Beneath the Bark: The Hidden Chemistry Sustaining the Red Mangrove

Keywords: reactive oxygen species, phenolic antioxidants, heavy metal sequestration, iron plaque formation, cellular homeostasis, osmolytes, salt-stress gene regulation

Abstract

Beneath the beautiful red bark of the red mangrove (Rhizophora mangle) is a network of chemical systems that sustain life in environments where few plants can survive. Constantly exposed to saltwater, sunlight, and pollution, Rhizophora mangle presents a coordinated chemical defense system to endure these conditions. Its phenolic compounds, flavonoids, and tannins neutralize reactive oxygen species before they can damage the tree, while its roots immobilize toxic metals through sequestration and iron plaque formation. Within each cell, the Rhizophora mangle can precisely regulate redox, ionic, and osmotic balance to ensure that water and ion content remain stable despite fluctuations from the external environment. It also regulates genes encoding essential components of the salt-stress-induced pathways to maintain photosynthetic efficiency and fitness. These interconnected processes transform the red mangrove into a chemical system that can self-regulate. Rhizophora mangle tunes environmental stress into stability for itself.

Introduction

Rhizophora mangle grows in intertropical regions in North America, the Caribbean, South America, and West Africa (Takvorian, 2022). Their common name comes from the bright red color of the wood underneath the gray bark of the tree, as seen in Figure 1. Historically, Rhizophora mangle was first described in 1753 by the Swedish biologist Carl von Linnaeus in his published work Species Plantarum (Rhizophora mangle, n.d.). The genus Rhizophora comes from the Greek, rhizo meaning “root” and phora meaning “to carry”. It is a reference to its prop roots. Mangle comes from the Spanish word for mangrove (Rhizophora mangle, n.d.).

Fig. 1. The red color of Rhizophora mangle stem is from tannin (Heemsoth, 2020).

Red mangroves’ physical and chemical structure allows them to tolerate both fresh- and saltwater, although they are mostly found in saltier water (Takvorian, 2022). They inhabit coastal environments characterized by a tropical climate with direct sunlight and high humidity, shallow waters, and tidal currents. Rhizophora mangle can anchor itself below the water in the muddy soil with thick prop roots, while its stem and branches are above water (Takvorian, 2022).

Rhizophora mangle is a very persistent species of trees. It can thrive when submerged in water, which allows it to survive even severe flooding (Takvorian, 2022). Rhizophora mangle is mostly threatened by human developments and pollution on coastlines, which reduces their habitats significantly, thereby threatening the animals that rely on Rhizophora mangle for survival. Fish, birds, and invertebrates which rely on the tree for protection during breeding, mating, and feeding, for example. Red mangroves also stimulate primary production in their local ecosystem as their broken branches and decomposing leaves offer habitat and nutrient-rich food for microorganisms and fish, thus supporting the entire food chain. Therefore, to conserve the Rhizophora mangle population, governments started initiatives to preserve these essential trees. For example, the government of Bermuda added Rhizophora mangle to their Protected Species Amendment Act of 2011 to protect Bermuda’s coastal habitats (Takvorian, 2022).

Rhizophora mangle has been used for multiple purposes in the past thanks to the tannin in its bark, leaves, and roots, which gives mangroves their color and health benefits (Heemsoth, 2020). Medicinally, Rhizophora mangle have antifungal, antibacterial, and antiseptic properties thanks to their bark and leaves (Rhizophora mangle, n.d.). Rhizophora mangle extracts can also be used as insecticides. The wood and timber are also used for canoes, fences, firewood, and fishing spears. Tannin can be extracted from the bark to create a reddish-brown dye, while bark fibers create ropes (Rhizophora mangle, n.d.).

The Use of Antioxidants

All plant life is dependent on photosynthesis where light energy from the sun is converted into chemical energy stored in glucose. This process is summarized in Equation 1 as follows:

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ (1)

The oxygen released through this process not only supports surrounding animal and human life but also serves the plant itself during respiration, a complementary process that converts glucose back into usable cellular energy (ATP). The process of respiration can be summarized in Equation 2:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (2)

During these processes, some of the molecular oxygen undergoes partial reduction, forming reactive oxygen species (ROS) such as superoxide (O₂×^-), hydrogen peroxide (H₂O₂), and hydroxyl radicals (×OH) (Thatoi et al., 2013). Each of these intermediates is highly unstable and reacts rapidly with nearby biomolecules. They can oxidize lipids, proteins, and nucleic acids, thereby damaging the cell’s essential structures and functions. This is a biochemical imbalance known as oxidative stress (Thatoi et al., 2013).

