Fish in the Bay – September 2022, part 2 – Seaweed & Seagrass Beds inhibit Red Tides!

This is a follow-up post regarding Harmful Algal Blooms (HABs) and the recent H. akashiwo red tide in San Francisco Bay.  As mentioned in the last report, vast armies of naturally algicidal bacteria are known to inhibit and even kill phytoplankton that cause red tides.  At least some of these bacteria exist as biofilms over the surface of seaweeds (macroalgae) and seagrasses. These bacteria inhibit growth of epiphytic algae in a symbiotic relationship with the host plant.  Thus, seaweed and seagrass beds can serve as constant sources of algicidal bacteria into the local microbiome. 

If the idea seems far-fetched to you, keep reading!  This post is just a bibliography and summary of recently published papers investigating these algicidal properties and documenting examples of their use in HAB control.

All editorial comments are shown in Red Font.

Ulva & Gracilaria (green and red macroalgae) by themselves may inhibit Red Tide explosions.

Wang et al. (2012) Allelopathic growth inhibition of Heterosigma akashiwo by the three Ulva species (Ulva Pertusa, Ulva Linza, Enteromorpha intestinalis) under laboratory conditions   Abstract.  Allelopathic effects of several concentrations of fresh tissue, dry powder and dry tissue of three bloom-forming green macroalgae Ulva pertusa, Ulva linza and Enteromopha intestinalis on the red tide microalga Heterosigma akashiwo were evaluated in microcosms systems.

… The resultant microcosms assay showed that the growth of H. akashiwo was strongly inhibited by using fresh tissues, dry powder or dry tissue of these three macroalgae, while aqueous and methanol extracts of both macroalgae had strong inhibitory effects on the growth of H. akashiwo, and the EC50 values for methanol extract of U. pertusa, U. linza or E. intestinalis were 0.016, 0.028×10−12 or 0.033×10−12, respectively. While the other three organic solvent extracts (acetone, ether and chloroform) had no apparent effect on its growth, this suggests that the allelochemicals from these three macroalgae had relatively high polarities.

… These three macroalgae’s culture medium filtrate exhibited no apparent growth inhibitory effect on the microalga under initial filtrate addition whereas the growth of H. akashiwo was significantly inhibited under semi-continuous filtrate addition, which suggests that continuous release of small quantities of rapidly degradable allelochemicals from the fresh tissue of both macroalgae was effective in inhibiting the growth of H. akashiwo.”

Wang, Zhao, and Tang (2009)  Effects of two species of macroalgae—Ulva pertusa and Gracilaria lemaneiformis—on growth of Heterosigma akashiwo (Raphidophyceae)  “The effects of fresh thalli, culture filtrate, water-soluble extract and dry powder of two species of macroalgae, Ulva pertusa (Chlorophyta [a green algae]) and Gracilaria lemaneiformis (Rhodophyta [a red algae]), on the growth of a bloom-forming microalga, Heterosigma akashiwo, were studied in co-culture under controlled laboratory conditions. Both fresh thalli and culture filtrate of U. pertusa and G. lemaneiformis, particularly in the form of fresh thalli, significantly inhibited microalgal growth; indeed, the microalga was completely killed during the course of the experiment.  … The results of the present study suggest that U. pertusa and G. lemaneiformis, especially in the form of fresh thalli, effectively inhibit the growth of H. akashiwo and could thus be potential candidates for use in the control and mitigation of H. akashiwo blooms.”

Jeong et al (2000) Algicidal activity of the seaweed Corallina pilulifera against red tide microalgae  Abstract  Extracts of seaweeds from the coast of Korea have been tested in vitro for algicidal activity against the growth of the toxic microalga Cochlodinium polykrikoides.  “Inhibition of growth resulted from methanol-soluble extracts of the seaweeds Corallina pilulifera, Ulva pertusa, Ishige foliacea and Endarachne binghamiae. Growth inhibition also resulted from the water-soluble extract of C. pilulifera. Powder and dry tissue from the seaweed C. pilulifera also inhibited cell growth of C. polykrikoides. The active algicidal products of C. pilulifera showed stable activity when boiled, exposed to light, or when treated under alkaline condition. Corallina pilulifera had no regional and seasonal variations in this algicidal activity. A powder of the seaweed C. pilulifera, the most potent species, showed algicidal activity against several red tide microalgae, especially C. polykrikoides, Gymnodinium mikimotoi, G. sanguineum, Heterosigma akashiwo, Prorocentrum triestinum and Pyraminonas sp.”

