It is hypothesized that the success of plants in colonizing terrestrial environments is coupled with the advent of flavonoids, due to their ubiquitous distribution and incredible functional diversity (Kenrick and Crane, 1997; Weng and Chapple, 2010). A few classes of flavonoids and their biosynthesis are summarized in Figure 1 . Flavonoids are involved in biotic and abiotic stress responses (Fini et al., 2011; Dong and Lin, 2021), hormonal regulation (Peer et al., 2004), and signaling for microbial and animal interactions (Rahme et al., 2014; Shimada et al., 2003; Dong and Lin, 2021). There are two classic hypotheses for the initial role performed by flavonoids in early land plants. One hypothesis is that the importance of flavonoids in the colonization of terrestrial environments comes from their UV-B screening properties (Li et al., 1993). UV-B radiation decreases photosynthetic efficiency by causing damage to photosystem II (Szilárd et al., 2007) and leads to the production of reactive oxygen species (ROS), which can damage DNA (Li et al., 2002). This hypothesis was based on flavonoid-deficient mutants, showing an increase in sensitivity to UV-B radiation (Li et al., 1993) and the accumulation of flavonoids in the epidermis in response to UV-B exposure (Tevini et al., 1991).

There are several pieces of evidence that oppose this hypothesized initial function of flavonoids. The efficacy of flavonoids as UV-screening compounds requires their accumulation in high quantities, which would not have been possible without certain enzymes to transport them to the cell wall and vacuole, and these enzymes likely would not have existed when flavonoids first arose (Stafford, 1991). This does not discredit the importance of UV-B defense for survival on land, but non-flavonoid compounds may have fulfilled this role, with the UV-B screening role of flavonoids being an exaptation of these molecules when novel enzymatic functions allowed them to accumulate at high levels. Indeed, hydroxycinnamates, which also belong to the phenylpropanoid class of secondary metabolites, can effectively absorb UV-B radiation and play a role in UV-B screening alongside flavonols (Burchard et al., 2000). Lignins may also have played a role in photoprotection in early land plants: lignin epitopes have been identified in the cell walls of bryophytes and are not associated with vascular or mechanical tissues (Ligrone et al., 2008). Non-flavonoid-producing organisms, including fungi, cyanobacteria, algae, lichens, and dinoflagellates, employ mycosporine-like amino acids to protect photosystem II against UV damage (Taira and Taguchi, 2017) and macromolecules against ROS, thereby playing the same role in this stress response as flavonoids in plants. Their biosynthesis also seems to be derived from the shikimate pathway, further tying them to the phenylpropanoids, which are derived from the chorismate produced by this pathway.

The second hypothesis is that flavonoids first arose to function as signaling molecules, specifically in the regulation of auxins (Stafford, 1991). Stafford argued for this role of flavonoids in light of the arguments outlined above, as flavonoids would have been able to perform signaling functions at low cytoplasmic concentrations, and other compounds existed in the ancestors of land plants that functioned in UV-B screening. Many flavonoid-deficient mutant angiosperms show altered auxin-related development, including dwarfing, altered root development, and loss of pollen viability under temperature stress (Dare and Hellens, 2013; Muhlemann et al., 2018; Maloney et al., 2014), though these traits and their severity vary between flavonoid-deficient mutants (Burbulis et al., 1996). Flavonols, specifically, are important in regulating auxin transport during nodulation to promote organogenesis (Zhang et al., 2009; Ng et al., 2015), and flavonoids are also involved in the microbial signaling required with root nodulation (Abdel-Lateif et al., 2012). The formation of microbial symbiotic interactions and improved nutrient acquisition were important innovations for the success of plants in terrestrial environments, with flavonoids being involved in both, as reviewed by Hassan and Mathesius (2012). The relationship between auxin transport and flavonoids has also been observed in gymnosperms (Ramos et al., 2016), but there is no evidence that flavonoids play similar roles in the developmental processes of bryophytes, with flavonoid-deficient liverwort mutants showing normal development (Clayton et al., 2018). It therefore cannot be assumed that the ancestral role of flavonoids was auxin regulation.