Most terrestrial plants can manage moderate ROS production using enzymatic antioxidants such as dismutase (SOD) and catalase (CAT). However, for Rhizophora mangle, the production of ROS is much more intense. This is attributable to its habitat, under bright sunlight, where it partakes in photosynthetic activity for longer periods of time, and high salinity, which disrupts the cellular ion imbalance and accelerates ROS production (Thatoi et al., 2013). Therefore, a natural balance is more difficult to maintain. Ironically, the same processes that sustain the tree’s life can also threaten it.

This creates a dilemma: how can a tree depend on oxygen and light for energy, even though they can harm it? The Rhizophora mangle’s solution is a sophisticated non-enzymatic antioxidant system composed of phenolic compounds, flavonoids, and tannins. These molecules form a chemical defense that helps Rhizophora mangle maintain itself even in harsh coastal conditions (Thatoi et al., 2013).

Phenols

Phenols are organic compounds that contain at least one ×OH group directly attached to a benzene ring. This arrangement makes the O-H bond weaker and more polarizable than in regular alcohols, because the ring’s pi-electron system stabilizes the resulting phenoxyl radical through resonance (Platzer et al., 2022). This structure is visualized in Figure 2.

Fig. 2. The conjugated pi-system of the phenoxyl radical (Own work by Raghad Gharbi).

Phenolic acids such as gallic acid (Fig. 3A) and ferulic acid (Fig. 3B) are both abundant in Rhizophora mangle bark.

Fig. 3. Skeletal structures of phenolic acids present in Rhizophora mangle bark. (A) Gallic acid. (B) Ferulic acid (Own work by Raghad Gharbi).

They follow a typical hydrogen-atom transfer reaction as shown in Equation 3 (Platzer et al., 2022):

ArOH + R^. → ArO^. + RH (3)

ArOH denotes a phenolic compound and R× a free radical. The newly formed phenoxyl radical (ArO×) is stabilized through resonance delocalization of the unpaired electron over the aromatic pi-system, dramatically lowering its reactivity (Platzer et al., 2022). This mechanism interrupts radical-chain propagation reactions such as lipid peroxidation, preserving the Rhizophora mangle’s cellular membranes (Thatoi et al., 2013).

These phenolic compounds form the first non-enzymatic, chemical line of defense in Rhizophora mangle, acting as small but highly reactive radical scavengers. To quantify the number of protective compounds the mangrove produces, researchers have performed spectrophotometric assays. The standard reference used in this method is gallic acid equivalents (GAE), a simple and well-characterized phenolic compound which allows the results to be expressed in comparable units (González-Ocampo et al., 2022). Leaf extracts of Rhizophora mangle typically yield 2.95 mg GAE per gram of dry tissue, while bark extracts contain about 6-7 mg GAE per gram of dry tissue (González-Ocampo et al., 2022). For reference, most non-holophytic (non-salt-tolerant) terrestrial plants, typically have an average phenolic content of 1-2 mg GAE per gram of dry tissue (González-Ocampo et al., 2022). Evidently, Rhizophora mangle possesses higher phenolic concentrations than freshwater species, highlighting its biochemical adaptation to oxidative stress caused by high salinity and sunlight exposure.

The antioxidant potency of these extracts is further confirmed through the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging assays. DPPH is a stable nitrogen-centered free radical that appears deep violet due to its strong absorbance at 517 nm (Rehrl et al., 2025). Its structure is visualized in Figure 4.

Fig. 4. DPPH skeletal structure (Own work by Raghad Gharbi).

When an antioxidant donates an electron or hydrogen atom to DPPH, the radical is reduced, and the solution becomes pale yellow. The magnitude of this color is quantified spectrophotometrically and expressed as the half-maximal inhibitory concentration (IC₅₀), the amount of extract needed to reduce 50% of the DPPH radicals present in the solution. Rhizophora mangle bark exhibits a low IC₅₀ value of 89.83 μg/mL, meaning that just 89.83 μg of extract in one millilitre of solution can scavenge half of the free radicals (Mata-Torres et al., 2026). By comparison, bark extracts from the non-halophytic beech tree exhibit a much higher DPPH IC₅₀ value of 831 μg/mL, indicating substantially weaker radical-scavenging efficiency (Rehrl et al., 2025). As such, the phenolic metabolism of Rhizophora mangle is highly efficient.

Flavonoids

Building upon these simple phenolics, Rhizophora mangle also synthesizes larger and more complex antioxidant molecules known as flavonoids (González-Ocampo et al., 2022). These compounds share the same aromatic backbone as phenolics but contain additional rings that allow electrons to move freely across the molecule through conjugation (Fig. 5) (González-Ocampo et al., 2022).

Fig. 5. The general structure of flavonoids (De Luna et al., 2020).