Bacteria that live on macroalgae and seaweeds inhibit Red Tide explosions.

Imai et al (2010) Algicidal bacteria isolated from the surface of seaweeds from the coast of Osaka Bay in the Seto Inland Sea, Japan.  Abstract.  Algicidal bacteria offer a promising tool for the prevention of red tides, because they are able to play a key role in terminating blooms in coastal areas. This study details the detection of vast numbers of algicidal bacteria attached to the surface of seaweeds such as Ulv a sp. and Gelidium sp. (of the order of 106 cells g−1 wet weight in some cases). Algicidal bacteria were isolated from Ulva sp. and Gelidium sp. from the coast of Osaka Bay from April to September 1999, and their algicidal properties were assessed using the prey microalgae Karenia mikimotoiHeterosigma akashiwoFibrocapsa japonica and Chattonella antiqua... It is therefore possible that seaweed beds play a significant role as providers of algicidal bacteria in preventing red tides to coastal waters.

Ichiro et al (2006)  Eutrophication and occurrences of harmful algal blooms in the Seto Inland Sea, Japan.  … The incidents of red tides dramatically increased in frequency and scale in the Seto Inland Sea along with serious eutrophication in the 1960s and 1970s. The maximum incident of 299 was recorded in 1976, but the incident has since shown a clear decreasing trend, reaching about 100 per year in the late 1980s … The important red tide organisms causing huge fishery damages by fish-kill are Chattonella antiquaC. marinaC. ovata and Heterosigma akashiwo (Raphidophyceae), and Karenia mikimotoi and Cochlodinium polykrikoides (Dinophyceae).

… As environment-friendly mitigation strategies for red tides, biological controls using algicidal bacteria and viruses are proposed.  A new finding of the abundant existence of algicidal bacteria on the surface of seaweeds suggests that co-culturing fish and seaweed is a prevention strategy for harmful algal blooms by virtue of the continuous release of many algicidal bacteria to the surrounding seawater. The artificial development of seaweed beds would also be effective as a prevention strategy for red tides.


From Imai et al (2017): Schematic representation of algicidal bacteria dispersed from a seagrass bed (Zostera marina) killing a harmful phytoplankton bloom. 

Imai et al (2017) Environmentally friendly strategies for the prevention of harmful algal blooms using algicidal bacteria associated with seagrass beds   AbstractAlgicidal bacteria (AB) are promising agents which have been isolated and tested from coastal waters. ABs contributed to the rapid termination of HABs in the coastal waters such as the Seto Inland Sea. Large numbers of ABs were found to attach onto the surface of the seagrass Zostera marina (commonly 107 algicidal bacteria per gram wet weight) but without occurrences of any algal blooms. The algicidal bacteria Alteromonas strains S and K were detected as more numerous in seawater in seagrass beds than in offshore seawater, strongly suggesting that their origin is from seagrass beds in coastal environments. The presence of large number of ABs indicates the potential for preventing HAB occurrences. …

… Abundant existence of algicidal bacteria in seagrass beds. In seaweed beds, it was found that a large number of algicidal bacteria inhabited the biofilm on the surface of seaweeds (Imai et al. 2002, 2006, 2012). High densities of about 105 ~ 106 cells g-1 (wet weight) were detected for bacteria lethal to the dinoflagellate Karenia mikimotoi and the raphidophytes Fibrocapsa japonica and Heterosigma akashiwo. Those algicidal bacteria also showed high abundances in the seawater near the seaweed beds. Therefore, seaweed beds are probably supplying algicidal bacteria to the surrounding seawater.


Prodigiosin is the prodigious bacteria that exudes an antimicrobial red dye or pigment. 

Prodigiosin excreted by bacteria is known to inhibit H. akashiwo. 