The first hypothesis has since been adopted, placing the emphasis on the involvement of flavonoids in UV-B stress on their antioxidant activity, rather than their screening properties (Fini et al., 2011; Agati et al., 2013). The ROS-scavenging function of flavonoids also explains their production in response to a host of biotic and abiotic stresses, with chalcone synthase, which catalyzes the first committed reaction in the synthesis of all flavonoids, being upregulated in response to heat and cold stress (Shvarts et al., 1997), UV-B radiation and blue light (Christie and Jenkins, 1996), wounding (Richard et al., 2000), and biotic stress (Dao et al., 2011), which all lead to the accumulation of reactive oxygen species. Acting as ROS-scavenging compounds would not require the accumulation of flavonoids in the vacuole or cell wall but would still allow flavonoids to reduce the damage sustained as a result of increased UV-B exposure when the ancestors of land plants moved to a terrestrial environment.

Like many other pathways of secondary metabolism, gene duplication events are important in the expansion of flavonoid metabolism and the origin of new enzymes and branchpoints. Some of the important biosynthetic genes arose from enzymes involved in primary metabolism, such as CHI, which is most likely descended from a non-catalytic fatty acid-binding protein (Ngaki et al., 2012; Kaltenbach et al., 2018). This also allows for neofunctionalization, with stilbene synthases having arisen from CHS multiple times independently, a process that requires only a few missense mutations (Tropf et al., 1994). Legume-specific type II CHI enzymes also originate from the typical type I CHI, which allows for the synthesis of flavonoid compounds required for microbial signaling (Shimada et al., 2003). FNS I likely arose from F3H, allowing for flavone synthesis in the Apiaceae (Martens et al., 2003; Gebhardt et al., 2005).

The enzymes involved in flavonoid biosynthesis, shown in Figure 1 , are arranged in a metabolon on the cytoplasmic face of the endoplasmic reticulum. The arrangement of enzymes within the metabolon differs between species but is also determined by spatial–temporal expression differences, with some proteins not being present in the metabolon and increased expression of certain enzymes in certain tissues allowing them to outcompete certain other enzymes for shared binding sites (Crosby et al., 2011). The patterns of these interactions are important for the branchpoints of flavonoid metabolism: for example, DFR and FLS compete for dihydroflavonol substrates at the branchpoint between anthocyanin and flavonol synthesis and additionally compete to interact with CHS within the metabolon (Crosby et al., 2011). These competitive interactions are important in determining the flux of intermediates to different branches of the pathway (Watkinson et al., 2018), especially considering the catalytic promiscuity exhibited by many of the enzymes involved in flavonoid metabolism. For some of the enzymes, like F3H, the catalytic promiscuity is more apparent in vitro, as protein–protein interactions within the metabolon aid in channeling the appropriate substrates from one enzyme to the next in the pathway. It can therefore be hypothesized that the ability to interact with other proteins of the metabolon is what shapes the pathway, rather than the substrate and product specificity of the individual enzymes. These factors have allowed for the expansion of flavonoid metabolism when plants adapt to new environments and the advent of novel useful flavonoid functions, which is likely why flavonoids are so well conserved in the land plants and could well have benefited the ancestors of seagrasses when returning to a marine environment.

Seagrasses present an interesting case when discussing the various roles performed by flavonoids, as these aquatic higher plants are descended from terrestrial ancestors, with flavonoid production being an ancestral trait and an adaptation toward life on land. Generally, flavonoids perform similar functions in seagrasses compared to land plants, being involved in oxidative stress responses and UV screening —the versatility of these compounds has allowed them to remain relevant outside of the environment in which they originally arose, with the versatility of the biosynthetic pathway allowing for new compounds to evolve.

