Assessment of heavy metal pollution in the coastal sediments of an urbanized atoll in the central Pacific: Majuro Atoll, the Marshall Islands.

Pacific atolls are extremely vulnerable to the effects of climate change. Coral reef ecosystems, which are responsible for the island formation and maintenance, can potentially keep pace with rising sea levels. Such ecosystems are sensitive to pollution; however, the sources and levels of atoll pollutants caused by urbanization have rarely been investigated. In this study, we assessed the heavy metal pollution (Cr, Mn, Ni, Cu, Zn, Cd, and Pb) of coastal sediments to evaluate the effects of urbanization on Majuro Atoll, the Marshall Islands. The densely populated area had the most significant pollution with high levels of Pb, Mn, Zn, and Cu due to road traffic activity. Domestic wastewater, a major pollution source in Pacific atolls, was not identified. Remarkably, the Zn and Pb levels in the lagoonal coasts of the remote island area were 697 - 1539 and 22 - 337 times higher, respectively, than in the natural area of Funafuti Atoll, Tuvalu. Thus, the remote island and sparsely populated areas were significantly polluted because of the maritime traffic activity in the lagoon and debris accumulation in/around the lagoon. This pollution resulted from improper municipal solid waste management of the main island. The contamination factor, pollution load index, and geo-accumulation index indicated high levels of heavy metal pollution in these areas. Urbanization of the atoll clearly resulted in a distinct heavy metal composition and high pollution levels compared with Funafuti Atoll. These findings emphasize the importance of pollution management in the conservation and rehabilitation of urbanized atolls threatened by future sea-level rises.

Comprehensive assessment of multiple biomarker mechanisms in the brackish water clam Corbicula japonica exposed to polystyrene microplastics using structural equation modeling.

Using structural equation modeling (SEM), we investigated multiple biomarker mechanisms in terms of biochemical and individual marker responses in the brackish water clam Corbicula japonica following acute exposure to polystyrene microplastic (PS-MP). This study is the first to comprehensively explore multiple biomarker responses in bivalves using SEM. The model revealed that PS-MP accumulation was an independent biomarker, exhibiting significant direct effects on superoxide dismutase (SOD) and catalase (CAT) among the biochemical markers. Although CAT generally interacts closely with SOD, no significant relationship was identified between them, indicating that CAT may have independently responded to PS-MP stress. Among individual markers, significant indirect effects were observed on clearance rate (CR), reflecting feeding activity and valve open rate, indicating excretion activity via SOD and CAT. Finally, the carbon-based scope for growth was significantly influenced by CR. SEM is efficient and useful for identifying significant direct and indirect pathway relationships and for uncovering uncommon relationships in unified multiple biomarker mechanisms in aquatic studies.

Antioxidant capacity and carbon-based scope for growth of brackish water clams Corbicula japonica under the combined effects of natural and anthropogenic factors.

Changes in natural estuarine environment and anthropogenic disturbances are becoming significant threats to organisms, particularly bivalves. A deeper understanding of the relationship between biochemical- and individual-level responses is necessary to assess the combined effects of natural and anthropogenic factors on bivalves. To the best of our knowledge, this is the first study where the oxygen radical absorbance capacity (ORAC) and carbon-based scope for growth (C-SFG) were applied as biomarkers to evaluate the response of the brackish water clam Corbicula japonica to four spatiotemporally varying environmental factors. High water temperature and food availability supported C-SFG while high salinity inhibited it. Most of wastewater (WW) treatments resulted in negative C-SFG values because of a reduced clearance rate and increased excretion rate. In particular, high food availability with WW treatment resulted in the lowest C-SFG value of -114 µg C·ind-1 h-1. The ORAC was activated in response to high salinity with WW treatment (p < 0.05). To ascertain the combined effects of the natural and anthropogenic factors, principal component and cluster analyses were performed on the ORAC and C-SFG data. Anthropogenic WW was found to have different effects on the physiological and biochemical biomarkers according to the natural conditions. A roughly negative correlation was observed between ORAC and C-SFG because activation of the antioxidant capacity can influence the growth potential of the clams through the additional use of available metabolic energy. However, some exceptions were observed where both the ORAC and C-SFG values were either high or low, which could be because the C-SFG response varies depending on different metabolic behaviors even when the ORAC response remains the same. These results indicate that the biochemical-level response (i.e., ORAC) of C. japonica can be interpreted using individual-level response (i.e., C-SFG), but careful attention must be given to over- or underestimation.

