Introduction

Microalgae, in the strictest definition, are eukaryotic, unicellular microorganisms that are photosynthetic and typically have an aquatic lifestyle1. Although cyanobacteria are prokaryotic and therefore not true algae, they are usually included because they share a similar physiology and ecology with eukaryotic microalgae. Moreover, photosynthesis originated around 3.5 billion years ago in cyanobacteria, with eukaryotes emerging much later, following 1–1.5 billion years of prokaryotic evolution2. This photoautotrophic ability enables microalgae to be widely distributed in aquatic environments with low organic nutrient levels, such as epilithic and endolithic niches3,4. For example, microalgae have been widely identified as pioneer colonizers on outdoor stone monuments and historical buildings5,6,7, particularly when water is available8,9 (Fig. 1a–e). Subsequently, their growth provides sufficient organic nutrients for heterotrophic colonizers, which are usually harmful microorganisms (e.g., archaea, bacteria, and fungi) to stone heritage10,11,12. However, case studies on microalgal colonization of heritage sites have been reported frequently, especially in stone heritage (Fig. 1f). Therefore, developing an efficient mitigation strategy to control microalgal growth is essential for the conservation of stone heritage.

Fig. 1: Examples of microalgal colonization at the archeological sites.
Fig. 1: Examples of microalgal colonization at the archeological sites.
Full size image

a Microscopy of microalgal colonization of stone materials. b Microalgal blooming at the Jinsha earthen site, Chengdu, China. c Microalgal colonization at the gate tower of Ming Xiaoling Mausoleum, Nanjing, China. d Microalgal growth on the brick wall of ancient tombs, Luoyang, China. e Cyanobacterial colonization of the Longmen Grottoes, Luoyang, China. f Statistics of microalgae-related cases at the archeological sites. Based on Scopus, bibliographic databases were curated using the following search algorithms: TITLE-ABS-KEY (cyanobacteria) OR TITLE-ABS-KEY (alga) AND TITLE-ABS-KEY (heritage) OR TITLE-ABS-KEY (monument) OR TITLE-ABS-KEY (Archaeological). Images courtesy of X.L., J.Q., F.W., and R.Y.

Over the past two decades, numerous bacterial species capable of lysing algal cells or suppressing algal growth—particularly those associated with harmful algal blooms—have been isolated and identified13,14. These bacteria are collectively referred to as algicidal bacteria and can inhibit or kill algae directly or indirectly15,16. Algicidal bacteria are highly diverse in their ecological roles, including shaping phytoplankton species composition in pelagic environments17. Their algicidal modes have been primarily categorized into two types17,18: direct attack, which requires bacteria to actively seek out and attach to microalgal cells19; and indirect contact, which depends on the secretion of algicidal substances20. The variety of algicidal mechanisms and the diversity of algicidal metabolites open up the possibility of developing eco-friendly mitigations for the biocontrol of harmful microalgae that colonize stone heritage.

Despite the achievements of applying algicidal bacteria to control harmful algal blooms, several issues and challenges remain, including the development of algicidal-specific predation strategies and molecular mechanisms, the mechanisms of action of algicidal substances, and the development of effective algicides17,18. Moreover, there are few studies on the application of algicidal bacteria or their metabolites in the conservation of stone heritage. These issues have largely impeded the application of algicidal bacteria in controlling harmful microalgae on stone heritage.

Here, we provide a comprehensive overview of the diversity and distribution of algicidal bacteria, their mechanisms of action, and their potential applications as eco-friendly biocontrol agents, aiming to offer a reference framework for future research on sustainable protection of stone heritage from harmful microalgae. In this review, cyanobacteria are treated operationally as microalgae for ecological and heritage-colonization purposes, while remaining taxonomically distinct.

Diversity and distribution of algicidal bacteria

Algicidal bacteria are mostly Gram-negative bacteria that belong to the three phyla Proteobacteria, Bacilli, and Bacteroidetes18. Also, some Gram-positive bacteria from the Actinobacteria and Firmicutes phyla have been reported to be algicidal. Currently, algicidal bacteria are found to belong to the genera Bacillus, Vibrio, Alteromonas, and Pseudoalteromonas (Fig. 2).