This structure makes flavonoids exceptional at stabilizing free radicals and absorbing excess light energy. Therefore, they not only neutralize oxidative molecules inside cells but also protect tissues from the damaging effects of UV radiation, acting as the plant’s photoprotective ‘chemical sunscreen’ (González-Ocampo et al., 2022). By absorbing high-energy light before it can reach sensitive chloroplasts, flavonoids reduce the formation of ROS during photosynthesis in the first place (Agati et al., 2013). The same compounds can then act again as antioxidants within the cell, providing a two-layer defense against oxidative stress.

A key advantage of these molecules is their ability to interact with metal ions (Kejík et al., 2021). In saline or waterlogged soils, iron often cycles between Fe²⁺ and Fe³⁺, catalyzing the Fenton reaction, in which ferrous iron (Fe²⁺) reacts with hydrogen peroxide (H₂O₂) to produce hydroxyl radicals (×OH), one of the most damaging ROS. Flavonoids bind to these metal ions, forming stable complexes that prevent this reaction from occurring (Fig. 6).

Fig. 6. Iron chelation by flavonoid phenolic structures. (A) Fe²⁺ coordination by phenolics stabilizes the reduced iron state and prevents its participation in the Fenton reaction. (B) Fe³⁺ coordination forms a more stable iron complex (Sabiha, 2013).

This is known as metal chelation, and it is particularly useful for Rhizophora mangle that grows in iron-rich mud. By chemically trapping these metals, Rhizophora mangle slows down ROS production before it even begins (Kejík et al., 2021).

Tannins

While flavonoids provide protection for the softer tissues that are exposed to light, Rhizophora mangle also reinforces its wood with another class of antioxidants, tannins (Ozogul et al., 2025). In Rhizophora mangle, condensed tannins are found in high quantities in the bark and roots, where oxidative and microbial stressors are the most persistent (Ozogul et al., 2025). These condensed tannins are large polyphenolic polymers composed of flavan-3-ol units linked by C-C bonds (Ozogul et al., 2025). They can also neutralize multiple radicals at once because they have many hydroxyl groups in their structure (Fig. 7).

Fig. 7. Generalized structure of condensed tannins as polymers of flavan-3-ol units (Own work by Raghad Gharbi).

More importantly, the tannins are also able to bind strongly to proteins and cell wall components, forming a tough barrier that resists microbial decay and minimizes oxygen diffusion into deeper tissues (Huang et al., 2024). This explains why wood of Rhizophora mangle is very resistant to rot and decomposition even when submerged in seawater.

Since tannins have a dual role, being antioxidant and antimicrobial, they are one of the most ecologically valuable compounds produced by Rhizophora mangle. In coastal environments, where oxygen levels fluctuate with tides, the accumulation of tannins ensures that root and bark tissues remain chemically stable despite constant oxidative and biological stress (Gourlay et al., 2022).

Tannins are also responsible for the distinctive reddish coloration of the inner bark. Condensed tannins (proanthocyanidins) possess large, highly conjugated structures that absorb a wide range of visible wavelengths, giving them an inherently brownish hue (Mata-Torres et al., 2026). As they scavenge radicals and become exposed to oxygen, these molecules become oxidized further and form dark red products that accumulate in the phloem tissue (Mata-Torres et al., 2026). Because tannins are present majorly in the inner bark, tannins give the inner bark of the Rhizophora mangle its red color that distinguishes it as the ‘red mangrove’. Its color can be observed in Figure 8.

Fig. 8. The distinctive reddish color of Rhizophora mangle bark (Heemsoth, 2020).

Mitigation of Heavy Metals Toxicities

Apart from oxidative stress, Rhizophora mangle must also adapt to environmental stresses as it lives between land and sea. Toxic inorganic pollutants and heavy metals accumulating in coastal areas from oil pollution, industrial effluents, and other sources result in significant health consequences for aquatic ecosystems (Rahman et al., 2024). Heavy metals transported by water and wind flow accumulate in sediments. Rhizophora mangle, present in tropical and subtropical regions, adapted multiple mechanisms to mitigate heavy metal pollution and survive despite the increasing toxic environment caused by anthropogenic activities. An excessive number of heavy metals hinders photosynthesis and cellular respiration, which restricts growth, reproduction, germination and the genetic makeup of the mangrove tree, as illustrated in Figure 9 (Rahman et al., 2024).

Fig. 9. Heavy metal toxicity effects on Rhizophora Mangle (Rahman et al., 2024).