There have been several studies characterizing the flavonoid contents of various species of seagrasses, with sulfated flavonoids being of particular prevalence in this group (McMillan et al., 1980). Sulfated flavones have been characterized in the genera Zostera, Thalassia, Halophila, Enhalus, and Phyllospadix, with some other genera producing sulfated non-flavonoid phenolic acids (McMillan et al., 1980). Sulfation greatly increases the solubility of these compounds, even more so than the corresponding glycone, which may be related to the function of these compounds (Grignon-Dubois and Rezzonico, 2018). Sulfated flavonoids may be important for the storage of inorganic sulfates in a more soluble form, facilitating transport through the plant cell and allowing for extrusion (Harborne, 1977; Grignon-Dubois and Rezzonico, 2023), as the toxic sulfide ions present in high concentrations in seawater has little effect on the health of certain seagrass species (Hasler-Sheetal and Holmer, 2015). Posidonia oceanica, for example, is much more sensitive to sulfides than Zostera marina or Thalassia testudinum, which both produce sulfated flavonoids (Grignon-Dubois and Rezzonico, 2018).

Sulfated flavonoids are not limited to seagrasses and are strongly associated with plants growing near water bodies rich in mineral salts, indicating that these compounds might act as an adaptation for dealing with high salt exposure (Teles et al., 2018; Harborne, 1975). Most plant families that produce these compounds are not closely related, indicating that this trait arose multiple times independently through the evolution of different sulfotransferases, though flavonols and flavones are generally the targets for sulfation (Teles et al., 2018). The link between flavonols and salt stress has been noted in the past, with increased levels of the flavonols quercetin and kaempferol being noted in Apocynum venetum seedlings following high salt exposure, as well as the upregulation of flavonol biosynthesis genes (F3H, F3′H, and FLS) coupled with the downregulation of general flavonoid biosynthesis genes (CHS and CHI) (Xu et al., 2020). This is consistent with observations by Walia et al. (2005), who noted that a rice genotype susceptible to salt stress had increased expression of CHS, CHI, F3′H, and DFR (the latter of which represents the branchpoint between flavonol and anthocyanin biosynthesis) in response to salinity stress compared to a tolerant line, correlating to the finding that susceptible varieties produced overall less flavonoids and phenolics when exposed to salt stress (Minh et al., 2016). Overexpression of FLS in Arabidopsis greatly increased the tolerance to salt stress, with the transgenic plants suffering far less membrane damage and displaying an overall improved phenotype under salt stress, with improved seed germination, growth, and chlorophyll content compared to the wild type (Guo et al., 2023). These findings seem to indicate that, although some flavonoids can increase tolerance to salinity stress, managing the competing branches of flavonoid biosynthesis is very important in inducing the desired phenotypic response.

Sulfated flavonoids have also been found to play a role in herbivore deterrence by reducing the attractiveness of sugars in Zostera noltei to the sea urchin Paracentrotus lividus (Casal-Porras et al., 2021), which is likely to be a secondary advantage conferred by these compounds in addition to their general function in aquatic higher plants. Sulfation may be important in modulating flavonoid function (Roubalová et al., 2015; Gidda and Varin, 2006) and has also been implicated in the signaling function of flavonoids —quercetin 3-sulfate reverts the auxin efflux inhibition caused by quercetin in Flaveria bidentis, a terrestrial dicot (Ananvoranich et al., 1994), though research regarding this function is limited (Chan et al., 2019), especially in marine angiosperms.

Overall flavonoid content has been observed to change according to the environmental conditions faced by seagrasses, changing with depth, location in the meadow, and season, with higher flavonoid contents generally being associated with increased environmental stress and competition (Casal-Porras et al., 2023; Sævdal Dybsland et al., 2021; Grignon-Dubois and Rezzonico, 2018). The relationship and ratios between flavonoids and non-flavonoid phenolics, especially rosmarinic acid, are typically assessed in these studies, with an increased flavonoid/rosmarinic acid ratio in Z. marina being attributed to more variable and stressful environments, such as the outskirts of a seagrass meadow when compared to the more sheltered interior (Sævdal Dybsland et al., 2021). Although flavonoids fulfill photoprotective functions in land plants (Agati et al., 2013), the importance of flavonoids in photoprotection differs between species of seagrasses, with some species seeming to rely more on rosmarinic acid for screening harmful radiation: in Z. noltei, there is a significant relationship between UV exposure and rosmarinic acid accumulation, but not flavonoid accumulation (Casal-Porras et al., 2023). Similarly, for Halophila johnsonii, flavonoid content was actually found to be higher in shaded plants, even though flavonoids were localized in epidermal cells where they could function as screening compounds (Gavin and Durako, 2012). The decrease in flavonoid content with increasing depth in Z. marina observed by Sævdal Dybsland et al. (2021) was therefore attributed to reduced general stress exposure with the buffering effect of the water, rather than reduced light exposure.

