Sunlight's hidden influence: how light alters the chemical makeup of Tokyo Bay's coastal waters

Coastal seawaters, especially those near large urban centers like Tokyo, are a complex mixture of natural and human-derived substances. This dissolved organic matter, or DOM, is chemically diverse and strongly influenced by municipal effluents. The waters of Tokyo Bay are known to be particularly abundant in sulfur-containing compounds from these sources. A recent study examined how sunlight affects the molecular composition of this unique coastal water, providing new information on the environmental fate of these compounds.

A Closer Look at Coastal Water Chemistry

Tokyo Bay receives a substantial amount of treated wastewater, enriching its DOM with a high proportion of sulfur-containing molecules. These compounds can affect water quality, and their behavior in the marine environment has been a subject of scientific inquiry. A research team led by corresponding author Qinglong Fu of the China University of Geosciences and Tokyo Institute of Technology set out to investigate the photochemical transformations of this complex organic matter when exposed to light.

Simulating Sunlight's Effects

To understand these processes, the scientists, including Shixi Wu of the China University of Geosciences and Manabu Fujii of the Tokyo Institute of Technology, conducted a series of light irradiation experiments. They collected seawater samples directly from Tokyo Bay and exposed them to natural sunlight as well as different wavelengths of ultraviolet light. Using an exceptionally sensitive technique known as Fourier transform ion cyclotron resonance mass spectrometry, they analyzed the precise molecular changes that occurred in the DOM after irradiation.

The Fate of Sulfur Compounds

The analysis confirmed that a significant portion of the organic matter in Tokyo Bay contains sulfur, making up 33 percent of the total molecular formulas identified. The experiments showed that UV light had a considerable effect on these molecules. Between 44 and 67 percent of the complex, high-molecular-weight sulfur-containing compounds were preferentially broken down by UV exposure. This degradation occurred mainly through processes called decarboxylation, where carbon is lost, and desulfonation, where sulfur is removed from the molecule.

The Formation of Halogenated Compounds

The study also examined how light drives the formation of new compounds containing halogens like bromine and iodine. Researchers observed that while UV light exposure increased the formation of organic brominated compounds, the UVA portion of natural sunlight was more suited to forming organic iodinated compounds. An interesting observation was that although the original seawater was rich in sulfur, very few of the newly formed halogenated molecules contained sulfur.

A New Tool for DOM Analysis

In their work, the researchers proposed a new molecular index, which they named DBE-0.5O. This new parameter is designed to offer a more accurate measurement of the degree of unsaturation in the carbon skeleton of DOM molecules. The authors contend that this index provides a more sensitive and chemically reasonable way to characterize complex organic matter compared to previously used indicators.

Environmental Significance

These findings provide valuable information on the natural processing of organic matter in coastal ecosystems. The photochemical breakdown of sulfur-rich DOM affects both the carbon and sulfur cycles in these environments. The results suggest that in the surface waters of Japan, photobromination is likely a more common process than photoiodination. This detailed molecular investigation improves the scientific community's understanding of how sunlight modifies the chemical composition of coastal seawater influenced by urban activity.

Corresponding Author:

Qinglong Fu

Original Source:

https://doi.org/10.1007/s44246-023-00083-z

Contributions:

Qinglong Fu performed the study's investigation, experimental design, methodology, data analysis, correspondence, funding acquisition, review and editing. Shixi Wu conducted the data analysis and drafted the manuscript. Manabu Fujii contributed to the funding acquisition, review and editing. All authors read and approved the final manuscript.