Fig. 2: Phylogenetic tree of typical algicidal bacteria.
Fig. 2: Phylogenetic tree of typical algicidal bacteria.
Full size image

The tree topology was inferred using maximum-likelihood methodology and constructed from 16S rRNA sequences of the representative species. Bootstrap values (expressed as percentages of 1000 replications) are shown at the branch nodes. The species that kill microalgae by direct attack are highlighted in blue, while the others kill them indirectly.

Vibrio

Vibrio alginolyticus is one of the most common algicidal bacteria in marine environments21. It has been found to kill or inhibit a wide range of marine microalgae, such as Chaetoceros22, Akashiwo23, Alexandrium24, and Heterosigma25. Polysaccharides constitute a significant portion of both dissolved and particulate organic matter in the ocean, and the degradation of polysaccharides plays a central role in the marine carbon cycle26. Many studies suggest that marine Vibrio species can utilize extracellular enzymes to degrade algal cell polysaccharides, such as glucosidases and chitinases26,27. Moreover, algicidal Vibrio species, such as Vibrio sp. W13, Vibrio sp. NJ-04 and V. splendidus OU02 possess genes encoding alginate lyase, an enzyme that can degrade alginate into unsaturated oligosaccharides by cleaving β-glycosidic bonds, thereby breaking down the algal cell wall27,28,29.

In addition to secreting extracellular enzymes to lyse algal cell walls, the Vibrio species can use other algicidal mechanisms. Wang et al.30 co-cultured two algicidal Vibrio species, V. brasiliensis and V. tubiashii, and found that their metabolites increased reactive oxygen species (ROS) levels of Akashiwo sanguinea. This ultimately led to cell membrane damage and further inhibited the antioxidant system, resulting in a sharp decrease in catalase (CAT) activity and ultimately the death of algal cells.

Alteromonas and Pseudoalteromonas

In Proteobacteria, Alteromonas and Pseudoalteromonas are primarily algicidal bacterial genera distributed in marine environments. Pseudoalteromonas is one of the most widely distributed and extensively studied algicidal genera31. Initially, many species of Pseudoalteromonas were members of the genus Alteromonas, but in 1995, a taxonomic reorganization of the genus Alteromonas was carried out based on phylogenetic analysis32. They are often the algicidal bacteria of diatoms and dinoflagellates.

The algicidal activity of species from the genera Alteromonas and Pseudoalteromonas primarily occurs through indirect action involving extracellular exudates. This algicidal characteristic may help bacterial cells compete for nutrients, regulate algal blooms, and defend against surface predators31,33,34. Sun et al.35 isolated a novel alginate lyase from Pseudoalteromonas sp. Alg6B, which showed a hydrolysis rate of up to 97% for Laminaria japonica within 24 h. Wang et al.36 reported that Pseudoalteromonas sp. LD-B6 showed intense algicidal activity against Noctiluca scintillans, achieving an algicidal rate of 90.5% within 12 h. Also, Alteromonas FDHY-03 and A. abrolhosensis JY-JZ1 have been shown to produce an algicidal substance that disrupts algal cell structures by digesting cell wall polysaccharides, leading to intracellular leakage37,38.

Bacillus

Bacillus spp. are Gram-positive, rod-shaped bacteria widely distributed in soil and water environments. Importantly, they are simple nutritional requirements and can form spores under unfavorable environmental conditions. Bacillus can secrete various antimicrobial substances and extracellular hydrolases, which have been reported to inhibit the growth of microalgae, especially cyanobacteria.

Most Bacillus species inhibit algal growth by suppressing photosynthesis and stimulating oxidative stress in host cells. For example, Bacillus altitudinis G3 exhibits high algicidal activity against Microcystis aeruginosa by secreting algicidal substances that can partially block the photosynthetic electron transfer from primary (QA) and secondary (QB) quinone electron acceptors39. Furthermore, the interruption of photosynthesis, especially the inhibition of electron transfer downstream of QA−, generates excessive ROS, leading to a significant increase in malondialdehyde (MDA) content, which is responsible for severe lipid oxidation and, ultimately, oxidative stress in M. aeruginosa. Additionally, Bacillus pumilus SU8S0818 has also been reported to kill cyanobacteria through algicidal substances that produce excessive ROS, leading to their oxidative stress death40.