Spatial variation in flavonoid profiles can actually be used to ascribe certain species of seagrasses to distinct chemical phenotypes (Grignon-Dubois and Rezzonico, 2018), reflecting the importance of flavonoids in the ability of seagrasses to become widely distributed and combat different stresses along that distribution. In Z. noltei, chemotypes vary based on the dominant flavonoid, with 71%–83% of the flavonoid content in the Cadiz Bay population consisting of apigenin 7-sulfate, in contrast to the Arcachon Bay population where diosmetin 7-sulfate constituted 85%–93% of the total flavonoid content (Grignon-Dubois and Rezzonico, 2012). Both populations grow in intertidal meadows but are separated by approximately 1,000 km. The authors hypothesize that this dramatic difference is due to low expression of F3′H in the Cadiz population, which reflects the plasticity of flavonoid metabolism and the dramatic impact of a single gene on the metabolic flux of the pathway. The study was extended to include several other populations around the coast of Europe and North Africa (Grignon-Dubois and Rezzonico, 2018), which allowed them to define a third chemotype with high levels of apigenin 7-sulfate, diosmetin 7-sulfate, and luteolin 7-sulfate, though the ecological significance of these three chemotypes is not quite clear.

Changes in flavonoid and non-flavonoid phenolic content can also potentially act as an indicator of the health of seagrass meadows. As long as the phenolic profile is well understood under stable conditions, sudden deviations from the dynamics of certain indicator compounds can indicate shifts in the stress exposure of the meadow (Grignon-Dubois and Rezzonico, 2023; Astudillo-Pascual et al., 2021). As phenolic content can change quickly and specifically in response to certain stress conditions, such as the dynamics in the ratio of flavonoids to rosmarinic acid under different conditions in Z. marina studied by Sævdal Dybsland et al. (2021), the stresses faced by a certain population can be assessed based on these compounds and their relative abundances. Seagrasses require a degree of phenotypic plasticity to combat the multiple stresses they experience as mostly clonal populations (Pazzaglia et al., 2021), which makes polyphenolics and flavonoids very useful, as alternate branches of the pathway can be exploited to respond to differing stresses (Di Ferdinando et al., 2014). These ratios can also be used to easily asses the health of a seagrass meadow (Sævdal Dybsland et al., 2021), which can prove to be a very valuable tool in monitoring the level of stress experienced by a meadow due to anthropogenic climate change.

Anthropogenic climate change will affect marine ecosystems through ocean warming, acidification, marine heatwaves, and changes in storm patterns. Flavonoids, through their diverse stress response actions, can help mitigate the impact of some of these stress factors. Anthropogenic climate change is likely to increase seagrass exposure to toxic sulfides indirectly through enhancing the effects of eutrophication. Phytoplankton blooms not only limit the light availability for seagrass meadows and create hypoxic conditions but also increase organic matter mineralization in the sediment through enhanced sulfate reduction (Holmer and Bondgaard, 2001; Seidel et al., 2021). Ocean warming leads to further reduced oxygen levels (Schmidtko et al., 2017), further favoring anaerobic metabolic pathways and thereby increasing sulfite levels (Broman et al., 2017). Seagrass species that produce sulfated flavonoids may therefore be more resilient than other species in areas afflicted by eutrophication, leading to changes in distribution and meadow composition. Increased levels of sulfate stress resulting from eutrophication might have an impact on flavonoid signaling if this exists in seagrasses (Roubalová et al., 2015). Upregulation of sulfated flavonoid production could divert more flavonoid pathway intermediates which could interfere with other flavonoid functions, which may require more investment in flavonoid metabolism.