Prodigiosin is a red pigment exuded by certain bacteria.  The compound was named for its ‘prodigious’ or marvelous/astonishing effects … Researchers have discovered that Prodigiosin can also cure Red Tides caused by Heterosigma akashiwo and other phytos: 


Zhang, Zheng, and Wang (2020) Physiological response and morphological changes of Heterosigma akashiwo to an algicidal compound prodigiosin  Abstract  Harmful algal blooms (HABs) occur all over the world, producing severely negative effects on human life as well as on marine ecosystems. The algicidal compound, prodigiosin, secreted by algicidal bacteria Hahella sp. KA22 can lyse the harmful alga Heterosigma akashiwo. This study is aimed to investigate the algicidal mechanism of prodigiosin against H. akashiwo by detecting physiological and morphological responses of H. akashiwo to presence of prodigiosin. The results indicated that prodigiosin showed strong algicidal effects on H. akashiwo at the concentration of 3 μg/mL. Chlorophyll-a and protein levels of the microalgae decreased significantly while malonaldehyde levels increased at this concentration. Contents of ascorbic acid and activities of superoxide dismutase and peroxidase increased fast with the quick decrease of the reactive oxygen species (ROS). For the 3 μg/mL prodigiosin treatment group, transcription of genes related to photosynthesis and respiration were significantly inhibited at 12 h while respiration-related genes increased at 24 h. Collectively, the results indicated that prodigiosin could kill the microalgae by inducing ROS overproduction which could destroy the cell integrity and change the antioxidant system levels and functional gene expression. Our results demonstrated that prodigiosin is an effective algicide for the control of harmful algae.

Zhang et al (2017) Toxic Effects of Prodigiosin Secreted by Hahella sp. KA22 on Harmful Alga Phaeocystis globose [a Haptophyte]   Abstract  Application of algicidal compounds secreted by bacteria is a promising and environmentally friendly strategy to control harmful algal blooms (HABs). Years ago prodigiosin was described as an efficient algicidal compound, but the details about the effect of prodigiosin on algal cells are still elusive. Prodigiosin shows high algicidal activity on Phaeocystis globosa, making it a potential algicide in HAB control. When P. globosa were treated with prodigiosin at 5 μg/mL, algae cells showed cytoplasmic hypervacuolization, chloroplast and nucleus rupture, flagella missing, and cell fracture, when observed by scanning electron microscope and transmission electron microscopy. Prodigiosin induced a reactive oxygen species (ROS) burst in P. globosa at 2 h, which could result in severe oxidative damage to algal cells. Chlorophyll a (Chl a) fluorescence decreased significantly after prodigiosin treatment; about 45.3 and 90.0% of algal cells lost Chl a fluorescence at 24 and 48 h. The Fv/Fm value, reflecting the status of the photosystem II electron flow also decreased after prodigiosin treatment. Quantitative polymerase chain reaction (PCR) analysis psbA and rbcS expression indicated that photosynthesis process was remarkably inhibited by prodigiosin. The results indicated that the inhibition of photosynthesis may produce excessive ROS causing cell necrosis. This study is the first report about algal lysis mechanism of prodigiosin on harmful algae. …

Kim et al (2008) Red to red – the marine bacterium Hahella chejuensis and its product prodigiosin for mitigation of harmful algal blooms  Abstract Harmful algal blooms (HABs), commonly called red tides, are caused by some toxic phytoplanktons, and have made massive economic losses as well as marine environmental disturbances. As an effective and environment-friendly strategy to control HAB outbreaks, biological methods using marine bacteria capable of killing the harmful algae or algicidal extracellular compounds from them have been given attention. A new member of the gamma-Proteobacteria, Hahella chejuensis KCTC 2396, was originally isolated from the Korean seashore for its ability to secrete industrially useful polysaccharides, and was characterized to produce a red pigment. This pigment later was identified as an alkaloid compound, prodigiosin. During the past several decades, prodigiosin has been extensively studied for its medical potential as immunosuppressants and antitumor agents, owing to its antibiotic and cytotoxic activities. The lytic activity of this marvelous molecule against Cochlodinium polykrikoides cells at very low concentrations (1 ppb) was serendipitously detected, making H. chejuensis a strong candidate among the biological agents for HAB control. This review provides a brief overview of algicidal marine bacteria and their products, and describes in detail the algicidal characteristics, biosynthetic process, and genetic regulation of prodigiosin as a model among the compounds active against red-tide organisms from the biochemical and genetic viewpoints.