Apart from the above-mentioned algicidal bacteria, some species of the genera Pseudomonas, Bdellovibrio, Spirillum, Flavobacterium, and Lysozymimonas, which belong to the phyla Bacteroidetes and Proteobacteria, can also act as microalgal predators through direct and/or indirect interactions to kill algae (Table 1). Unfortunately, the algicidal potential of these bacteria is not well explored, indicating that further work is required to unravel their novel algicidal processes that might be applied to biocontrol.

Table 1 Other bacteria with algicidal potential
Full size table

Mechanisms of algicidal action

Generally, algicidal modes of action in bacteria are categorized into indirect and direct mechanisms. Indirect action mainly relies on the secretion of extracellular active substances to lyse algae or competition with algae for limited nutrients, as exemplified by Vibrio spp. and Bacillus spp. However, direct action requires physical contact, in which algicidal bacteria adhere to algal cells and may even penetrate the intracellular space to exert their algicidal effects, a strategy predominantly employed by Myxobacteria spp. and Bdellovibrio spp.

Indirect action

Indirect algicidal contact is the primary mode of algicidal action reported for most bacteria. Algicidal bacteria release specific or non-specific extracellular substances that act as algicides, indirectly killing algae or inhibiting their growth and reproduction. These substances include proteins, peptides, amino acids, fatty acids and their derivatives, alkaloids, and antibiotics. Common bacteria that exhibit this activity include Vibrio41, Pseudomonas42, Flavobacterium43, and Alteromonas44.

Algicides derived from different natural product categories have been shown to primarily exhibit three main algicidal actions: 1) disruption of the cell membrane system, 2) oxidative stress, and 3) inhibition of photosynthetic respiration17.

Disruption of the cell membrane system

For indirect algicidal contact, algicidal bacteria release substances that disrupt the cell wall, membrane, and thylakoids of algal cells. This leads to loss of cell integrity and inhibits algal growth, resulting in lysis and cell death.

In Vibrio, some extracellular hydrolases could dissolve algal cells by digesting the cell wall and cell membrane. Due to its amphipathic properties, the algal plasma membrane can interact with cationic peptides via electrostatic interactions, thereby facilitating membrane permeation. The chloroplast membrane of algal cells can also be similarly disrupted by cationic peptides (Fig. 3a)45. Park et al.45 determined the cationic and hydrophobic content by altering amino acid arrangements, ultimately synthesizing four hybrid peptides with algicidal activity. Among them, glycine, proline-derived cyclic dipeptides, and diketopiperazines are significant classes of antimicrobial compounds that inhibit the growth of Microcystis aeruginosa by compromising cell wall integrity and intracellular structures46.

Fig. 3: Modes of algicidal action through disruption of the cell membrane system.
Fig. 3: Modes of algicidal action through disruption of the cell membrane system.
Full size image

a Degradation of the cell membrane system by extracellular enzymes. b Inhibition of photosynthesis through disruption of the chloroplast membrane. c Interference with membrane protein synthesis through the introduction of exogenous amino acids.

Exogenous polyunsaturated fatty acids can be incorporated into bacterial phospholipids, altering the plasma membrane’s ultrastructure46. This induces changes in membrane permeability and the dissociation of phycobilin from the thylakoid membrane, leading to potassium ion leakage through the cell wall. The leakage of ions from the plasma membrane through enzymatic hydrolysis or pore formation is considered irreparable damage16.

Wu et al.47. found that indole alkaloids can disrupt the cell membrane and chloroplast membrane, deteriorating the internal materials of the thylakoid membrane, interrupting the electron transfer in photosystem II, reducing the effective quantum yield, and ultimately leading to the failure of photosynthesis, thereby inhibiting the growth of cyanobacteria (Fig. 3b). Kodani et al.42 isolated a tricyclic β-carboline, an indole alkaloid, from an algicidal freshwater Pseudomonas species, inhibiting various cyanobacteria growth. Moreover, 2-n-pentyl-4-quinoline, a quinoline alkaloid produced by marine Pseudoalteromonas and marine Pseudomonas species, can inhibit the growth of cyanobacteria and diatoms32.

