| 其他摘要 | Due to the massive production and use of plastic products, coupled with the current
lack of efficient treatment and recycling methods, plastic waste has been detected in
environments across the globe. A portion of this waste is in the form of microplastics
(particle sizes ranging from 1 μm to 5 mm), which are widely distributed throughout
global ecosystems. It is estimated that approximately 80% of plastic fragments originate
from land and enter the oceans, where microplastics are subjected to various physical,
chemical, and biological influences. These factors cause microplastics to undergo
diverse transformations, which in turn affect their transport, fate, and ultimate destiny
in coastal areas. Floating microplastics often develop a biofilm on their surface, altering
their surface properties and physicochemical state. The formation of biofilms is
influenced by various environmental factors such as temperature, nutrients, and
dissolved oxygen, which predominantly determine the biological composition of the
biofilm on microplastics. In addition, oysters in coastal raft aquaculture systems, known
for their effective water filtration capabilities, inevitably come into contact with
microplastics in the water during filtration. Oysters can incorporate microplastics into
their feces or pseudo-feces, thereby influencing the vertical distribution of microplastics
in nearshore environments. Therefore, this study selected typical floating polyethylene
(PE) microplastics as the research object and conducted incubation experiments in three
different types of study areas: a seawater bathing beach, a seawater pond, and a raft
aquaculture area. The study aimed to elucidate the formation process of biofilms on the
surface of microplastics, analyze changes in density and sedimentation induced by
biofilm formation on microplastics, and further investigate the environmental factors
such as temperature, nutrients, dissolved oxygen, and pH that influence biofilm
formation on microplastics in the coastal waters of the Yellow Sea and Bohai Sea.
Moreover, bio-deposition traps were designed and deployed to collect biodeposits
produced by cultured oysters, and a Laser Direct Infrared Imaging (LDIR) system was
employed to analyze microplastics. This allowed for the examination of the vertical
transport capacity of microplastics by oysters. The study also utilized a Generalized
Linear Mixed Model (GLMM) to explore the main factors influencing microplastic
abundance in coastal sediments, such as water depth, distance from the shore, and
proximity to oyster farming areas. Additionally, indoor simulation experiments were
conducted to investigate the biodeposition effects of oysters on microplastics of
different concentrations, particle sizes, and polymer types. The research findings reveal
the significant role of marine mollusks in the transport of microplastics in coastal areas,
providing essential data to support the management and control of microplastic
pollution in coastal environments. The main conclusions of this study are as follows: (1) This study investigated the impact of biofilm formation on the sedimentation behavior of aged and non-aged microplastics. After subjecting microplastic samples to
UV aging for 15 days, scanning electron microscopy revealed the presence of fine
cracks and particles on the microplastic surfaces, which increased their roughness.
Compared to the original microplastics, the aged samples showed a significant increase
in surface oxygen content, along with the appearance of additional carbonyl stretching
peaks, indicating that the aging process induces the formation of oxygen-containing
functional groups and causes oxidation effects on the microplastics. Moreover,
naturally aged microplastic fibers in field conditions exhibited additional C-O bond
peaks, suggesting that different aging methods may lead to varying aging characteristics
in microplastics. The aging state of microplastics has a certain influence on microbial
attachment. Optical microscopy images showed that the surface of non-aged
microplastics had less biofouling attachment and a more dispersed distribution. In
contrast, aged microplastics displayed more cracks and scratches on their surfaces, with
biofouling more concentrated and tending to form clusters. The amount of biofilm on
the plastic surfaces increased with exposure time. In the later stages of microplastic
incubation, the biofilm content followed the trend of seawater ponds > aquaculture
areas > bathing beaches, with significant differences observed among different
environments. Aged microplastics exhibited greater biofilm content than non-aged
microplastics. An analysis combining biofilm biomass on microplastics with
environmental factors revealed that silicate and dissolved oxygen were the main
environmental factors influencing biofilm content. Silicate concentration distinguished
the different stages of biofilm development between mid and late stages at different
sites, while dissolved oxygen primarily differentiated biofilm content among different
habitats. Even at the end of incubation (12 weeks), the density of the aged microplastics
with the highest biofilm content only reached 0.97 g cm⁻³, which was insufficient to
cause sedimentation. This indicates that biofilm formation on microplastics of this
particle size has a relatively minor effect on changing their density, and a longer
incubation period is required for sedimentation to occur.
