Seagrass: Comparison
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ٍSeagrass is a type of aquatic plants that grow in a wide variety of marine habitats. Seagrasses are specifically adapted to thrive in shallow and transparent seawater environments to access ample sunlight for their photosynthetic processes. This unique botanical group plays a crucial ecological role in coastal ecosystems and is distinguishable from macroalgae due to its vascular plant characteristics and its capacity to form underwater meadows in coastal waters.

  • biorefinery
  • green energy
  • pretreatment
  • seagrass
  • marine biomass

41. Introduction

Seagrass plays a crucial role in coastal marine ecosystems globally, serving as a foundational element. It provides sustenance for marine creatures such as dugongs and turtles and functions as an essential nursery for various fish and prawn species. It serves as a habitat for a wide range of small marine organisms. Thus, the presence and health of seagrass beds are vital for maintaining the overall ecological balance and biodiversity of coastal marine environments. These meadows are essential to shoreline communities, supporting diverse marine life that sustains local livelihoods. In recent decades, human-induced factors such as diminished water quality, rising temperatures, amplified sedimentation, and intensified grazing pressure have resulted in a worldwide decrease in seagrass populations and the expanse of seagrass beds [65,66][1][2]. On the other hand, the overgrowth of seagrass can have negative impacts on the coastal marine ecosystems and the surrounding environment. For instance, the non-native salt marsh plant Spartina alterniflora expanded its range deeper into the intertidal zone within China’s Yellow River Delta, which posed a threat to the natural habitat of the native seagrass Zostera japonica [67][3]. Thick and overgrown seagrass beds can impede water circulation and flow, leading to stagnant areas with reduced oxygen levels, which can harm aquatic life. In addition, the excessive seagrass growth can alter the physical structure of coastal habitats. It may create overly dense and monotonous underwater landscapes, reducing the availability of open spaces needed by various species for feeding, reproduction, and movement. While seagrass beds are known for their biodiversity, an overgrowth can lead to a decline in species diversity, as some species may thrive in the dense seagrass while others that require more open habitat may suffer. Moreover, the overgrown seagrass can trap sediments and organic matter, leading to increased turbidity (cloudiness) in the water, negatively affecting light penetration and hindering the growth of other marine plants, such as macroalgae or even corals. Furthermore, it may provide excessive shelter for herbivores like sea urchins and parrotfish, making it difficult for them to graze and control algal growth. This can lead to simultaneous algal overgrowth, negatively impacting the coastal area. In order to maintain a healthy balance, it is important to monitor and manage seagrass ecosystems carefully. Proper management practices may involve seagrass restoration, selective thinning, and maintaining water quality to prevent excessive growth while ensuring the continued ecological benefits of these vital coastal habitats. Simultaneous management of seaweeds has the potential to help maintain the equilibrium of seagrass ecosystems, particularly in temperate and tropical regions where seagrass decline has been linked to the proliferation of macroalgal bloom [68][4].
Tropical seagrass meadows have gained recent research focus due to declines in iconic marine species like the dugong. Initiatives like neighborhood watch programs and extensive research efforts are vital for managing the uncertain future of coastal marine life. The seagrass habitats in tropical systems can be categorized into river estuary, coastal, deep water, and reef. These categories vary based on water depth and seabed type, influencing factors such as light availability crucial for photosynthesis. Deepwater coastal ecosystems, found between 10 and 70 m deep, contain sand-covered seafloors, while reef environments are formed by coral-derived calcium carbonate rock. Shallow water habitats, exposed during low tide, pose challenges to seagrass growth due to air exposure and intense light. The ongoing research in these diverse habitats contributes crucial knowledge for the conservation of coastal marine ecosystems [69,70][5][6].

2. Species Diversity

Seagrasses represent a polyphyletic group of marine angiosperms comprising approximately 60 species distributed across five families: Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae [71][7]. These seagrasses are classified within the order Alismatales, following the Angiosperm Phylogeny Group IV proposed system. Important coastal habitats are formed by seagrasses [72][8]. These species have different morphological, physiological, and ecological characteristics that allow them to adapt to various environmental conditions. Seagrasses can be found in a variety of habitats, such as sandy/muddy or rocky shores, kelp forests, coral reefs, estuaries, lagoons, and deep waters. Many factors, such as climate, light, nutrients, salinity, sedimentation, herbivory, and human activities, influence seagrass diversity. Seagrass diversity is important for maintaining the resilience and functioning of seagrass ecosystems and the services they provide. Seagrasses were recorded and found in 159 countries across six continents [73][9]. According to Short et al. [74][10], seagrasses exhibit five primary centers of high global diversity, all of which are situated in the eastern hemisphere. Among these centers, four are in the Tropical Indo-Pacific region, while the fifth is in southwestern Australia, falling within the adjacent Temperate Southern Oceans bioregion. The study reported that the largest and foremost center of seagrass diversity, boasting the highest number of seagrass species of 19, is situated across insular Southeast Asia and extends through north tropical Australia, including the Great Barrier Reef. A second, comparatively smaller center of diversity is identified in southeastern India, encompassing 13 exclusively tropical species. The remaining three high-diversity global centers are located in eastern Africa, southern Japan, and southwestern Australia. In East Africa, there are 12 seagrass species, with only one, Z. capensis, being of temperate origin, resulting in a predominantly tropical species mix. Southern Japan also boasts 12 seagrass species, with Z. japonica being the sole temperate species contributing to the diversity of this otherwise tropical region. Within the temperate Southern Oceans bioregion, southwestern Australia is home to 13 seagrass species, four of which are tropical in origin and contribute to its high diversity. A deeper insight into these diversity patterns and the specific distribution ranges of individual seagrass species has been discussed by Green and Short [75][11].