Avoid, Tolerate, and Adapt

Passive diffusion across the plasma membrane and active transport through metal ion transporters, according to specific concentration gradients, cause the uptake of heavy metals by mangrove roots (Rahman et al., 2024). Most trace elements, like Cu, Al, Co, Fe, and Ti, are non-toxic and even necessary for growth of plants and animals within a certain limit. However, some are non-essential and cause toxic effects, such as Zn, Hg, Pb, and Cr. Metalloids like Se and As are also toxic. To break down and immobilize the absorbed contaminants or even completely avoid the non-essential toxic heavy metal uptake, Rhizophora mangle uses mechanisms such as sequestration in sediments, iron plaque formation, and compartmentalization. Mangroves are thus able to retain the heavy metals inside their root cells before they’re transferred to the aerial plant parts. The trees’ high tolerance capacity towards heavy metals was proven by the higher bioconcentration of metals in the mangrove roots than in their leaves, as illustrated in Figure 10. Heavy metals like Cd, Cu, Cr, Pb, and Zn accumulate mainly in the root epidermis and cortex rather than in the leaves (Rahman et al., 2024).

Fig. 10. Heavy metals (red dots) transported from the roots of the red mangrove through its stem and to its leaves (Rahman et al., 2024).

As Rhizophora mangle plays an essential role in the local ecosystem’s nutritional distribution and water quality, its ability to adapt its biological and biochemical cycles is important for the tree to flourish among other species of plants and animals (Rahman et al., 2024). Indeed, red mangroves are one of the most productive ecosystems in the world as about 50% of their primary productivity is exported to the oceans as organic matter, influencing food chains near coastal waters (Bernini et al., 2010). They therefore have a role as ecosystem engineers, since red mangroves shape their environment and influence other species (Mangrove Forest). They trap sediments to create new land, reduce erosion from storms, create habitats for marine life, and alter nutrient flows, which feed coastal food webs (Mangrove Forest). They also improve water quality by filtering pollutants (Mangrove, 2025). Furthermore, red mangroves are also considered a keystone species because removing them can cause the collapse of entire coastal ecosystems due to the loss of food sources and protection. Red mangroves thus have a disproportionately large impact on the biodiversity of coastal ecosystems and play an important role in many species’ life cycles (Mangrove, 2025).

Multiple Mechanisms

Rhizophora mangle effectively acts as a local sink for heavy metals (Rahman et al., 2024). For example, they sequester heavy metals such as Cu, Zn, and Ni in sediments by releasing oxygen via their roots, which leads to the precipitation of iron oxyhydroxides (Fe(OH)₃) (Castro et al., 2022). These bind with heavy metals to immobilize them, and that leads to the production of bulk biomass (Rahman et al., 2024). Additionally, Rhizophora mangle decomposes organic matter into coastal sediments, which influences heavy metals’ mobility and deposition. Indeed, sediments that are rich in organic matter and have smaller particle sizes absorb higher concentration of heavy metals. This means that mangroves favor heavy metals’ deposition in sediments and thus restrict the further redistribution to the local ecosystem. Furthermore, mangrove sediments are acidic, which allows mangrove soils to retain heavy metals more effectively from wastewater. That is because an acidic soil can convert metals into more mobile and bioavailable ionic forms or use cation exchange mechanisms. Oxygen-deficient soils also accelerate the sulfur-reducing bacteria that convert sulfate into hydrogen sulfide, which binds heavy metal ions (Rahman et al., 2024). Thus, heavy metal sequestration is an effective strategy used by Rhizophora mangle to trap heavy metals before they damage the tree’s cells.

Rhizophora mangle develops iron plaque on the surface of their roots, as illustrated in Figure 11 (Rahman et al., 2024). First, oxidation-reduction reactions by microbes and root exudates (secretions into the soil that allow plants to communicate with their environment) transform iron elements in the rhizosphere into soluble Fe²⁺ (He et al., 2024). Then, O₂ is released by radial oxygen loss (ROL) through wide aerenchyma into the surrounding soil. Next, toxic Fe²⁺ from the surrounding anoxic environment is oxidized in water to Fe³⁺ (He et al., 2024). Finally, Fe³⁺ precipitates as iron oxides on the root surface (Rahman et al., 2024). This process forms the iron plaque and can be expressed by Equation 4 (He et al., 2024).

4Fe²⁺ + 10H₂O + O₂ → 4Fe(OH)₃ + 8H⁺ (4)

The iron plaque acts as a protective layer and immobilizes heavy metals and pollutants (Rahman et al., 2024). It restricts the heavy metal uptake in root tissue, which eventually increases the Rhizophora mangle’s tolerance ability. For example, Fe-plaque allows the removal of Zn, Mn, and Fe from seedlings. It can also act as a reservoir for trapped heavy metal pollutants (Rahman et al., 2024).

