Hisayuki Arakawa

Lethal effects caused by suspended particles and sediment load on zoospores and gametophytes of the brown alga Eisenia bicyclis

The effects caused by suspended particles in seawater and sediment cover of the substrate on the attachment to the substrate of zoospores, and on the subsequent growth and survival of gametophytes of the brown alga Eisenia bicyclis were examined in the laboratory. The attachment rate to the substrate of the zoospores was remarkably reduced by the sediment cover on the substrate, and it decreased to 3.8% of the no-sediment control by a slight sediment cover of 3.0 mg/cm2 (0.048 mm in thickness). The growth of gametophytes was not inhibited at 10 mg/cm2 or less of sediment cover, though it was stopped at 30 mg/cm2. The survival rate of gametophytes became 39.4% at 5 mg/cm2 on the substrate, and gametophytes were all blighted at 30 mg/cm2. The ratio of male of female of the surviving gametophytes became 81.5:18.5, when the sediment cover was 10 mg/cm2. The rate of total loss (TL [%]) of zoospores and gametophytes of Eisenia bicyclis was obtained from the following equation: TL=100 (1−exp[−0.0339 C] exp[−1.24 Q]), where, C(mg/L) and Q (mg/cm2) indicate the concentration of suspended particles in seawater and the sediment on the substrate, respectively.

An on-chip imaging droplet-sorting system: A real-time shape recognition method to screen target cells in droplets with single cell resolution

A microfluidic on-chip imaging cell sorter has several advantages over conventional cell sorting methods, especially to identify cells with complex morphologies such as clusters. One of the remaining problems is how to efficiently discriminate targets at the species level without labelling. Hence, we developed a label-free microfluidic droplet-sorting system based on image recognition of cells in droplets. To test the applicability of this method, a mixture of two plankton species with different morphologies (Dunaliella tertiolecta and Phaeodactylum tricornutum) were successfully identified and discriminated at a rate of 10 Hz. We also examined the ability to detect the number of objects encapsulated in a droplet. Single cell droplets sorted into collection channels showed 91 ± 4.5% and 90 ± 3.8% accuracy for D. tertiolecta and P. tricornutum, respectively. Because we used image recognition to confirm single cell droplets, we achieved highly accurate single cell sorting. The results indicate that the integrated method of droplet imaging cell sorting can provide a complementary sorting approach capable of isolating single target cells from a mixture of cells with high accuracy without any staining.

Biological half-life of radioactive cesium in Japanese rockfish Sebastes cheni contaminated by the Fukushima Daiichi nuclear power plant accident.

Since the Fukushima accident in March 2011 the concentration of radioactive cesium in Japanese rockfish (Sebastes cheni) has been decreasing slower than other fish species. The aim of this study was therefore to investigate the possibility of slow elimination rate (i.e., relatively longer Tb) as one of the reasons for the slow decrease in (137)Cs concentrations in Japanese rockfish (S. cheni). To do this, we reared twenty-three individuals of this species for a period of about 1 year, during which time we measured the (137)Cs concentrations and γ-ray spectra 14 times by using a high-efficiency NaI(Tl) scintillator. We then examined the relationship between the (137)Cs concentrations and the total length of each individual. We estimated the biological half-life (Tb, day) for each individual using the total number of (137)Cs counts in the energy region, and examined the effects of total length and (137)Cs concentration on Tb by generalized linear model (GLM). We also examined the effect of sex, total length, seawater temperature, and the (137)Cs concentration of seawater on temporal changes in the (137)Cs count reduction rate by GLM. There was no clear relationship between the corrected whole-body (137)Cs concentrations and the total length in females, however there was a significant positive correlation between these two variables in males. The difference between males and females may be attributable to variation in the degree of dilution because of variable growth of individuals, and suggests that the (137)Cs concentrations of small individuals may be greatly diluted because of faster growth. However, there was no significant difference in Tb between sexes. The mean Tb (±SD) in all individuals was 269 (±39) days; this Tb value is 2.7-5.4 times longer than past Tb values (marine fish: 50-100 days), and is thought to be one of the reasons for the slower decrease in (137)Cs concentrations in this species than other fish species on the coast of Fukushima. The GLM showed significant effects of both total length and (137)Cs concentration on Tb, which may reflect a reduction in the metabolic rate with increased body size (i.e., aging) and gradient of concentration against seawater. The GLM also showed a significant positive effect of seawater temperature on the reduction rate of the (137)Cs counts (D, day(-1)). Therefore, D was clearly related to seasonal variations in the temperature of seawater, and this relationship may be attributable to changes in the metabolic rate that are controlled by variations in the seawater temperature. From these measurements, we examined the processes that control reductions in (137)Cs radioactivity.