L-valine has been demonstrated to affect the morphology and integrity of Microcystis, by which exogenous L-valine can substitute for other amino acids and be incorporated into proteins, thereby producing defective proteins. This ultimately leads to the collapse or perforation of Microcystis cells (Fig. 3c)48. Additionally, the bacteriolysin of Bacillus amyloliquefaciens FZB42 can disrupt the plasma membrane of Microcystis aeruginosa49. An et al.50 found that (2-isobutoxyphenyl)amine from Brevibacterium sp. BS01 can alter the cell structure of Alexandrium tamarense, leading to cytoplasmic degradation and loss of organelle integrity.

Oxidative stress

The accumulation of ROS in algae is another common mode of algicidal action, which leads to oxidative stress in algal cells (Fig. 4). ROS include superoxide anion radicals (O2−·), hydrogen peroxide (H2O2), and hydroxyl radicals (OH·), which are well known to be associated with cell aging and death. In most eukaryotes, the primary sources of ROS are the mitochondrial electron transport chain and peroxisomes. However, plant cells contain chloroplasts with intense electron flow, leading to a high rate of ROS production. Due to the strong oxidizing nature of ROS, many essential biomolecules, including DNA, proteins, and lipids, cannot withstand oxidative damage, resulting in loss of cellular function14,51.

Fig. 4: Algicidal mechanisms involved in oxidative stress.
Fig. 4: Algicidal mechanisms involved in oxidative stress.
Full size image

a Some substances can enter the interior of algal cells through transport proteins, causing the chloroplasts to produce large amounts of ROS. b High concentrations of metals generate ROS through the Fenton and Haber-Weiss reactions. c With the onset of oxidative stress in algal cells, the antioxidant system is crucial, including antioxidant enzymes and antioxidants. The main antioxidant enzymes include superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Due to the dysfunction of the antioxidant system in dying cells, their activity usually increases in the early stages to remove accumulated ROS, then sharply declines in the later stages of the algicidal process. d ROS can lead to lipid peroxidation, which could inhibit photosynthesis by damaging the thylakoid membrane and/or oxidizing photosynthetic pigments (d1), damage the cell membrane and cause leakage of cellular contents (d2), and produce excessive malondialdehyde (MDA) that damages the membrane (d3). e ROS could damage essential biomolecules, such as DNA, RNA, and functional proteins. Red arrows indicate the damage effects to the corresponding cellular structures, whereas black arrows represent the inductive or promotive effects to the corresponding processes.

Chen et al.52 isolated pyocyanin from Pseudomonas aeruginosa as an algicidal compound, which exhibits high toxicity to eukaryotic microalgae and other aquatic organisms. In our previous studies, we found that Enterobacter cloacae NP23 and Gibberella moniliformis AM1 exhibited a high lysis rate for four algal species (i.e., Chlorella vulgari, Scenedesmus sp., Microcystis wesenbergii, and Chlorella pyrenoidosa) by inhibiting their host antioxidase activities13,14,53. These two algicidal bacteria would provide a new member for the biocontrol of microalgal populations in eco-technology54. An algicidal compound secreted by Bacillus sp. strain B1, N-acetylsalicylic acid, can induce excessive ROS production in Heterosigma akashiwo, affecting the antioxidant system, photosynthetic pigment contents, and protein levels, leading to the death of H. akashiwo51. L-lysine can induce oxidative stress in Microcystis aeruginosa, reducing chlorophyll content and disrupting the cell membrane in cyanobacteria55,56. Bacillus licheniformis strain sp34 can induce oxidative stress, lipid peroxidation, morphological damage in algal cells, DNA damage, and dysfunction of DNA repair mechanisms, weakening the photosynthetic system and inhibiting Microcystis aeruginosa54.

Inhibition of photosynthetic respiration

Photosynthesis depends on the absorption of sunlight by chlorophyll molecules (e.g., chlorophyll a) in photosystem I (PSI) and photosystem II (PSII). Microalgae can synthesize specific pigments (e.g., carotenoids) that help prevent photo-oxidative damage caused by highly reactive by-products of photosynthesis. However, algicidal bacteria could disrupt photosynthetic processes and activities in algal cells, including electron transfer in photosynthetic pigments, photochemical efficiency, and synthesis and maintenance (Fig. 5), thereby interfering with energy transformation and eventually leading to cell death55.