An interesting and troubling implication of climate change on seagrass survival is the reduction of phenolic compounds in response to ocean acidification (Arnold et al., 2012). As has been discussed thus far, seagrasses employ flavonoids and other phenolic compounds as versatile stress response compounds and increase flavonoid levels in more stressful, less stable environments. While increased atmospheric CO₂ increases the phenolic content of terrestrial plants, seagrasses growing near volcanic CO₂ vents experiencing low pH coupled with high CO₂ levels display a loss of phenolic compounds, which makes them more vulnerable to herbivory (Arnold et al., 2012). This vulnerability to herbivory due to alterations in secondary metabolism is made more concerning in light of the shifting range of herbivores in response to climate change, with tropical herbivores shifting poleward and establishing in temperate seagrass meadows (Campbell et al., 2024). Herbivores can greatly affect the health of seagrass meadows and alter the food web by feeding directly on seagrasses instead of seagrass detritus (Lal et al., 2010; Longo, 2024; Hyndes et al., 2016; Campbell et al., 2024). The degree of this loss of phenols does seem to vary between populations, however, as found by a study comparing the metabolic response to elevated CO₂ of two eelgrass populations (Zayas-Santiago et al., 2020). This study found that, while the primary metabolism of both populations benefited from increased CO₂ concentrations, the population from a colder climate displayed less reduction in defensive compounds derived from the shikimate pathway, including phenolics. The spatial variation in phenolic content in seagrass species may therefore be an important factor in the perseverance of some seagrass meadows and populations over others in the acidifying and warming oceans.

The impact of the shift in herbivore distributions in response to ocean warming on seagrass species will also be affected by the accumulation of sulfated flavonoids. Sulfated flavonoids act as herbivore deterrents in some seagrass species (Casal-Porras et al., 2021), while other species lack these compounds (Grignon-Dubois and Rezzonico, 2018; Astudillo-Pascual et al., 2021), which could leave them vulnerable to herbivory. Seagrasses that do not produce sulfated flavonoids would therefore be confronted with a combination of increased sulfate toxicity and increased herbivory due to ocean warming, which would place them at a competitive disadvantage against other species. Additional monitoring would therefore be necessary for these species, which may have already been threatened due to anthropogenic activity. It is possible for new flavonoid sulfotransferase enzymes to arise within these species convergently, especially as ocean warming is expected to increase the rate of flowering and sexual reproduction in some seagrass species (Marín-Guirao et al., 2019; García-Escudero et al., 2024), which provides more opportunity for recombination from which improved genetic variants may arise. Alternatively, seagrass species which lack sulfated flavonoids may utilize alternative flavonoids or phenolpropanoids for herbivore deterrence and managing the damage caused by sulfates, which can lead to more distinct chemotypes. Shifts in flavonoid metabolism in species lacking sulfated flavonoids can be used to monitor how well different populations are responding to these stress factors and prioritize conservation efforts accordingly.

Seagrasses need to be able to adjust to fluctuations in salinity, especially those growing in estuarine environments where these fluctuations can be significant and rapid (Touchette, 2007). Climate change exacerbates these salinity changes beyond normal levels, with heavy rainfall and flooding leading to the loss of seagrass meadows due to increases in turbidity and decreases in salinity (Campbell and McKenzie, 2004; Chollett et al., 2007), while accelerated evaporations and influx of saltwater into lagoons from rising sea levels will likely lead to increased salinity (Jakimavičius et al., 2018; Shen et al., 2022). Flavonols may therefore be a flexible tool for seagrasses to tolerate the stress associated with increased salinity fluctuations caused by anthropogenic climate change. Flavonol metabolism is often in competition with anthocyanin metabolism, with the two pathways competing for pathway intermediates, metabolon interactions, and transcription factors (Owens et al., 2008; Luo et al., 2016; Yuan et al., 2016; Crosby et al., 2011). Anthocyanin production is likely to decrease under conditions of ocean warming, as this pathway is upregulated in response to cold stress (Sullivan and Koski, 2021), which would allow for increased levels of flavonols under these conditions. Increased levels of evaporation in estuarine habitats, which would lead to increased salt concentrations, would also increase the level of UV exposure faced by these seagrasses. Both flavonols and anthocyanins are antioxidants involved in combatting UV damage, but under conditions of ocean warming and increased salinity in estuaries, flavonols may become more important than anthocyanins in fulfilling a UV-protective function.