Limited Recovery of the Kelp Eisenia bicyclis After Population Reduction of the Sea Urchins Hemicentrotus pulcherrimus and Anthocidaris crassispina on Kii Peninsula, Southwestern Japan

ABSTRACT We gauged the effect of the sea urchins Hemicentrotus pulcherrimus and Anthocidaris crassispina on an Eisenia bicyclis kelp bed on the Kii Peninsula in southwestern Japan. We monitored sea urchin density, size, and vertical algal distribution every 2 months from September 2000 to March 2002 and in September 2002 in a 40-m2 plot that stretched from the intertidal (reef) zone to the subtidal zone at depth of up to 2 m. The sea urchin densities were more than 40 individuals/m2 (H. pulcherrimus) and more than 5 individuals/m2 (A. crassispina) during September 2000 to May 2001. These densities decreased abruptly to less than 20 and less than 1 individual/m2 in October 2001, respectively, coincident with low salinities of less than 23 practical salinity unit (PSU) in late June because of river water inflow. Until May 2001, adult E. bicyclis grew only on the intertidal reef, and articulated coralline turf was predominant in the subtidal zone. From January 2002, the kelp bed expanded slightly to the upper subtidal zone, but most young kelp growing at a depth of more than 1.5 m disappeared, despite the low sea urchin density. It is likely that abiotic factors resulting from the river water inflow inhibited the growth of E. bicyclis.

Effects of sediment influx on the settlement and survival of canopy-forming macrophytes

Kelp forests on coastal rocky shores are negatively impacted by sudden sediment loads that can occur with storms and floods. Using laboratory experiments, we studied the effects of sediment deposition on the survival of the large brown alga Eisenia bicyclis juveniles (zoospores and gametophytes) to quantify the potential impacts of particulate matter on kelp forests. The zoospore adhesion rate and the gametophyte survival and growth rates all declined markedly with increasing sediment load, particularly with smaller particle diameter. Using experimental results, we derived an equation to calculate the rate of initial kelp depletion with sediment load based on the quantity and size distribution of sediment particles. The equation enabled the estimation of E. bicyclis depletion rates in the field by measurement of particle quantity and diameter distribution of sediments on the reef substrate.

Effect of Wavelength of Intermittent Light on the Growth and Fatty Acid Profile of the Haptophyte Isochrysis galbana

The haptophyte Isochrysis galbana is widely used as a food source for bivalve aquaculture as it is rich in fatty acid. In this study, the effect of different colors of intermittent light on the multiplication of cells of I. galbana and their fatty acid composition was investigated. I. galbana was cultivated under conditions of white, blue (dominant wavelength; 470 nm), green (525 nm), or red (660 nm) intermittent light (frequency; 10,000 Hz, duty ratio; 50%, light intensity; 52 μmol photons m−2 s−1, water temperature; 20°C).

Distribution of Radioactive Material in Marine Ecosystems Off the Fukushima Coast: Radioactive Cesium Levels in Fukushima Marine Organisms

As a result of the earthquake and tsunami of March 2011, a large quantity of radioactive material was emitted into the sea from the Fukushima Daiichi Nuclear Power Plant (FNPP). Seawater, sediments, and many marine organisms with high radiation levels have been reported from coastal areas in Fukushima Prefecture. Currently, fishing in Fukushima Prefecture has been banned. This projected study will examine the following issues: (1) diffusion of radioactive material via the food chain, (2) diffusion via migration/movement of various organisms, and, as a long-term project, (3) the biological half-life of selected radionuclide cesium within the fish body. Sampling of marine organisms was carried out between November 2011 and May 2013 by scuba diving, gillnetting, and seine netting. A rocky shore and a sandy beach at Yotsukura (35 km south of FNPP; depth, 0.5–1 m), Ena (50 km south of FNPP; depth, 5–6 m), and Souma (50 km north of FNPP; depth, 3–7 m) were selected as sampling locations. Sampled species included 15 seaweeds, 25 invertebrates, and 43 fish species. Concentrations of radioactive cesium (134+137Cs) were measured using a germanium semiconductor detector. In addition, an ultrasonic pinger was used to investigate the ranging behavior of the rockfish Sebastes cheni in an area contaminated with high concentrations of cesium. Immediately after the accident at FNPP, very high levels of radioactive cesium were recorded in coastal marine organisms, but these decreased with time. At present, the concentrations found in demersal coastal fish are higher than in pelagic species. The ranging behavior of rockfish, contaminated at comparatively high levels, was very limited, with the fish remaining within a small territorial area and thus limiting the diffusion of radioactive cesium. It is 3 years since the Fukushima accident; however, continuous monitoring of radioactive cesium concentrations in benthic and coastal fish is still required to implement suitable management policies.