Fig. 11. Iron plaque (IP) formation from the redox cycle of Fe occurring outside the root, which is shown as a cross-section. It protects plant roots from heavy metals (HMs) (He et al., 2024).

Another mechanism Rhizophora mangle uses to adapt to heavy metals is compartmentalization (Madhavan et al., 2025). Metal compartmentalization allows these trees to keep the metal levels below the toxicity threshold. For example, specific heavy metals are deposited in tissues, like the roots, that are more resistant to their effects than others. Within the root, different tissues also accumulate different metals. In the case of Rhizophora mangle, Al, Pb, As, Fe, and Cr concentrations were higher in root tissues, whereas Mn was higher in leaf tissues. Another adaptation used by Rhizophora mangle is to deposit heavy metals into senescent leaves, which are in the last stage of their life cycle before detaching from the plant. For example, metals such as Cd, Cr, Mn, Pb, and Hg accumulated in pre-abscission senescent leaves, while Cu was compartmentalized in mature leaves and buds (Madhavan et al., 2025). Thus, Rhizophora mangle can mitigate heavy metal toxicities through various mechanisms that it adapted in response to the environmental conditions of its ecosystem.

Cellular Homeostasis

Rhizophora mangle not only needs to adapt to heavy metals polluting its ecosystem, but also to the high salinity of the water. Rhizophora mangle thrives in intertidal zones where conditions such as salinity and temperature constantly fluctuate. In its habitat, water salinity may vary from close to 0 ppt to over 70 ppt, although the tree thrives around 35 ppt (Biber, 2006). Such extreme conditions would be detrimental to most plants, causing hyper-ionic and hyper-osmotic stress which harms the cells. This is why Rhizophora mangle has developed systems to maintain redox, ionic, and osmotic homeostasis to counter its ever-changing environment (Nizam et al., 2021).

Redox Homeostasis

Redox homeostasis is essential for survival of Rhizophora mangle. In normal conditions, there is a low amount of reactive oxygen species (ROS) which include superoxide anions (O₂⁻), hydrogen peroxide (H₂O₂), hydroxyl radicals (OH·), and singlet oxygen (¹O₂) (Nizam et al., 2021). In these conditions, the ROS act as chemical messengers. H₂O₂, for example, travels through aquaporins in cell membranes to regulate processes like stomatal closure, helping conserve water in the tree. High heat and salinity, however, cause the overproduction of ROS. The buildup of these species can be harmful to the cell, causing a loss of membrane integrity and metabolic failure because of the effects the species have on cellular components. For example, they can oxidize membrane lipids, denature proteins and fragment nucleic acids (Nizam et al., 2021). Thus, maintaining redox homeostasis is essential for the tree to survive.

The first line of defense involves superoxide dismutase (SOD). These SOD enzymes convert the harmful superoxide radicals into something beneficial. For example, SOD buildup can help improve structural resistance by lignifying the roots. Another example is how O₂^- can be converted into hydrogen peroxide (Equation 5), which can then be used to help strengthen the cell wall (Nizam et al., 2021).

2O₂ + 2H⁺ → H₂O₂ + O₂ (5)

When superoxide dismutase works with catalase, another enzymatic antioxidant, they create an enzymatic detox system that converts harmful ROS into water and oxygen. Furthermore, ferritin within the cell can store excess Fe, which prevents destructive metal-catalyzed ROS from forming. Finally, glutathione S-transferase is an antioxidant enzyme that uses glutathione to neutralize peroxides (Nizam et al., 2021). Figure 12 illustrates this process, showing the effects of salinity stress on redox reactions within mangrove cells.

Fig. 12. Cellular homeostasis pathways in connection to ROS induced anatomical variations (Nizam et al., 2021).

Ionic Homeostasis

Salt stress entails excess sodium (Na⁺) and chloride (Cl^-) ions in the surrounding environment, producing an ionic imbalance. Studies show, however, that mangrove cells living in 150 mM NaCl can restore Na⁺ concentration level to normal through a series of ion-tolerant mechanisms.

The roots of Rhizophora mangle have developed in such a way that they can filter out these ions while still allowing the uptake of water (Sudhir et al., 2022). This mechanism allows the tree to maintain ionic homeostasis by stabilizing intracellular ion concentrations. When Na⁺ and Cl^-enter the cell, they increase osmotic pressure, which is relieved by secreting the ions through vacuoles or actively exporting them from the cell. To make this possible, Na⁺/H⁺ antiporters (membrane proteins that allow Na⁺ and H⁺ to enter and exit the cell) remove excess Na⁺by an electrochemical gradient that was produced by H⁺-ATPases that hydrolyze ATP. Furthermore, (calcium) Ca²⁺ signaling plays a key role in maintaining ionic homeostasis. In cases of high salinity, signaling is amplified. This means regulating in the cell membrane and vacuolar ion transport is increased, which helps rectify the ionic imbalance. Additionally, the ferritin produced by the red mangrove contributes to ion homeostasis by storing extra Fe ions, like its role in redox homeostasis (Sudhir et al., 2022).