(2) This study elucidated the significant role of cultured Pacific oysters
(Crassostrea gigas) in the vertical transport of microplastics in nearshore
environments. Using custom-designed sediment traps, we collected biodeposits
produced by the oysters in situ. The results demonstrated that oysters possess a strong
capacity for microplastic deposition, with individual oysters capable of depositing up
to 15.88 items per individual per day (items ind⁻¹ d⁻¹). Previous studies have
underestimated the oysters' ability to vertically transport microplastics. The
abundance of microplastics in the biodeposit group was significantly higher than in
the control group (which used empty oyster shells as a filler), by a factor of 3.54.
Compared to natural sedimentation, oysters significantly increased the deposition of
positively buoyant microplastics and smaller particles (<50 µm) but showed no
preference or selection for different types of microplastics. The variety of plastic polymers in the biodeposit group was more diverse, with fewer polymer types found
in the seawater and control groups than in the biodeposit group. Microplastics smaller
than 50 μm dominated all identified microplastic samples, accounting for 85.17%,
81.95%, and 79.12% in the seawater, control, and biodeposit groups, respectively.
The biodeposition activity of aquaculture organisms like oysters may further alter the
vertical distribution pattern of microplastics in nearshore environments. In Yantai
coastal sediments, the average microplastic abundance was 100.04 ± 56.78 items/g
dry weight (d.w.), with the lowest recorded abundance being 36.25 items/g d.w. and
the highest 271.25 items/g d.w. The average microplastic abundance in aquaculture
areas (113.59 items/g d.w.) was higher than in non-aquaculture areas (78.35 items/g
d.w.). The oysters' deposition of microplastics created a sedimentation hotspot, and
the best-fit generalized linear mixed model indicated that shellfish aquaculture
activities have a significant impact on the spatial and temporal distribution
characteristics of microplastics in sediments. The oysters' efficient deposition of
microplastics may substantially influence the spatiotemporal distribution patterns of
microplastics in nearshore environments. Additionally, we measured the abundance
of microplastics in the water column and found that the abundance was significantly
underestimated. Traditional microplastic identification techniques, such as Fourier
transform infrared spectroscopy (FT-IR) and Raman spectroscopy, overlooked a
substantial amount of microplastics that are difficult to observe with the naked eye,
while the laser direct infrared imaging system (LDIR) provided a more accurate
estimate of microplastic abundance in the water.
(3) In laboratory experiments, we investigated the biodeposition of
microplastics by Pacific oysters under varying concentrations, particle sizes, and
polymer types. The results showed that the oysters' microplastic deposition at high
concentrations (1000 items/L) was significantly greater than at the other two
concentrations (200 items/L and 500 items/L). The amount of microplastic deposition
by oysters increased with the exposure concentration, indicating that oysters do not
reject microplastic ingestion even at higher concentrations. When exposed to
different particle sizes, the oysters exhibited the highest daily deposition rate for
smaller microplastics (50 μm), reaching up to 39.58 ± 3.61 items per individual per
day (items ind⁻¹ d⁻¹). The deposition of smaller and medium-sized microplastics (100
μm) was significantly higher than that of larger microplastics (180 μm). As the
particle size increased, the amount of microplastic deposition decreased
correspondingly. The oysters' daily deposition of different polymer types ranged from
14.58 to 21.88 items, with no significant differences in deposition rates across various
polymer types, suggesting that the oysters did not show selective deposition based
on the polymer type of the microplastics. |
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