Radioactive cesium concentrations in coastal suspended matter after the Fukushima nuclear accident

Radioactive cesium concentrations in the suspended matter of the coastal waters around the Fukushima Daiichi Nuclear Power Plant (FDNPP) were investigated between January 2014 and August 2015. The concentrations of radioactive cesium in the suspended matter were two orders higher in magnitude than those determined in the sediment. In addition, we discovered highly radioactive Cs particles in the suspended matter using autoradiography. The geometrical average radioactivity of particles was estimated to be 0.6 Bq at maximum and 0.2 Bq on average. The contribution ratio of highly radioactive Cs particles to each sample ranged from 13 to 54%, and was 36% on average. A major part of the radioactive Cs concentration in the suspended matter around the FDNPP was strongly influenced by the highly radioactive particles. The subsequent resuspension of highly radioactive Cs particles has been suggested as a possible reason for the delay in radioactive Cs depuration from benthic biota.

Physical Factors Involved in the Isoyake (Seaweed Forest Depletion) at Mio, Pacific Coast of Central Japan

Physical Factors Involved in the Isoyake (Seaweed Forest Depletion) at Mio, Pacific Coast of Central Japan In Japan, seaweed forest depletion is known as isoyake. We conducted a study on the marine environment to clarify the cause and the persistence of the isoyake in the sea area at Mio, on the west coast of the Kii Peninsula, central Japan. In the sea area at Mio, a layer of high-turbidity seawater formed on the sea surface (lowest Secchi disc depth: 1m) during and after precipitation. The average accumulation of sediment on the bedrock was 6.7 mg/ cm2. This value was approximately four times that recorded in the sea area at Noshima, where seaweed communities were present. These results imply that the large amounts of suspended particles in the seawater and sediment particles that settled on the bedrock were the major factors contributing to the continuation of the isoyake at Mio.

Visual contrast threshold of striped beak-perch Oplegnathus fasciatus

Underwater visibility is lowered with increasing suspended materials in coastal waters. Low contrast threshold and high visual acuity enable a fish to locate precisely its prey and find the presence of a predator. The striped beak-perch Oplegnathus fasciatus is widely found in the coastal waters of Japan and often used in the laboratory experiments to examine the visual ability of fish. In spite of intensive studies on the visual acuity of this fish, no study on the contrast threshold has been conducted. In the present study, the contrast threshold by behavioral response of the fish to prey was examined. The principle of the experimental equipment followed Nakamura. The equipment was installed in a darkroom. As shown in Figure 1, a rectangular parallelepiped water tank was divided into a waiting tank of the length 60 cm, and a response tank of the length 140 cm. A gate to restrict the movement of fish was installed at the border between the waiting tank and the response tank. Opening and closing of the gate was carried out by remote control. The water tank was filled with filtered sea water (beam attenuation coefficient: ca. 0.5 m-1 at 486 nm) to a depth of 30 cm. Filtered sea water was supplied to the end of the waiting tank at a rate of 6–8 L/min. The sea water was discharged from the back of the response tank so that a weak flow (less than 1 cm/ s) from the waiting tank to the response tank could be maintained. By this method, the behavior of fish is not influenced by the smell of a target. The temperature of filtered sea water was 21–24°C during the experiment. The irradiance of the water surface of the tank was kept uniform at ca. 60 lx. A white piece of mantle tissue of frozen Japanese common squid was used as the target and its shape was a circle (diameter 5 mm; thickness 4–5 mm). The size of the target did not influence the visual recognition. In order to generate various contrasts for the target, black to white background plates with six grades of lightness were placed behind the target. The apparent contrast C(r), for the target against the background, at the distance r can be expressed by the following equation.