Fig. 5: Mechanism of photosynthetic inhibition by algicidal chemicals.
Fig. 5: Mechanism of photosynthetic inhibition by algicidal chemicals.
Full size image

Tryptophan could bind to the enzyme FtsH, which degrades the protein D1, thereby inhibiting its degradation. Also, some molecules, such as 2-hydroxychalcone, arginine A, and β-diketone, could inhibit microalgal photosynthesis by impeding electron transfer.

Recent studies demonstrated that a novel algicidal bacterium, Pseudomonas fragi YB2, could significantly decrease chlorophyll content by 4.74 times in 120 h and the activity of succinate dehydrogenase by 103 times in 36 h56. Su et al.57 found that Acinetobacter sp. J25 could reduce chlorophyll a in algae by up to 90% after a 3-day treatment. Also, arginine A, which is a potent anti-cyanobacterial compound produced by Sphingomonas sp. M-17 could act as a photosynthetic inhibitor, interrupting the electron transport chain before PSII58. Additionally, tryptophan hinders photosynthesis by inhibiting the degradation of the D1 protein by the FtsH protease during PSII quality control in chloroplasts and kills algae by significantly reducing photosynthetic efficiency and carbon assimilation (Fig. 5), inhibiting photochemical electron transfer, and increasing the number of closed reaction centers and energy loss59.

Based on the type of electron acceptors on the reduced (acceptor) side of the photosystems, the reaction centers (RC) of the PSI and PSII core complexes are divided into FeS type and quinone type. In photosynthetic reaction centers of PSII, the photoinduced charge separation is terminated by an electron transfer between the primary (QA) and secondary (QB) quinones. Therefore, Zhang et al.60 found that 2-hydroxychalcone can replace HCO3– as a ligand for non-heme iron, thereby disrupting the electron transfer from QA to QB (Fig. 5). Based on this, Yilimulati et al.61 further inferred that β-diketones can bind to the Fe-S cluster of Fd, inhibiting photosynthesis.

However, these three modes of action are not mutually exclusive. ROS attacks the polyunsaturated fatty acids in biological membranes, leading to lipid peroxidation and the production of lipid peroxides in algal cells. At the same time, the accumulation of ROS disrupts pigment synthesis and membrane integrity, inhibiting or ultimately killing algal cells62. Observations showed that the supernatant of Brevibacterium sp. BS01 inhibits psbA gene expression, blocks the electron transport chain, and significantly increases ROS levels and the excessive activity of the antioxidant system. The accumulation of ROS disrupts pigment synthesis and membrane integrity, inhibiting or killing algal cells63. On the other hand, impaired photosynthesis also promotes ROS production17. Dysfunction of the PSII system transfers excitation energy to singlet oxygen, which then forms ROS. Decreased chlorophyll a and carotenoid levels reduce the capacity to inhibit ROS generation64. The pyoluteorin secreted by Hahella sp. KA22 completely inhibits photochemical reactions and the carbon-fixation system in PSII, thereby suppressing photosynthesis, promoting excessive ROS production, inducing severe oxidative damage to cells and organelles, and ultimately killing algal cells64,65.

Direct action

Some algicidal bacteria can directly contact algal cells and digest algal cell walls by releasing hydrolases17. Enzyme activity depends on site specificity, protein concentration, and bacterial cell density. They mainly correspond to two types of enzyme activities: glycoside hydrolases (carbohydrate hydrolases) and proteases, based on their functions16.

Carbohydrate enzymes target the degradation of polysaccharides that constitute the algal cell wall and are involved in cell adhesion. It has been reported that extracellular algicidal glycoside hydrolases produced by bacteria include α-glucanases, β-glucanases, or cellulases16. β-glucanases primarily act on the digestion of algal cell walls, while α-glucanases mainly act on the degradation of intracellular starch38,66. The activities of amylase, cellulase, and xylanase are also associated with the degradation of the algal cell wall67.

