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Environmental Sciences
Due to climate change and human activities, seagrass is in crisis as the coverage of seagrass declines at an accelerated rate globally. The eutrophication in coastal waters and discharge of pollutants such as sulfide, heavy metals, organic matter and microplastics caused by human activities are important reasons for seagrass loss. In addition, environmental stressors lead to reduced immunity and decreased resistance of seagrass to various pathogens, leading to seagrass wasting diseases.
- seagrass
- coastal water
- plastic
- heavy metal
- seagrass diseases
1. Introduction
Seagrass is the only species of flowering plant that lives entirely in the marine environment, mainly in marine habitats between sub-arctic and tropical latitudes. Sea grass bed is the most widely distributed coastal ecosystem on the earth, providing a variety of important ecological service functions, such as biological conservation, coastline protection, sediment stability, water purification and nutrition cycle. Seagrass meadows are breeding and feeding places for many fish and invertebrates with important economic value, and also important habitats for threatened species [1]. The seagrass meadow has high primary productivity, with aboveground biomass (mainly leaves) accounting for 50% of the total biomass. In addition, the seagrass bed ecosystem possess abundant microbial diversity and diverse microbial communities, which play an important role in the offshore nutrient cycle [2,3]. Besides, seagrass ecosystems filter and remove the bacteria pathogens, thereby reducing the exposure of humans, fish and invertebrate to pathogens [4]. The seagrass debris with highly inert organic carbon, makes an important contribution to the global blue carbon storage and sequestration [5].
Seagrass meadows store a large amount of organic carbon, which is an important component of marine blue carbon. The organic matter reservoirs in shallow marine ecosystems are composed of exogenous organic matter, microalgae and macroplants. and seagrass sources in the waters near estuaries contribute 65% of surface sediment lignin [6]. A large number of macroalgal blooms occur due to organic matter input. The macroalgae can co-metabolize with the microbial community by providing available energy and resources, and further promote the remineralization of refractory components in seagrass debris. And this may contribute to the reduction of blue carbon storage in seagrass meadows [5]. Nutrient loads and reduced light caused by macroalgal blooms are the main reasons for the seagrass decline worldwide. Though the serious degradation trend worldwide was observed because of ocean warming and eutrophication, seagrass blue carbon is still able to effectively eliminate the CO2 emissions and is considered as an effective natural solution to mitigate global climate change [7]. Thus, reducing coastal eutrophication could increase the conservation of seagrass meadows and further mitigate global climate deterioration. In addition, coastal environmental pollutants, such as heavy metals, refractory organic matter and microplastics, can not only pose a serious threat to the life activities of primary marine animals and plants, but also accumulate in marine food products, thus damaging human health.
2. Pollution Status of Seagrass Beds
2.1. Eutrophication
Coastal nutrient input is one of the key factors contributing to the seagrass degradation worldwide. Nutrient enrichment resulted in the excessive reproduction of epiphytic algae and macroalgae in the seagrass bed, along with light occlusion by the high density of algae organisms, which caused decreased photosynthetic rate of seagrass and insufficient photosynthetic oxygen production. Besides, the oxidation process of high-concentration sulfide is further limited by oxygen deficiency, which inhibits the growth and metabolism, or even leads to death of seagrass [5]. Seagrass debris, as well as large amounts of debris produced by the death of saprophytic and macroalgae, would float on the seawater surface or be deposited in surface sediments and transported by wind and water to coastlines. Algal debris is usually composed of unstable organic carbon (LOC) and is more easily ingested and utilized by microorganisms. Eutrophication could lead to the increase of sediment organic carbon from algae in seagrass beds, increase the composition of active organic carbon in sediments, promote the growth and metabolic activity of microorganisms, and accelerate the utilization and transformation of sediment organic carbon [17]. Therefore, coastal eutrophication could alter the uptake, conversion, and storage function of coastal blue carbon in the seagrass bed ecosystem.
2.2. Sulphides
The coastal sediments inhabited by seagrass are characterized by low concentration of oxygen and high concentration of toxic and reducing substances (such as iron, manganese and sulfide) [18]. Sulfide even at low concentrations (1 to 10 μmol/L) is toxic to the cells of eukaryotes such as seagrass, while seagrass could still survive in such high sulfide ranges [19]. Seagrass can avoid root oxygen hypoxia and sulfide invasion through root oxygen leakage (radial oxygen loss). In other words, oxygen produced by photosynthesis spreads through the ventilated tissue to breathe and leaks through the root tip to maintain oxygen in the rhizosphere [20,21,22]. Zostera marina has formed two major sulfide detoxification strategies with the help of seagrass microbes. Sulfide is oxidized and precipitated to elemental sulfur in the aerating tissue, or to thiols and sulfate in the plant. Then elemental sulfur and thiols are stored in the rhizomes and roots, while sulfate is transported from the tissue underground to that overground. Besides, the underground tissue possess the highest detoxification capacity (86%), especially the rhizome (61%), as the main buffer for detoxifying sediment sulfide, could protect the fragile meristem in the leaves [19]. However, due to climate change and human activity, future increases in surface water temperature, hypoxia and sediment sulfide levels would further elevate the sulfide pressure in seagrass bed ecosystem, possibly even exceeding the sulfide tolerance and detoxification [19,23,24]. It is known that microorganisms respond quickly to changes in their living environment. By detecting the abundance of sulfur cycling genes, such as sulfate reduction genes, in the sediment microbial communities of seagrass.