Nakashima et al (2006) Producing mechanism of an algicidal compound against red tide phytoplankton in a marine bacterium gamma-proteobacterium  Abstract  Strain MS-02-063, gamma-proteobacterium, isolated from a coast area of Nagasaki, Japan, produced a red pigment which belongs to prodigiosin members. This pigment, PG-L-1, showed potent algicidal activity against various red tide phytoplanktons in a concentration-dependent manner. An understanding of a mechanism of PG-L-1 production by this marine bacterium may yield important new insights and strategies for preventing blooms of harmful flagellate algae in natural marine environments. Therefore, we analyzed the mechanisms of PG-L-1 production. In our previous study, the pigment production by this marine bacterium was completely inhibited at 1.56 microg/ml of erythromycin or 3.13 microg/ml of chloramphenicol, while minimal inhibitory concentrations for cell growth of erythromycin and chloramphenicol against this bacterium were >100 and 25 microg/ml, respectively. It is interesting to note that the ability of the pigment production in erythromycin-treated bacterium recovered by an addition of homoserine lactone. In fact, the pigment production was inhibited by beta-cyclodextrin that inhibits autoinducer activities by a complex with N-acyl homoserine lactones. N-acyl homoserine lactones with autoinducer activities are ubiquitous bacterial signaling molecules that regulate gene expression in a cell density dependent process known as quorum sensing. Therefore, it was suggested that PG-L-1 produced by strain MS-02-063 is controlled by the homoserine lactone quorum sensing. It is speculated that this quorum sensing is involved in the production of algicidal agents of other marine bacteria. This bacterium and other algicidal bacteria might be concerned in regulating the blooms of harmful flagellate algae through the quorum sensing system.

Jeong et al (2005) Genomic blueprint of Hahella chejuensis, a marine microbe producing an algicidal agent  Abstract Harmful algal blooms, caused by rapid growth and accumulation of certain microalgae in the ocean, pose considerable impacts on marine environments, aquatic industries and even public health. Here, we present the 7.2-megabase genome of the marine bacterium Hahella chejuensis including genes responsible for the biosynthesis of a pigment which has the lytic activity against a red-tide dinoflagellate. H.chejuensis is the first sequenced species in the Oceanospiralles clade, and sequence analysis revealed its distant relationship to the Pseudomonas group. The genome was well equipped with genes for basic metabolic capabilities and contained a large number of genes involved in regulation or transport as well as with characteristics as a marine heterotroph. Sequence analysis also revealed a multitude of genes of functional equivalence or of possible foreign origin. Functions encoded in the genomic islands include biosynthesis of exopolysacchrides, toxins, polyketides or non-ribosomal peptides, iron utilization, motility, type III protein secretion and pigmentation. Molecular structure of the algicidal pigment, which was determined through LC-ESI-MS/MS and NMR analyses, indicated that it is prodigiosin. In conclusion, our work provides new insights into mitigating algal blooms in addition to genetic make-up, physiology, biotic interactions and biological roles in the community of a marine bacterium.


Two recently published reviews, one from Japan & the other funded by NOAA, condense current knowledge regarding HAB control. 

  • Reading both papers is highly recommended! – The papers are linked below.

  • For those who don’t time to read the full papers, key points are extracted and quoted here.


Illustration of HAB control using seaweed and seagrass beds as part of the Japanese “Sato-Umi” concept.

Review of HAB control strategies from Japan – 2021

Imai, Inaba, and Yamamoto (2021) Harmful algal blooms and environmentally friendly control strategies in Japan   Abstract  The presence and status of harmful algal blooms (HABs) in Japan are reviewed, revealing a decrease in red tides; however, toxic blooms are found to be increasing in western Japan. Environmentally friendly control strategies against HABs are also compared with integrated agricultural pest management. Very high densities (105–108 CFU/g) of algicidal and growth-inhibiting bacteria were found in biofilm on seagrass and seaweed surfaces and in surrounding coastal seawater. … These findings offer new insights into the ecology of influential bacteria and harmful algae, suggesting that protection and restoration of native seagrasses and seaweeds in coastal marine environments should be implemented to suppress HABs. Diatom blooms were successfully induced with bottom sediment perturbation to prevent the occurrence of harmful flagellates such as Chattonella spp. and Alexandrium catenella in the Seto Inland Sea; however, this method requires robust and reproducible verification. …

In H. akashiwo red tides, algicidal bacteria also increased after the bloom peak in the coastal waters of Hiroshima Bay, and a predator–prey relationship of abundance was observed (Imai et al. 1998a; Kim et al. 1998; Yoshinaga et al. 1998). Similar relationships were also observed in the South Carolina brackish detention ponds in the USA (Liu et al. 2008).