Proteases may have dual functions of algicidal activity and promoting the utilization of host polymers through extracellular degradation. For example, a marine Pseudoalteromonas species that is algicidal to diatoms can produce two proteases (probably belonging to the peptidase families S8 and M11, respectively) responsible for cell wall digestion during the cell life cycle68. In addition to their lytic activity, bacterial proteases can also reduce microalgae activity by affecting cell motility69,70.

Importantly, algicidal bacteria that employ direct action are usually predatory, and their modes of action can be divided into endobiotic and epibiotic predation19. Endobiotic predators can enter the periplasm of the prey cell, utilizing its biomass to reproduce and lyse the prey cell from within71. In contrast, epibiotic predators induce lysis of the prey from the outside and feed on the released biomass72.

Epibiotic predation

Epibiotic predation is a mode by which bacteria attach to the surface of host cells and consume the cytoplasm from the outside19. In the 1980s, Guerrero et al. described the attack by Vampirococcus on various Chromatium species as a typical epibiotic predatory strategy73. In direct contact with algicidal activity, epibiotic predation is more common. For example, some studies found that Tenacibaculum sp. GD3 could aggregate and bind to the cell membrane of Karenia mikimotoi, then kill the host through lipid peroxidation and membrane protein lysis (Fig. 6a)15. As the culture time increased, CAT activity and MDA levels decreased, indicating the collapse of the algal defense system (Fig. 4). Furthermore, all chlorophyll fluorescence parameters declined significantly, indicating damage to the photosynthetic system. The expression of genes encoding glutathione S-transferase (GST), pheophorbide a oxygenase (PAO), pabA1, cellulase, and cyclin B was significantly inhibited, leading to irreversible rupture of cells and chloroplasts after 8 h. Moreover, Shi et al.74 showed that Bacillus cereus DC22 can secrete mucus that binds algicidal bacterial cells to cyanobacterial cells and lyses the latter. Additionally, Saprospira sp. PdY3 lyses cyanobacteria through direct contact: it exhibits distinct group behavior on solid media, forming orderly bundle-like group structures; however, when co-cultured with Anabaena in a liquid medium, PdY3 also shows group behavior, forming a three-dimensional mesh structure and lysing Anabaena75.

Fig. 6: Modes of algicidal action through cell predation.
Fig. 6: Modes of algicidal action through cell predation.
Full size image

a Epibiotic predation through lipid peroxidation and membrane protein lysis. b Endobiotic predation through cytolysis and/or periplasmic invasion. c Endobiotic predation through cytolysis in Cyanoraptor togatus LGM-1.

Endobiotic predation

Endobiotic predation is a mode by which algicidal bacteria penetrate host cells and consume them from the inside73. In 1984, Caiola76 isolated a bacterium that could enter Microcystis aeruginosa, causing the host cell to lyse and die. In 1985, Moulder referred to the process in which predatory cells directly enter the host cell cytoplasm as cytolysis77, while the process in which predatory cells invade and grow in specific compartments of Gram-negative cells was called periplasmic invasion (Fig. 6b)73.

One of the most famous predatory bacteria is the endobiotic Bdellovibrio bacteriovorus19,71. During the attack phase, Bdellovibrio signals the bacterial population to release deacetylase, which alters the host cell wall. Once prey is located, it releases enzymes such as glycanases and peptidases to form a “reinforced circular pore” through which the predator enters the host cell. Inside, B. bacteriovorus degrades periplasmic components and consumes them, preventing the host cell from self-replicating. After an extended phase, Bdellovibrio separates into single cells and exits the prey by producing lysozyme. This specifically destroys the previously deacetylated host cell wall, thereby preventing the predator’s self-destruction. The progeny released by Bdellovibrio then begins the cycle anew.

In addition, Julie et al.78 isolated and identified a strain of endobiotic predatory algicidal bacterium, Cyanoraptor togatus LGM-1. Under the microscope, LGM-1 docks with the sheath of microalgae, enters the interior of the microalgae, and undergoes morphological changes. After multiple divisions, a fibrous cocoon forms around the bacteria, while the peptidoglycan transverse walls of the microalgae gradually degrade. In the mid-to-late stage of predation, extracellular propagules of LGM-1 appear outside the fibrous cocoon and, finally, the microalgae lyse (Fig. 6c).