While marine environments can be more stable than terrestrial ones with water screening out UV-B radiation and modulating the temperature, intertidal areas can be particularly variable, with strong fluctuations in light intensity requiring photosynthetic organisms to maximize photosynthetic efficiency under low light conditions and protect photosynthetic machinery under extreme levels of irradiance (Kohlmeier et al., 2017; Léger-Daigle et al., 2022). Anthocyanin biosynthesis is subject to extensive negative regulation, as reviewed by Lafountain and Yuan (2021), often involving various plant hormones, which allows for rapid increase in anthocyanin production upon the reception of a signal. This, combined with their UV-screening ability, makes them very useful for seagrasses which can experience variable levels of UV exposure, on account of differences in depth (Dattolo et al., 2014), meadow density (Schubert et al., 2015), and tides (Kaewsrikhaw and Prathep, 2014). Leaf reddening has been observed in a limited number of seagrass species growing in intertidal regions and shallow waters and is associated with stress exposure (Novak and Short, 2010). Anthocyanin production can be upregulated for photoprotection when exposed to high light conditions (Ma et al., 2021b), while energy is not wasted in conditions where UV-screening mechanisms are unnecessary.

The accumulation of anthocyanins to compensate for the shortcomings of other photoprotective mechanisms has also been observed in terrestrial plants (Zhu et al., 2018). In several species of seagrasses, leaves develop red coloration due to anthocyanin accumulation following high light exposure, with the anthocyanins acting as UV-B- screening compounds or compensating for reduced capacity of photoprotective strategies (Buapet et al., 2017; Zhu et al., 2018). The depth and density of the seagrass meadow also influences the degree of light exposure and thereby the degree of anthocyanin accumulation (Kaewsrikhaw et al., 2016; Dattolo et al., 2014). This variation in pigment content between seagrasses can actually discriminate between species by remote sensing and can therefore be a useful tool in monitoring the health of seagrass meadows (Fyfe, 2003). In T. testudinum, foliar anthocyanin accumulation can be a permanent trait or induced in response to light (Novak and Short, 2012), with red leaves displaying increased levels of UV-absorbing compounds besides anthocyanins and maintaining better photosynthetic efficiency than green leaves at midday (Novak and Short, 2011). Interestingly, a study by Kaewsrikhaw and Prathep (2014) found that anthocyanin content in Halophila ovalis was significantly higher during the rainy season, when light intensity was much lower. This contradicts the hypothesis of the involvement of anthocyanins in UV screening in seagrasses, which seems to hold for some other species, like Zostera capricorni and T. testudinum (Fyfe, 2003; Novak and Short, 2011). While anthocyanins are useful in photoprotection in seagrasses, green leaves do not necessarily suffer greater photoinhibition than red leaves in all seagrass species, as other photoprotective mechanisms can still be effective in high light environments (Buapet et al., 2017; Dattolo et al., 2014). A later study by the same authors (Kaewsrikhaw et al., 2016) found that H. ovalis growing in intertidal areas had higher anthocyanin content compared to individuals growing in subtidal areas. The latter population also showed increased chlorophyll content to mitigate the effects of lower light at greater depths. This observation once again supports the role of anthocyanins in photoprotection and UV screening and somewhat contradicts their earlier thoughts regarding anthocyanin accumulation during the rainy season (Kaewsrikhaw and Prathep, 2014). This might in fact be important for managing cold stress during the rainy season, which is a known function of anthocyanins in some plants (Ahmed et al., 2015; Christie et al., 1994; Schulz et al., 2016).

In the shift to aquatic environments, some seagrasses like Z. marina have lost genes involved in UV sensing and resistance, due to the screening effect of seawater rendering these genes redundant (Olsen et al., 2016). Among these lost genes are photoreceptors, specifically CRY2, which play an important role in upregulating anthocyanin biosynthesis genes and anthocyanin accumulation under high light conditions (Jiang et al., 2016), which results in some species of seagrasses being especially vulnerable to photoinactivation of the oxygen-evolving complex (OEC) under harsh light conditions (Wang et al., 2022; Zhao et al., 2021).