At one seaweed bed in Obama Bay, Fukui Prefecture, algicidal bacteria against the red tide raphidophytes Chattonella spp. (C. antiquaC. marina, and C. ovata), H. akashiwo, and Fibrocapsa japonica were found at high densities despite the absence of these red tide raphidophytes (Imai and Yoshinaga 2002).

Theories and criteria for HAB controlling strategies. In the field of agriculture, pest control is crucial for maintaining and improving production, and a great deal of research effort has been devoted to it. When farmed marine organisms are regarded as crops, the pests are equivalent to harmful algae. There are many similarities between HAB control in fisheries and pest control in agriculture, and applying the lessons from increasing crop productivity to marine resources can provide a number of useful perspectives.

In the past, Japanese agriculture adopted a defense-oriented strategy with a focus on plant protection from pests. A variety of organisms inhabited paddy and upland fields, passively preserving the biodiversity; however, novel synthetic pesticides developed after World War II changed the approach to pest control from “defensive” to “offensive.” With the exception of agricultural crops, pest control was carried out based on a “disinfection policy,” which does not permit the existence of pests and useful organisms, even beneficial insects and animals. As a result, serious issues have arisen, such as the appearance of drug-resistant organisms, dangers of residual pesticides in crops and the environment, and induction of abnormal pest occurrences. These problems have caused major destruction of the natural biota and ecosystem (Kiritani 1979; Naba 2001).

It is fundamentally important to prevent the occurrences of HABs over long timescales by the creation of water environments that naturally suppress the growth of HAB-causative organisms. The restoration and creation of seagrass and seaweed beds have been proposed as environmentally friendly strategies to prevent the occurrence of HABs (Imai et al. 2006a2017b; Imai 2015). Furthermore, the effects of combined aquaculture of fish and seaweeds may be promising (Imai et al. 200220122020; Imai 2019; Inaba et al. 2020). These effects take advantage of the natural function of seaweeds and seagrasses, providing inhabitable substrates to algicidal and growth-inhibiting bacteria, which are then supplied to the surrounding waters. It is expected that the coastal marine ecosystem itself will self-maintain in a healthy state, with relatively higher densities of bacteria limiting algal growth, and in which phytoplankton communities containing HAB species do not proliferate into overscale and destructive blooms (Imai and Yamaguchi 2012; Imai 2015; Imai et al. 2017b).

(“Sato-Umi” is the Japanese “Village-Sea” concept that has been adopted as the national restoration policy.  Much of the recent HAB research in Japan has been conducted in the context of “Sato-Umi.”)


From Coyne, Wang, and Johnson (2022): FIGURE 5. Potential application strategies of algicidal bacteria for HAB prevention and mitigation. Current and proposed strategies include 1) direct dispersal of algicidal bacteria and/or their algicides, 2) application of immobilized algicidal bacteria, 3) deployment of multi-function systems, and 4) restoring seagrass/seaweed beds to recruit algicidal bacteria.


NOAA Funded Review of HAB control strategies – 2022

Coyne, Wang, and Johnson (2022)  Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms  

Introduction. Phytoplankton, or single-celled microalgae, play key roles in biogeochemical cycles. They transform energy from sunlight into biomass, forming the basis of the food web in aquatic environments while providing a substantial sink for both inorganic nutrients and CO2.