2.3. Heavy Metals
Seagrass meadows have been seriously polluted by a series of pollutants including heavy metals and pesticides, and are becoming an important sink of anthropogenic pollutants in coastal areas [26,27]. Simultaneously, as a significant habitat and food source for a variety of marine animals such as green turtles, seagrass meadows could transfer heave metals accumulated to consumers with higher nutritional levels as well [28]. Trace metal pollution in estuarine seagrass meadows has been observed worldwide, with most studies conducted in the Caribbean [29], Italy [28], India [30], Fiji [31], Australia [32] and Republic of Korea [33]. Marine angiosperms generally have a high bioaccumulation capacity for trace metals because of the interactions between waters and sediments in marine environment directly through leaves and root-rhizomes, where ion uptake occurs [34]. Seagrass is confronted with greater anthropogenic pressure than other marine communities. Changes in seagrass coverage, decreased growth rate and slow development signify environment variation [29,34]. Therefore, trace element level in seagrass beds can be a useful indicator of harmful pollutants in seagrass ecosystems.
2.4. Refractory Organic Compounds
Coastal environmental pollution, especially persistent organic pollution, led to severe decline of seagrass meadows, among which polycyclic aromatic hydrocarbons (PAHs) attract much attention due to their persistence, toxicity, mutagenicity and carcinogenicity. The lipophilic character of organic pollutants makes it easy to penetrate the cytoplasmic membrane to accumulate in marine organisms. Human activities such as combustion of fossil fuels cause terrestrial import of PAH, leading to increased PAH content in coastal sediments and seagrass. PAHs are transmitted along the food chain and ultimately damage human health [36]. The abundant microbial communities distributed in seagrass bed ecosystems play a key role in organic substance degradation, certain consortia of which could degrade hydrocarbon through nitrogen fixation [37]. The effect of PAHs addition on bacterial communities in the sediment of Enhalus acoroides seagrass showed that different strains and bacterial group behaved differently in response to PAH exposure. It is worth noting that microbial community structure of seagrass sediment was sensitive to PAH-induced stress and susceptible to PAHs contamination, which can be used as a potential indicator of PAHs contamination [38]. The fungi in sediments of seagrass Enhalus acoroides, such as the phylum Ascomycota and Basidiomyces, had the potential to degrade PAHs. It was found that low concentration (100 mg/kg) PAHs contamination can increase the fungal diversity in a short time. While the fungal diversity reduced with high concentration (1000 mg/kg) PAHs contamination in a short time, which however could be utilized in a long term (7~28 d) by the fungi [39]. Thus, microbial community in seagrass sediment could be used as an early monitoring indicator to identify PHAs contamination in seagrass bed ecosystems.
2.5. Microplastics
Marine microplastics is an important pollutant in the global coastal and marine environment. Plastics floating on the surface or submerged in water have serious negative effects on marine life and marine ecosystems. Plastic adhered to contaminated organic matter and sediment particles would be sink to the sediment surface colonized by seagrass and macroalgae plants. These macrophyte would accumulate in large amounts and create a new micro-habitat furtherly by changing the nonbiological conditions (such as environmental pH, temperature and oxidation-reduction), microbial community composition and biogeochemical cycle process [42,43,44,45]. This poses a threat to the survival of some marine organisms, and disrupts balance of the seagrass bed ecosystem through the cascade and amplification effect.
Marine organisms living in seagrass bed ecosystems that feed on seagrass leaves can accumulate microplastics in the body through feeding, and transmit through the food chain to higher organisms. This will eventually damage human health seriously by accumulating high content of microplastics in human body. Existing studies have detected the adverse effects of microplastics on marine organisms in the physiological, metabolic and genetic levels [47]. Thus it can be predicted that the biotoxicity and environmental risks would amplify with the increase of microplastics abundance. Furthermore, coastal microplastics could be input mainly by terrestrial and marine routes. The former mainly input microplastics including microplastic debris and chemical fiber products as well as cosmetic wastes rich in plastic particles into the offshore environment through rivers, sewage discharge and other ways. While the marine input mainly exists in fishing activities such as aquaculture and marine transport. Therefore, it is of great significance to figure out and block the input routes of microplastics in the specific coastal areas as well as develop new degradable plastic products for the protection of coastal seagrass ecosystems and human health.
2.6. Pathogenic Bacteria
Due to human impacts and global climate change, emerging infectious marine diseases are becoming more widespread and serious. At present, four known eukaryotic genera that can cause seagrass diseases are Labyrinthula, Phytophthora, Halophytophthora and Physoyxea [50]. Among them, the genus Labyrinthula is a heterotrophic and halophytic protist causing seagrass wasting diseases (SWD). It is also the most studied seagrass pathogen to date. Labyrinthula strains isolated from seagrass leaves showed varying degrees of virulence using laboratory infection tests and related phylogenetic analysis. Isolates with high virulence were able to invade leaf cells of living plants and cause black leaf lesions, a diagnostic feature of SWD [49,50,51]. However, Labyrinthula is also a ubiquitous symbiont that decomposes marine plants and algal wastes, one of which hosts the lawn grass [51]. Since the 1930s, the North American and European Atlantic coastal Zostera has been severely affected by consumptive disease, when it killed 90% of the North Atlantic Zostera population [52]. Muehlstein et al. [53] (1991) first identified L. zosterae (Labyrinthulomycetes) as the pathogen causing wasting diseases of Zostera, and later found that in many countries such as Australia, Mexico, and Republic of Korea. However, the research on seagrass consumptive disease and its pathogenic bacteria in China is still blank.
This entry is adapted from the peer-reviewed paper 10.3390/w15213754
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