Because more than 80% of species of gamete-spawning corals, including most Acroporidae species, do not inherit Symbiodiniaceae from their parents, they must acquire symbiont cells from sources in their environment. To determine whether photosynthetically competent Symbiodiniaceae expelled as fecal pellets from giant clams are capable of colonizing corals, we conducted laboratory experiments in which planula larvae of Acropora tenuis were inoculated with the cells in fecal pellets obtained from Tridacna crocea. T. crocea fecal pellets were administered once a day, and three days later, cells of Symbiodiniaceae from the fecal pellets had been taken up by the coral larvae. T. crocea fecal pellets were not supplied from the 4^th day until the 8^th day, and the cell densities in the larvae increased until the 8^th day, which indicated the successful colonization by Symbiodiniaceae. The control group exhibited the highest mean percentage of larvae (100%) that were successfully colonized by culture strains of Symbiodiniaceae, and larvae inoculated with fecal pellets reached a colonization percentage of 66.7 ~ 96.7% on the 8^th day. The highest colonization rate was achieved with the fecal pellets containing cells with high photosynthetic competency (Fv/Fm). Interestingly, the genetic composition of Symbiodiniaceae in the larvae retrieved on the 8^th day differed from that in the fecal pellets and showed exclusive domination of the genus Symbiodinium. A minor but significant population of the genus Cladocopium in the fecal pellets was not inherited by the larvae. These experiments provided the first demonstration that the Symbiodiniaceae from tridacnine clams provided via fecal pellets can colonize and even proliferate in coral larvae.

Harmful algal blooms (HABs) occur frequently in the Seto Inland Sea, bringing significant economic and environmental losses for the area, which is well known as one of the world’s most productive fisheries. Our objective was to develop a quantitative model using in situ hyperspectral measurements in the Seto Inland Sea to estimate chlorophyll a (Chl-a) concentration, which is a significant parameter for detecting HABs. We obtained spectra and Chl-a data at six stations from 12 ship-based surveys between December 2015 and September 2017. In this study, we used an iterative stepwise elimination partial least squares (ISE-PLS) regression method along with several empirical and semi-analytical methods such as ocean chlorophyll, three-band model, and two-band model algorithms to retrieve Chl-a. Our results showed that ISE-PLS using both the water-leaving reflectance (R_L) and the first derivative reflectance (FDR) had a better predictive ability with higher coefficient of determination (R²), lower root mean squared error (RMSE), and higher residual predictive deviation (RPD) values (R² = 0.77, RMSE = 1.47 and RPD = 2.1 for R_L; R² = 0.78, RMSE = 1.45 and RPD = 2.13 for FDR). However, in this study the ocean chlorophyll (OC) algorithms had poor predictive ability and the three-band and two-band model algorithms did not perform well in areas with lower Chl-a concentrations. These results support ISE-PLS as a potential coastal water quality assessment method using hyperspectral measurements.

Sea-bottom plowing is originally a method used to oxidize sediment by stirring the bottom with a trawl fishing tool, and its effect in increasing primary productivity of the water column was investigated in the western part of the Seto Inland Sea of Japan. Preliminary field sampling showed that diatom resting stage cells were abundant in the sediment of the tested area at 1.5–2.6 × 10⁵ MPN g^-1 wet sediment. When the sediment was added to filtered seawater, diatom cells emerged from the sediment after one day and increased more drastically under a light level corresponding to sunny weather than cloudy weather. In the actual trials of sea-bottom plowing on the field for a continuous period of 4 years, dissolved inorganic nutrients increased at the bottom layer after the plowing and promoted photosynthetic activities of the phytoplankton communities in 2018 and 2021. Chlorophyll a concentration at the middle layers increased 1.06–2.15 times after plowing throughout the trials for 4 years. Diatoms contributed to 67–99% of this chlorophyll a concentrations and included the genera Skeletonema and Chaetoceros, which formed resting-stage cells in the sediment. Pseudo-nitzschia spp. often increased after the plowing, which was assumed to be of seawater origin. Estimated primary productivities of the middle layers dropped once the following day due to turbid water caused by the plowing but increased 2.03–4.41 times after two or five days in 2018, 2019, and 2021. These results suggest that sea-bottom plowing has an enrichment effect on diatoms and could be a possible measure to fertilize the sea.

Automobile tires are a major source of pollution, releasing complex mixtures of chemicals into aquatic environments. Although the toxicity of tire leachate has been frequently documented, the specific chemical drivers affecting marine microalgae remain largely unidentified. This study investigated the effects of water-soluble leachates from two passenger car tires (Tire A and Tire B) on four marine phytoplankton species: the diatoms Skeletonema costatum and Chaetoceros lorenzianus , and the flagellates Chattonella antiqua and Karenia mikimotoi . Leachates from both tires inhibited growth and photosynthesis in all species, with flagellates showing greater sensitivity than diatoms. Tire B leachate exhibited significantly higher toxicity. Chemical analysis using liquid chromatography-mass spectrometry (LC-MS/MS) identified the vulcanization accelerator 1,3-diphenylguanidine (DPG) at 11.37 mg L ^− 1 in the more toxic Tire B leachate, which was not detected in the Tire A leachate. Subsequent exposure experiments with pure DPG confirmed its high toxicity, with 96-hour EC ₅₀ values for growth inhibition ranging from 0.93 to 2.59 mg L ^− 1 . The flagellate Chat. antiqua was the most sensitive species. DPG was estimated to contribute 49.4–78.5% of the overall toxicity of Tire B leachate, confirming it as a primary toxicant for the tested species. A potential toxic mechanism is proposed in which the cationic form of DPG electrostatically interacts with negatively charged microalgae cell surfaces, disrupting membrane integrity. These findings identify DPG as a key toxicant from tire wear particles for foundational marine producers, warranting further investigation into its ecological risk.