As hosts to their bacterial cohort, algae provide a rich source of dissolved organic material in the form of fixed carbon and other organic material, while the bacteria may provide essential cofactors such as cobalamin (vitamin B12) or phytohormones (auxins) to stimulate algal growth and maintain the algae-bacteria relationship (e.g., Paerl, 1982Seyedsayamdost et al., 2014Cooper and Smith, 2015Durham et al., 2015Johansson et al., 2019). The essential nature of these interactions is further supported by evidence that algae actively recruit favorable bacterial colonizers to their surface by the secretion of selective chemicals (e.g., Shibl et al., 2020). Algae may also accrete bacteria that express different benefits dependent on conditions, including stresses due to light, iron, and temperature (Johansson et al., 2019). In many cases, algae suffer without their acquired bacterial mutualists and perform poorly in axenic cultures (Biondi et al., 2018). However, interactions between phytoplankton and bacteria in marine and freshwater environments are both complex and dynamic. They span a range of modes with mutualistic symbiosis on one side of this strategic spectrum, where each species benefits (reviewed by Cooper and Smith, 2015), to the opposite side of the spectrum in which pathogenic or algicidal bacteria lyse algae for access to their intracellular contents. These complex inter-kingdom relationships can fluctuate with changes in their physical and chemical environment, and are often mediated by species composition and abundance (Zhang et al., 2021c).

Recent work by Shibl et al. (2020) used metabolomics combined with transcriptomics to show that the diatom Asterionellopsis glacialis responds to bacteria by producing secondary metabolites that promote attachment of beneficial bacteria while suppressing attachment of opportunistic bacteria. Transplanted bacteria isolated from the phycosphere of different species of Pseudonitzschia also showed algicidal activity against non-native hosts, while enhancing the growth of its native host (Sison-Mangus et al., 2014). This may be the result of co-adaptation of host and bacterial symbiont, and/or the ability of algicidal bacteria to switch from a mutualistic to parasitic relationship dependent on the host.

On the other hand, taxonomic specificity of algicidal interactions also results partially from defenses by the targeted alga and its mutualistic microbiome. In an example of endogenous defenses against algicidal attacks, the diatom Chaetoceros didymus secretes oxylipins that inhibit the growth of the algicidal bacterium Kordia algicida (Meyer et al., 2018). In other cases, an alga’s bacterial cohort can provide it with some relief from attacks by algicidal bacteria. Karenia brevis maintained without its native bacterial microbiome, for example, lost resistance to algicidal bacterium Flavobacteriaceae sp. S03 (Roth et al., 2008a)

In another recent example, Quan et al. (2021) described the effects of two algicidal compounds isolated from Bacillus B1 on raphidophyte alga, Heterosigma akashiwo.

Effects of Bacterial Algicides on Algae.  A wide range of outcomes that occur over time scales of minutes to days have been observed for algal species after exposure to algicidal bacteria (Figure 1). These effects may lead irrevocably to cell death, as in algal cell lysis, or they may be reversible, as in formation of temporary algal cysts. Other effects may be algistatic, resulting in slowed growth of the alga in response to algicidal compounds such as cell cycle inhibitors.

Rupture of the algal cell, or lysis, is perhaps the most commonly observed effect in algicidal interactions (e.g., Zhang et al., 2017Jeong and Son, 2021). Lysis may be an externally driven event (necrosis), induced by physical or chemical interactions resulting in the loss of membrane integrity without involving a metabolic or physiological response by the algal cell (Franklin et al., 2006). For example, mycosubtilins produced by bacteria in the genus Bacillus interact with the cytoplasmic membrane, resulting in increased ion permeability and lysis of both dinoflagellate and raphidophyte species (reviewed by Jeong and Son, 2021). Benzoic acid, produced by algicidal Thalassospira sp. also induced cell lysis in HAB dinoflagellate Karenia mikimotoi, possibly by passing through the cell membrane and acidifying the algal cytoplasm (Lu et al., 2016). Alternatively, lysis may occur after prolonged exposure and can be the culmination of internally driven mechanisms (reviewed by Wang et al., 2020a). For example, algicidal compounds may induce production of reactive oxygen species, leading to peroxidation of membrane lipids and cell lysis (Tan et al., 2016).

Control of Harmful Algal Blooms by Algicidal Bacteria in the Environment.  …  Free-living (Sakami et al., 2017) and epiphytic bacteria with algicidal and antifouling properties are abundant on macroalgae or seagrasses (Inaba et al., 2017Imai et al., 2021), and there is evidence that they may regulate biofouling by algae (reviewed by Tarquinio et al., 2019). Negative correlations between seagrass beds and algal abundance further suggested that algicidal bacteria associated with these beds may have some control over the growth of algae in the environment (Inaba et al., 2018Onishi et al., 2021), and the potential for development of management strategies to control HABs (Imai et al., 2021), as discussed in more detail below.

