of proteins as TCA-insoluble materials. Results indicated that particulate protein constitutes a significant fraction (∼67%) of cellular organic matter produced via prokaryotic dark CO2 fixation. The microcentrifuge method allowed us to estimate the rate of total protein synthesis via CO2 fixation. Time-series data proved useful for determining the optimum incubation period, and for estimating the rate of protein synthesis by regression analysis. The total prokaryotic dark CO2 fixation rate (10.3 ng C l−1 h−1) estimated by these methods was 2.7-times the rate of particulate CO2 fixation, which was comparable to the CO2 fixation rate estimated by methods used in previous studies. Thus, total prokaryotic dark CO2 fixation appears to be more important in the marine carbon cycle than previously thought.

CO2 Uptake in the Shallow Coastal Ecosystems Affected by Anthropogenic Impacts

Shallow coastal ecosystems (SCEs) are generally recognized as not only significant organic carbon reservoirs but also as sources for CO2 emission to the atmosphere, thus posing a dilemma regarding their role in climate change mitigation measures. However, we argue that SCEs can act as sinks for atmospheric CO2 under a given set of biogeochemical and socioeconomic conditions. The key properties of SCEs that show net uptake of atmospheric CO2 are often characteristic of human-dominated systems, that is, high nutrient inputs from terrestrial systems, input of treated wastewater in which labile carbon has been mostly removed, and the presence of hypoxic waters. We propose a new perspective on the potential of human-dominated SCEs to contribute to climate change mitigation, both serving as carbon reservoirs and providing direct net uptake of atmospheric CO2, in light of human systems–ecosystem interactions. Namely, if we view the land and a SCE as an integrated system, with appropriate management of both wastewater treatment and SCE, we will be able to not only suppress CO2 release but also capture and store carbon.