3. Cell Wall Structure

Genome data has revealed that significant alterations in cell wall composition are essential for organisms to adapt successfully to diverse marine environments [76,77][12][13]. However, the composition and characteristics of seagrass cell walls remain relatively enigmatic. Beyond the ancestral traits inherited from land plants, seagrasses are expected to have undergone a habitat-driven adaptation process to their unique environment. This environment is marked by various abiotic factors, such as high salinity, as well as biotic factors, like diverse seagrass grazers and bacterial colonization, all of which contribute to the stressors faced by these plants. Seagrass cell walls are composed of polysaccharides that are familiar from angiosperm land plants, i.e., cellulose, despite the lack of current information [31][14]. However, sulfated polysaccharides (SP) are characteristic of the macroalgae. They are also present in the cell walls of certain seagrasses [32,33,78][15][16][17]. Another distinctive characteristic is the presence of peculiar pectic polysaccharides known as apiogalacturonans in seagrass cell walls. The peculiar monosaccharide apiose (Apif) is substituted with significant numbers of low-methyl esterified galacturonic acid (GalAp) units [35,36][18][19]. Seagrasses feature highly glycosylated arabinogalactan-proteins (AGPs) that are of particular interest due to their dual role in wall architecture and cellular regulatory processes [79,80][20][21]. These AGPs are characterized by their complex structure, featuring extensive polysaccharide components consisting of arabinogalactans (AGs). AGPs were recently isolated from seagrasses and structurally described for the first time [81][22]. In seagrasses, phenolic polymers (i.e., lignin) responsible for the mechanical strengthening of the wall have also been detected, even though in much lower amounts than angiosperm land plants [30,82,83,84][23][24][25][26]. As a result, the cell walls of seagrasses appear to be intriguing fusions of traits from marine macroalgae and angiosperm land plants with novel structural components. More details about the cell wall structure of seagrasses have been previously discussed [81][22]. However, it is imperative to deepen the comprehension of seagrass cell walls, especially from a technical perspective, given their potential utility in various applications, such as biofuel production, biodegradable materials, and as a source of valuable compounds for pharmaceutical and industrial purposes.

4. Seagrass Cultivation

Seagrass cultivation can be performed in different ways, depending on the species, the environmental conditions, and the project’s objectives. Some common methods of seagrass cultivation include seed collection and sowing [85][27]. This method involves collecting seagrass seeds from natural populations or the lab, then sowing them directly into the seabed or into biodegradable mats or bags that can be transplanted later. This method can produce large numbers of plants with high genetic diversity, but it requires careful timing, handling, and monitoring of the seeds and seedlings. Vegetative propagation is another cultivation method [86][28]. It involves cutting seagrass shoots or rhizomes from donor plants and planting them into the seabed or into pots or trays that can be transplanted later. This method can produce fast-growing plants with high survival rates, but it requires sufficient donor material and may reduce the genetic diversity of the plants. In addition to the aforementioned methods, tissue culture is another promising cultivation technique [87][29]. This method involves growing seagrass cells or tissues in a sterile laboratory environment and then transferring them to a greenhouse or a nursery for further growth and acclimation. This method can produce disease-free plants with high genetic diversity, but it requires advanced equipment/skills and is costly and time-consuming.
Overall, seagrass cultivation is a promising technique to enhance the conservation and restoration of seagrass ecosystems, which are threatened by global and local stressors such as climate change, pollution, coastal development, and overfishing. In addition, seagrass land-based co-cultivation with seaweeds can also provide benefits for human well-being, such as food security, shoreline protection, carbon sequestration, water filtration, and fisheries production [88][30]. The existing literature reveals a scarcity of studies focusing on cultivating seagrass for biomass production. In contrast to macroalgae, most research concerning seagrass biomass relies on natural cultivation systems. However, macroalgae have been relatively more extensively investigated for their potential biomass production through cultivation. The emphasis has been placed on understanding seagrass ecosystems within their natural habitats rather than developing dedicated cultivation strategies for biomass production. In that context, the co-cultivation of seagrass and macroalgae could be of significant ecological, environmental, and economic importance for carbon sequestration, wastewater treatment, enhanced biodiversity, food security, and biofuel production.

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