Microplastic Pollution in Marine Environment: Occurrence, Fate …
119
4.2 Effect of Microplastics on Nitrogen Cycles in Sediments
The ocean nitrogen cycle is a very important link in the earth’s elemental cycle.
The marine nitrogen cycle is a biochemical process consisting of a series of redox
reactions. Nitrogen fixation and nitrogen assimilation provide biological nitrogen
(ammonium salt) for the ecosystem. Ammonium salts form nitrates by nitrification
and can be converted into nitrogen by denitrification. The whole nitrogen cycle realizes the conversion between different nitrogen-bearing inorganic salts in the ocean.
Extensive studies had shown that microplastics in the ocean can either directly or
indirectly affect the nitrogen cycle. Microplastics indirectly affect the nitrogen cycle
by changing the abundance of biomes and microbiomes in sediments. Marine sediments at the bottom of the microorganisms are of great significance to the marine
nitrogen cycle.
In the process of microbial mediated catabolism, nitrification and denitrification
have an indispensable link in the nitrogen cycle, nitrification provides the material
basis for denitrification, while denitrification can reduce the NO 3
− of the ocean which
is helpful for removing the reactive nitrogen in the marine environment to decline
the toxicity for nitrate accumulation of marine life.
Since the coastal salt marsh environment is located at the demarcation line between
coastal and human-residential areas, it can flow directly into the ocean through land
runoff, rainwater discharge, sewage treatment plant discharge, and export. And its
vegetation is conducive to the deposition of suspended solids, organic matter, and
microplastic particles, so coastal salt marshes are the important area of geochemical
cycling [96]. Studies have manifested that the presence of microplastics changes
the composition of the microbial community and the nitrogen cycle process in the
sediment, and different microplastics have a significant impact on the nitrogen cycle
[96]. For instance, polyurethane foam and polylactic acid plastics can promote the
nitrification and denitrification processes, while polyvinyl chloride suppresses the
process.
Sulfides have been reported to suppress nitrification in marine sediments [99].
Studies have shown that the abundance of sulfate-reducing bacteria in the PVC
treatment process is high, and the sulfides produced by the sulfate-reducing bacteria
may inhibit the nitrification process [96], thus destroying the nitrogen cycle in the
marine environment.
Most of the existing researches only consider the influence of different kinds of
microplastics, but seldom include the influence of additives added in microplastics
on the nitrogen cycle. Polyvinyl chloride products containing plasticizers are often
used in the medical field [100] and have antibacterial properties that affect microbial
communities in sediments. The study of Cluzard et al. [101] shows that when PE
particles are added to the sediment, the content of NH 4
+ in the overlying water will
increase, thus affecting denitrification [101].
Microplastics can affect the nitrogen cycle not only by changing the microbial
community richness in the sediment, but also by influencing the regulation of nutrient
flux and nitrogen fixation through micro-benthic organisms living at the sedimentwater interface [96, 101].
119
4.2 Effect of Microplastics on Nitrogen Cycles in Sediments
The ocean nitrogen cycle is a very important link in the earth’s elemental cycle.
The marine nitrogen cycle is a biochemical process consisting of a series of redox
reactions. Nitrogen fixation and nitrogen assimilation provide biological nitrogen
(ammonium salt) for the ecosystem. Ammonium salts form nitrates by nitrification
and can be converted into nitrogen by denitrification. The whole nitrogen cycle realizes the conversion between different nitrogen-bearing inorganic salts in the ocean.
Extensive studies had shown that microplastics in the ocean can either directly or
indirectly affect the nitrogen cycle. Microplastics indirectly affect the nitrogen cycle
by changing the abundance of biomes and microbiomes in sediments. Marine sediments at the bottom of the microorganisms are of great significance to the marine
nitrogen cycle.
In the process of microbial mediated catabolism, nitrification and denitrification
have an indispensable link in the nitrogen cycle, nitrification provides the material
basis for denitrification, while denitrification can reduce the NO 3
− of the ocean which
is helpful for removing the reactive nitrogen in the marine environment to decline
the toxicity for nitrate accumulation of marine life.
Since the coastal salt marsh environment is located at the demarcation line between
coastal and human-residential areas, it can flow directly into the ocean through land
runoff, rainwater discharge, sewage treatment plant discharge, and export. And its
vegetation is conducive to the deposition of suspended solids, organic matter, and
microplastic particles, so coastal salt marshes are the important area of geochemical
cycling [96]. Studies have manifested that the presence of microplastics changes
the composition of the microbial community and the nitrogen cycle process in the
sediment, and different microplastics have a significant impact on the nitrogen cycle
[96]. For instance, polyurethane foam and polylactic acid plastics can promote the
nitrification and denitrification processes, while polyvinyl chloride suppresses the
process.
Sulfides have been reported to suppress nitrification in marine sediments [99].
Studies have shown that the abundance of sulfate-reducing bacteria in the PVC
treatment process is high, and the sulfides produced by the sulfate-reducing bacteria
may inhibit the nitrification process [96], thus destroying the nitrogen cycle in the
marine environment.
Most of the existing researches only consider the influence of different kinds of
microplastics, but seldom include the influence of additives added in microplastics
on the nitrogen cycle. Polyvinyl chloride products containing plasticizers are often
used in the medical field [100] and have antibacterial properties that affect microbial
communities in sediments. The study of Cluzard et al. [101] shows that when PE
particles are added to the sediment, the content of NH 4
+ in the overlying water will
increase, thus affecting denitrification [101].
Microplastics can affect the nitrogen cycle not only by changing the microbial
community richness in the sediment, but also by influencing the regulation of nutrient
flux and nitrogen fixation through micro-benthic organisms living at the sedimentwater interface [96, 101].