Application paradigm shift to heritage conservation and future perspectives

Currently, the application of algicidal bacteria as a biocontrol agent has been successfully demonstrated to mitigate harmful algal bloom18. This strategy is an eco-friendly treatment for environmental biological contaminants, urgently required for the sustainable development of our society. However, a comprehensive understanding of algicidal mechanisms will help develop additional resources for algicidal bacteria, thereby advancing the field of biocontrol agents of microorganisms15,54,55,58,65.

Moving forward, this mitigation strategy could serve as a successful paradigm for heritage conservation when the risk of on-site application is fully evaluated. In the future, multidisciplinary incorporation from microbiologists, ecologists, and heritage conservators would achieve this great goal through the following endeavors:

First, the prerequisite for effective control of harmful microalgae is that microbiologists provide accurate identification of the species present at the archaeological site. With this information, we can identify the corresponding algicidal bacteria that kill or inhibit the harmful microalgae. Here, we propose to curate a database or bio-archive of microalgae and their algicidal bacteria to support the development of an efficient biocontrol policy for sustainable heritage conservation79. Based on the algicidal mechanisms underlying the interaction between microalgae and algicidal bacteria, we can produce algicidal bacteria or their metabolites as algicides using current biotechnology approaches. However, more work is required to ensure that these insights from algal bloom control can be non-trivially translated to stone heritage systems. In contrast to aquatic bloom systems, subaerial biofilms on stone heritage markedly differ in hydration regimes, nutrient fluxes, surface chemistry, and ecological stability. Given these differences, we have to explicitly identify which algicidal mechanisms are likely transferable and which are not. For example, severe microenvironments (e.g., oligotrophy, limited water availability, and large temperature fluctuations) on stone surfaces could threaten the persistence and survival of algicidal bacteria, thereby reducing their algicidal effectiveness. To address these issues, microbiologists can focus on biosynthetic engineering of algicidal metabolites, in situ isolation of algicidal bacteria from stone heritage systems, or screening for algicidal bacteria that adapt to microenvironments. Nevertheless, it is more controllable in practice on stone surfaces than in the aquatic environment, as interactions between bacteria and phytoplankton in aqueous ecosystems are more complex and dynamic80, and water also reduces the density of algicidal bacteria or their metabolites17. Thus, whether direct or indirect mechanisms are more efficient depends on whether algicidal bacteria can survive on stone surfaces.

Next, our ecologists could track the timelines and patterns of microalgal blooms at the archaeological site. This provides us with accurate information on when and what algicidal bacteria should be used to address the microalgal bloom on site. Furthermore, a comprehensive assessment of the ecological effects of algicidal bacteria or their metabolites should be conducted to confirm that there is no environmental risk. For example, whether potential interactions with existing stone-associated microbial consortia (e.g., fungi and lichens) would exacerbate the biodeterioration of stone heritage must be considered. These potential side effects will constitute the pertinent regulatory and ethical barriers to on-site release of microorganisms, as one cannot expect to freely apply living organisms such as these to surfaces without some form of regulation17. As our treatment is based on microbial control, this ecological risk could be lower than that of chemical algicides, which are strictly forbidden for heritage conservation due to their proven ecological risks.

Lastly, but most importantly, heritage conservators are responsible for a comprehensive on-site assessment of the long-term effects on heritage materials themselves, for at least years. For one, microalgal biofilms can, in some cases, protect covered heritage materials, especially when natural weathering processes outweigh biodeterioration caused by microalgae81. In this case, killing or inhibiting microalgae could not be a positive strategy. For another, we should ensure that either algicidal bacteria used for microalgal biocontrol or algicides produced by algicidal bacteria do not damage heritage materials. Thus, the risks of unintended biodeterioration caused by metabolites (e.g., organic acids, enzymes, and extracellular polymeric substances) produced by algicidal bacteria must be evaluated. If these assessments are successful, we can deliver algicidal bacteria as an eco-friendly approach to mitigate microalgal blooms and support sustainable heritage conservation. Based on these endeavors, we will develop a comprehensive mitigation strategy to address the harmful microalgal bloom at the archaeological site.