Behavior of eukaryotic symbionts in large benthic foraminifers Calcarina gaudichaudii and Baculogypsina sphaerulata under exposure to wastewater.

Large benthic foraminifers (LBFs) are significant contributors to coral island formation in the Pacific Ocean. In recent years, the population of LBFs has decreased because of the increase in anthropogenic influences, such as wastewater (WW) discharge. To implement efficient mitigation measures, pollution tolerance in LBFs should be understood. However, the effects of WW on LBFs and their symbionts have not yet been demonstrated. This study examined the changes in the photosynthetic efficiency (Y[II]) of Calcarina gaudichaudii and Baculogypsina sphaerulata in response to WW by using a pulse-amplitude-modulation fluorometer. These LBFs were exposed to WW with different dilution levels for 22 days. The Y(II) values of the LBFs were found to deteriorate within 1-2 days. However, the Y(II) values both deteriorated and were enhanced in the experiments, thus indicating that WW contains both harmful and beneficial components. Baculogypsina sphaerulata showed an earlier response and greater sensitivity to WW and a higher epibiont infestation than C. gaudichaudii. This result can be attributed to the differences in the physiological and morphological responses of distinct LBFs. A sequencing analysis of 18S rDNA confirmed that the dominant eukaryotic symbionts in the two LBFs studied were Ochrophyta and Labyrinthulomycetes. These eukaryotic symbionts were released and attached as epibionts onto LBFs that were exposed to WW, thus leading to an increase in inactive LBFs. The Shannon-Weaver and Simpson diversity indices revealed that eukaryotic symbiont communities decreased in biodiversity after exposure to WW because of the abundance of algal symbionts. On the basis of these results, we conclude that WW, even with 10,000 × dilution, causes a decrease in active LBF populations owing to the release of eukaryotic symbionts, the decrease in biodiversity, and the infestation of epibionts even though Y(II) is temporarily enhanced. These responses are more significant in B. sphaerulata than in C. gaudichaudii.

Oxidative stress responses to feeding activity and salinity level in brackish water clam Corbicula japonica.

Three laboratory-scale experiments were conducted to assess the oxidative stress responses of brackish water clam Corbicula japonica to feeding activity and salinity level. Natural brackish water from Lake Hinuma was used in experiments I and II, while experiment III used artificial brackish water with cultured diatoms as the food source. During experiment I, the oxygen radical absorbance capacity (ORAC) varied greatly when the initial suspended solids (SS) concentration was 50 mg SS·L-1. As a result, no significant difference in ORAC was found between the initial SS concentrations of 5 and 50 mg SS·L-1 (p > 0.05). In contrast, during experiment II, ORAC decreased from 6.4 to 3.5 µmol Trolox Equivalent (TE)·mg protein-1 at the SS concentration of ~5 mg SS·L-1 (p < 0.05). The rate of carbon uptake in experiment I (SS concentration = 5 mg SS·L-1) was ~2.3 times greater than that in experiment II. These results indicate that SS availability has a great effect on ORAC in C. japonica. During experiment III, ORAC increased under initial SS concentrations of 0 and 40 mg SS·L-1 at salinities of 10 (p < 0.01) and 20 psu (p < 0.05), respectively. In contrast, ORAC decreased significantly decreased during the experiment for SS concentration = 80 mg SS·L-1 and salinity = 20 psu (p < 0.01) and for SS concentration = 120 mg SS·L-1 and salinity = 10 or 20 psu (p < 0.01); ATP content also decreased significantly (p < 0.01). A good correlation was found between the change in ATP content and ORAC. Together, the findings suggest that energy availability and salinity level have strong effects on antioxidant capacity in C. japonica.