Finally, Rhizophora mangle prevents toxic salt buildup by storing excess ions in vacuoles or by shedding ion-rich leaves, essentially removing accumulated ions from the tree (Sudhir et al., 2022). Therefore, through the use of filtration, ion transport regulation, and ion removal, Rhizophora mangle is able to efficiently maintain ionic homeostasis even in harsh environments.

Osmotic Homeostasis

Unfortunately, ion homeostasis alone cannot maintain a balance and prevent dehydration across the plasma membrane. The cell must also preserve osmotic equilibrium. To do this, mangroves make compatible solutes called osmolytes. These are small, uncharged molecules that lower the water potential of the cytoplasm. Some examples within Rhizophora mangle include pinitol, mannitol, betaine, proline, starch, polysaccharides, aspartic acid, and sterols (Nizam et al., 2021). Even when there is a lack of water, enzymatic activity must be maintained. Thus, the tree can build up these osmolytes to help stabilize membranes, which helps achieve osmotic homeostasis.

Secondary metabolite pathways also contribute to osmotic control. The phenylpropanoid biosynthetic pathway produces secondary metabolites such as lignin, flavonoids, anthocyanins, and phenols under salinity stress to keep osmotic balance inside plant cells. Moreover, the suppression of some aquaporins, specifically tonoplast intrinsic proteins, which are located in the membrane surrounding plant vacuoles, significantly reduces water loss through membranes in high salinity (Nizam et al., 2021).

The tree has developed several leaf-level adaptations to minimize water loss and maintain osmotic homeostasis. Firstly, trees growing in high-level saline areas reduce stomatal openings on their leaves to conserve water (Barr et al., 2009). Secondly, mangroves can increase leaf thickness to induce succulence during periods of high salinity. To increase succulence, the leaves developed an epidermal coating of wax (Fig. 13) to maintain water loss by lowering transpiration rates through the leaves and ultimately diluting absorbed salt. Finally, to minimize water loss, Rhizophora mangle changes the angle of its leaves’ faces to control the amount of sun and heat absorbed. The leaves experience over 1000 W/m² solar irradiance so the tree must adjust the amount of solar energy absorbed. When transpiration cooling at the leaf's surface is not effective enough, the tree adapts by physically changing the leaf angles, controlling the surface area in contact with the sun and therefore the amount of energy absorbed (Barr et al., 2009).

Fig. 13. Minerals on leaf epicuticular wax of Rhizophora mangle. (A) Salts on the abaxial leaf surface. (B) Control (SEM), square – leaf wax, lenticel located in elevation (circle) and stomata (arrows) on abaxial leaf surface. Adapted from Victório et al. (2025) (Victório et al., 2025).

Genetic and Molecular Adaptations to Salinity Stress

To survive in the high salinity of coastal waters, Rhizophora mangle not only adapted ways to maintain homeostasis, but also to improve its photosynthetic machinery. Soil and water salinity is one of the major factors affecting seed germination, growth, and productivity in plants. It is known that photosynthesis and CO₂ assimilation rates are severely affected under salinity stress in plants without effective countermeasures. Salt stress is caused by high concentrations of Na⁺ and Cl^- in water or soil, and it can induce three types of stress pathways: osmotic stress, ionic stress, and secondary stress. Osmotic stress results from the reduced water potential at the root surface, leading to significantly decreased water uptake in plants, and therefore, severe dehydration. Ionic stress consists of the reduced ability to take up essential ions into the cell because of the increased presence of Na⁺ (Yang & Guo, 2018). Na⁺ competes with essential ion uptake at the root plasma membrane, such as K⁺ and Ca⁺, since these ions all use the same transport channels or binding sites on the membrane. The high concentration of Na⁺ decreases the uptake of K⁺ and Ca⁺ and impairs enzyme and protein activity (Munns & Tester, 2008). Osmotic and ionic stress induce secondary stress in plants, including the accumulation of toxic compounds such as ROS. ROS species can oxidize pigments, proteins, and lipids if they are not detoxified or scavenged, thereby inducing oxidative stress (Yang & Guo, 2018).