Perhaps the most convincing evidence for control of HABs by naturally occurring algicidal bacteria in the environment is the presence and increased abundance of algicidal bacteria during late stages of blooms (e.g., Imai et al., 2001Kim et al., 2008Liu et al., 2008Park et al., 2010Zhang et al., 2012Inaba et al., 2014Scherer et al., 2017). Indeed, sampling the bacterial community during blooms is an effective approach to isolate algicidal bacteria from the environment (e.g., Rashidan and Bird, 2001Park et al., 2010Ismail and Ibrahim, 2017Shi et al., 2018Zhang et al., 2019Cho et al., 2021). For instance, Shi et al. (2018) recently described an algicidal gamma-Proteobacteria in the genus Alteromonas isolated during a bloom of HAB dinoflagellate P. donghaiense. Laboratory culture experiments subsequently showed that this bacterium upregulated beta-glucosidase expression in the presence of the dinoflagellate, resulting in digestion of the alga’s cell wall. The abundance of other algicidal bacteria may not fluctuate during blooms, but their prevalence in the microbial community suggests that they play a role in bloom dynamics (Scherer et al., 2017).

Application Strategies for Control of Harmful Algal Blooms.  With limited direct evidence for control of HABs by naturally occurring algicidal bacteria, there is growing interest in both research and management to develop application strategies for the use of algicidal bacteria or their products to control HABs (e.g., Imai et al., 2021Mehrotra et al., 2021). These application strategies include direct dispersal of bacteria and/or their algicidal products, deployment of immobilized algicidal bacteria for more targeted dispersal, the use of multi-functional systems, and deployment of substrates such as seagrass beds to recruit naturally occurring algicidal bacteria (Figure 5). It is unlikely that any one approach will be appropriate for all HABs. Management strategies to control HABs will require an assessment of cost, location, feasibility, social acceptance, and target species when deciding on an appropriate control method (Hudnell, 2010Brooks et al., 2016).

A viable alternative to the application of laboratory-cultured algicidal bacteria for control of HABs is enhancing the recruitment and growth of algicidal species to areas that are at risk of HABs. Seagrass and seaweed beds, as mentioned above, are hotspots for bacteria that inhibit the growth of microalgae (Sakami et al., 2017). Biofilms composed of complex communities of epiphytes including algicidal bacteria are recruited to the surfaces of seagrasses and macroalgae, which in turn provide a continuous supply of these species back into the surrounding seawater for HAB prevention and mitigation (also see detailed review by Imai et al., 2021). The surface of seagrass and macroalgae not only provide habitats for bacteria, but may also exude nutrients that promote bacterial growth (reviewed by Tarquinio et al., 2019). The density of algicidal bacteria inhabited in the seagrass leaves can be over 107 colony-forming units (CFU)/g of the leaf wet weight (e.g., Inaba et al., 2017). A study conducted by Inaba et al. (2020) using artificially created Ulva pertusa beds with floating cages demonstrated a much higher density of bacteria with algicidal activity against raphidophytes and dinoflagellates on the macroalgae beds compared to the surrounding seawater. A similar level of algicidal bacteria was found on the artificial macroalgae beds compared to those that are natural in the environment (Inaba et al., 2020). These results indicated the potential of using artificially introduced macroalgae beds to recruit algicidal bacteria as a boost to HAB mitigation in areas at risk for toxic blooms, such as shellfish harvesting sites. In addition to HAB mitigation, seagrass and seaweeds provide essential habitat for wildlife and reduce nutrients (reviewed by Morris et al., 2018) while also mitigating coastal hazards by reducing wave heights and preventing floods (reviewed by Morris et al., 2018Valdez et al., 2020). Overall, restoration of seagrass and macroalgae may provide an ecologically friendly means to mitigate HABs. A drawback to this approach compared to direct application of specific algicidal bacteria may be the time it takes for algicidal bacteria to become established. However, as mentioned by Imai et al. (2021), little effort is involved in maintaining seagrass and macroalgae beds once they are restored, so that this method may also represent a cost-efficient approach for HAB control efforts.


The Martians had no resistance to the bacteria in our atmosphere to which we have long since become immune. Once they had breathed our air, germs which no longer affect us began to kill them. … After all that men could do had failed, the Martians were destroyed and humanity was saved by the littlest things … 

War of the Worlds, 1953.

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