Anthocyanins provide a mechanism of resilience to cold stress and photooxidative damage, but as ocean water provides a buffering effect on both of these stresses, seagrasses may have a smaller set of stress response mechanisms (Olsen et al., 2016; Zhang et al., 2022). In a study by Zhang et al., 2022 it was found that low temperatures combined with high light damaged the photosynthetic machinery of three tropical seagrass species (Enhalus acoroides, Thalassia hemperichii, and Cymodocea rotundata), limiting their distribution to the Indo-Pacific convergence regions. Enhalus acoroides and T. hemperichii do experience leaf reddening (Apichanangkool and Prathep, 2014), and anthocyanins are probably an important mechanism in mitigating cold stress during emersion combined with high light in the absence of other mechanisms. As anthocyanins are cold-induced, they have been found to decrease under temperature increases in terrestrial plants (Sullivan and Koski, 2021), though most research pertaining to the subject is focused on anthocyanin content of grapes for the wine industry. Ocean warming and marine heatwaves may therefore have adverse effects on anthocyanin accumulation in seagrasses, which may be especially detrimental to species that rely heavily on anthocyanins as a photoprotective and cold stress resistance mechanism.

Seagrasses generally reproduce asexually through stolons, though sexual reproduction does occasionally occur, which is important in maintaining standing genetic variation in populations; however, the success rate is poor. Seagrasses generally rely on hydrophilous pollination and engage in various strategies to avoid self-pollination, including dioecy, reduced perianths, and separation of male and floral structures in monoecious species (Van Tussenbroek et al., 2016). Ocean warming and marine heatwaves affect the flowering behavior of different seagrass species differently, with the flowering of cold-adapted P. oceanica seemingly being triggered by marine heat waves (Marín-Guirao et al., 2019; García-Escudero et al., 2024), while Z. marina has displayed a decrease in flowering frequency and intensity as oceans warmed (Qin et al., 2020). Although the flowers of seagrasses are generally green or pale yellow, in accordance with the abiotic pollination strategy, a transcriptomics study in P. oceanica actually found anthocyanin biosynthesis genes to be upregulated in floral tissues, which results in a slight red pigmentation of the male reproductive structures appearing prior to pollen release (Entrambasaguas et al., 2017). The purpose of this anthocyanin accumulation is currently unclear, but anthocyanin production is generally downregulated in response to increased temperatures (Sullivan and Koski, 2021), which will likely lead to a loss of red pigmentation in these flowers as oceans continue to warm.

Anthocyanins are generally produced in the flowers of angiosperms to act as an attractive cue for pollinators, allowing them to be distinguished from the surrounding foliage visually and thermally (Harrap et al., 2017; Takács et al., 2009; Seymour and Matthews, 2006). The discovery of invertebrate-mediated pollination in T. testudinum (Van Tussenbroek et al., 2012, Van Tussenbroek et al., 2016), in combination with the discovery of floral anthocyanin accumulation in P. oceanica, indicates that the possibility of anthocyanins functioning in pollinator attraction in seagrasses cannot be ruled out. Alternatively, the floral anthocyanin accumulation may act as a strategy to deter herbivores, as anthocyanin accumulation can signal metabolic investment in a tissue or decrease its palatability (Gould, 2004). Though biotic pollination was until recently believed to not occur in aquatic ecosystems at all, this discovery in this one species of seagrass spurred additional research regarding pollination benefits arising from other observed biotic interactions involving red algae (Lavaut et al., 2022), which indicates that invertebrate pollination in aquatic ecosystems may be very ancient. Ocean warming is predicted to shift the ranges of herbivores poleward, which may negatively impact seagrass meadows losing invertebrate pollinators and positively affect the sexual reproduction of seagrass meadows that are introduced to this new layer of interaction. The accompanying loss of anthocyanins in the reproductive tissues of some seagrass species may make them more difficult to seek out by potential pollinators or make them more palatable to herbivores, with both scenarios negatively impacting sexual reproduction.