Rhizophora mangle is a highly salt-tolerant species. Experiments have been carried out in constant (Naidoo, 1985) and fluctuating (Lin & da S. L. Sternberg, 1993) salinity conditions and they show that in both conditions, Rhizophora mangle maintains a consistent CO₂ assimilation rate and stomatal conductance (Lopes et al., 2023). The results indicate a robust countermeasure to a wide range of salinity in Rhizophora mangle. On the molecular level, these countermeasures can be studied by observing gene expression of salt-regulated genes acting on photosynthesis, antioxidant defense, and sodium transport.

Stomatal conductance is a measure of how easily gases, mainly CO₂ and water vapour, pass through stomata, and it can be used to measure the efficiency of photosynthesis in plants (Faralli et al., 2019).

Improvements in Photosystem II

The maintaining of proper metabolism and fitness of Rhizophora mangle under salinity stress suggests the presence of adaptive measures to sustain a healthy photosynthetic rate. Photosynthetic CO₂ assimilation is mainly influenced by the concentrations of NADPH and ATP, both of which are products of the light-dependent reactions through the electron transport chain (ETC) and ATP synthase. The ETC includes membrane protein complexes such as I PSII, which are susceptible to oxidative damage by ROS. In Rhizophora mangle, PSII is adaptively protected, stabilized, and improved in high salinity conditions.

PSII is a large protein complex composed of over 20 subunits that plays a central role in the light-dependent reactions of photosynthesis (Fig. 14). It consists of a reaction center (RC) that is the main target of oxidative damage. The RC has a subunit named protein D1, which is encoded by the psbA gene. The efficient turnover of damaged D1 protein is crucial to PSII’s stability and hence increases oxidative stress tolerance. As an adaptive measure, the psbA gene has a twofold increase in transcription in Rhizophora mangle under high salinity conditions, since the transcriptional upregulation of the psbA gene increases the pool of D1 proteins available for damaged protein turnover. This response is triggered and regulated by the redox state of the plastoquinone (PQ) pool (Lopes et al., 2023), a key electron carrier in the electron transport chain (Borisova-Mubarakshina et al., 2018). The psbA gene is most expressed when the PQ pool is most oxidized, indicating a state of oxidative stress (Lopes et al., 2023). As a result, Rhizophora mangle upregulates psbA gene transcription to sustain photosynthetic CO₂ assimilation led by PSII stabilization and repair.

Fig. 14. (A) The PS II dimer with individually colored protein subunits in the right monomer. Cofactors are colored in left monomer. Orange beads represent water molecules (Umena et al., 2011). D1 protein is shown in cyan blue. (B) The structure of the reaction center is colored in grey. The catalytic cycle of OEC and the cofactors involved in the electron transfer in PS II are shown (Chen et al., 2022).

PSII includes an oxygen-evolving complex (OEC) that oxidizes water (Fig. 14). The proper function of the OEC is crucial to avoid the formation of oxygen radicals, a ROS that can induce oxidative damage. OEC prevents the formation and release of the intermediate oxygen radicals by using a mechanism called the Kok cycle. The Kok cycle is a four-step catalytic cycle in which the Mn₄CaO₅ cluster (Fig. 15) accumulates a single oxidizing equivalent in each step and ensures that partially oxidized intermediates are not released freely.

Fig. 15. The structure of the Mn₄CaO₅ cluster. Manganese, oxygen, and calcium atoms are marked on the structure as Mn1-4, O1-4, and Ca, respectively. Dotted lines represent ligations with the ligand water molecules (W1-4. The seven ligand residues are shown by D1-Asp170, D1-Glu189, D1-His332, D1-Glu333, D1-Asp342, D1-Ala344, and CP43-Glu354 (Saito et al., 2020).

Thus, like the reaction center subunit upregulation, Rhizophora mangle also exhibits an upregulation of the oxygen-evolving complex’s subunits under salt stress. The OEC is stabilized by efficient repair mechanisms that synthesize new subunits de novo to replace damaged ones. The PSBO2 gene encodes a subunit of the OEC, and the expression of this gene is upregulated significantly in Rhizophora mangle under high salinity stress (Lopes et al., 2023). This upregulation of the OEC subunit reveals yet another countermeasure of the Rhizophora mangle to cope with salinity. As a result, the maintenance and improvement of PSII in Rhizophora mangle by transcriptional regulation supports healthy production of NADPH and ATP, which are necessary to maintain fitness and essential energy demands under oxidative stress induced by osmotic, ionic, and secondary stress in saline conditions.

Salt-Stress Pathways Regulation

Plants exposed to high salinity conditions use three signaling pathways (Fig. 16) to reduce and control oxidative damage and maintain ion homeostasis. The salt overly sensitive (SOS) pathway is activated by high concentrations of Na⁺ in the cytosol and, in turn, activates the SOS1 Na⁺/H⁺ antiporter to allow excess Na⁺ to be expelled from the cell (Ji et al., 2013). The ROS pathway is activated by salt-stress-induced production of oxygen radicals or ROS that act as secondary messengers to trigger antioxidant enzymatic defenses. Though the intermediate oxygen radicals can be harmful to the cell if uncontrolled, the ROS signaling pathway protects the cell from oxidative damage (Rauf et al., 2024). The abscisic acid (ABA) pathway primarily responds to osmotic stress by increasing ABA protein synthesis to promote stomatal closure and to reduce water loss (Ng et al., 2014). All these pathways are present in Rhizophora mangle and are specifically regulated and adapted to the saline habitat condition of the tree (Fang et al., 2025; Feng et al., 2020; Lopes et al., 2023).

Fig. 16. The three major salt-stress induced pathways in plants and their components, ROS, SOS and ABA pathway (Henna Parveen et al., 2024).

In Rhizophora mangle, the ROS pathway is regulated through special vacuolar Na⁺/H⁺ antiporters. These antiporters are a type of secondary active transport driven by a proton gradient. The Na⁺/H⁺ antiporter moves Na⁺ inside the vacuole while moving H⁺ out, effectively compartmentalizing the Na⁺ and reducing the risk of accumulation of ROS. The vacuolar Na⁺/H⁺ antiporters are encoded by the NHX1 gene, whose transcription is related to increased salt tolerance. Rhizophora mangle exhibits a significant upregulation of the NHX1 gene in the site of high salinity, indicating a major role of vacuolar antiporters in the tree’s salt tolerance (Lopes et al., 2023). Building on the chemical defense provided by phenolic acids, Rhizophora mangle also has an effective enzymatic antioxidative defense system in response to the production of ROS. As previously mentioned, the enzyme SOD is the first enzymatic line of defense, and it catalyzes the disproportionation of superoxide radicals, a ROS, into H₂O₂ and water (Stephenie et al., 2020). The FSD3 gene encodes Fe-SOD, SOD with Fe as its cofactor, and it is also upregulated by Rhizophora mangle as a coping mechanism against salt stress, enabling ROS scavenging. Accordingly, an enhanced expression of the GAD gene was detected, which codes for the glutamate decarboxylase (GAD) enzyme that catalyzes the decarboxylation of L-glutamate to form γ-aminobutyric acid (GABA). GABA acts as an antioxidant or signaling molecule under stress and enables ROS scavenging by activating antioxidant enzymes (Wu et al., 2020). These mechanisms, along with other transcriptional regulations (Fig. 17), allow Rhizophora mangle to thrive in saline conditions.

Fig. 17. An overview of transcriptional adaptations of the salt-regulated pathways in Rhizophora mangle. a) The photosynthetic CO₂ assimilation rate is maintained by b) improving and stabilizing PSII and c) increasing the synthesis of ATP to accommodate the energy needs of the tree. d) RuBisCO activation is essential to maintain the Calvin cycle under stress. e) The gene encoding vacuolar Na⁺/H⁺ antiporters are upregulated to avoid ion toxicity. f) The antioxidative response system is induced which includes ROS scavenging and g) synthesis of GABA (Lopes et al., 2023).

Conclusions

Rhizophora mangle exists in an environment filled with conflict - salt-laden waters, intense sunlight, and metal-rich sediments – yet it thrives by chemically defending itself against each challenge.

Much like most trees, Rhizophora mangle depends on light as its source of energy. In its coastal habitat, light is both its greatest gift and greatest threat. Intense sunlight and high salinity generate ROS that could cripple photosynthesis. The mangrove solves this challenge using its elegant chemical shield composed of phenolic acids, flavonoids, and tannins that neutralize radicals before they can injure its cells. Furthermore, the tree must also use mud as its source of nutrients. However, this mud is laced with toxic metals. To gather what it needs without poisoning itself, Rhizophora mangle releases oxygen through its roots, forming an iron plaque that traps heavy metals on contact. In addition, the mangrove must absorb water, like all plants, yet the salty water contains enough ions to critically dehydrate its cells. Rhizophora mangle roots selectively filter ions before they enter the xylem, while Na⁺/H⁺ antiporters and vacuoles store or expel the rest, maintaining the tree’s delicate ionic balance. Even the photosystem, that all plants use, must adapt to the salt’s corrosive effects. Rhizophora mangle reinforces its PSII by upregulating psbA and PSBO2, ensuring that the oxygen-evolving complex is continually repaired.

Each of these adaptations is a direct response of Rhizophora mangle to a challenge posed by its harsh environment. Through these elegant chemical designs, Rhizophora mangle does not merely endure the intertidal world, it thrives within it and turns every challenge into proof of its remarkable design.