Use of immobilized algicidal bacteria to control natural freshwater diatom blooms

The purpose of this investigation was to assess the possible use of algicidal bacteria in conjunction with an immobilization technique for efficient termination of natural blooms of Stephanodiscus concomitant with minimization of adverse effects caused by a single application of bacteria. The performance of Pseudomonas fluorescens cells immobilized on cellulose sponges (CIS) was compared with that of freely suspended cells (FSC) of the organism at low water temperature (WT) of <10°C in co-cultures and natural microcosms. In the co-cultures, CIS resulted in more effective lysis of Stephanodiscus, irrespective of similar bacterivore (Spumella sp.) density, and significantly reduced the concentrations of nitrate and soluble reactive phosphorus (SRP), but not that of silicate, compared with FSC. In the microcosms containing natural freshwater, CIS reduced the densities of Stephanodiscus spp. and Aulacoseira spp. but had no effect on other phytoplankton. Compared with FSC, removal of nutrients by the CIS prevented secondary blooms caused by other phytoplankton. Our results indicate that the CIS affords effective protection of P. fluorescens from low WT and heterotrophs, and restrained regeneration of both SRP and nitrate. Thus, it was demonstrated that the CIS may be an attractive alternative to FSC for control of natural blooms.     

Isolation and characterization of a bio-agent antagonistic to diatom, Stephanodiscus hantzschii

AIMS: Identification of bacterium HYK0203-SK02 and its lysis of Stephanodiscus hantzschii. METHODS AND RESULTS: In an effort to identify a bio-agent capable of controlling S. hantzschii blooms, we used the algal lawn method to identify 76 bacteria in relevant water samples. Of these, the seven isolate showed algicidal activity against S. hantzschii; isolate HYK0203-SK02 exhibited the strongest algicidal activity, and was used for further analysis. 16S rDNA sequencing of this isolate allowed us to identify HYK0203-SK02 as a strain of Pseudomonas putida (99.2%). Growth of S. hantzschii was strongly suppressed by bacteria in all growth phases, with the strongest algicidal activity noted against diatoms in the exponential stage (5-18 days). Host range assays revealed that isolate HYK0203-SK02 also strongly inhibited the growth of Microcystis aeruginosa, but stimulated growth of the diatom Cyclotella sp., which has a similar structure to that of S. hantzschii. Biochemical assays revealed that the algicidal substance seemed to be localized in the cytoplasmic membrane of this newly identified algicidal bacterium. CONCLUSION: The algicidal bacteria P. putida HYK0203-SK02 caused cell lysis and death of not only diatom S. hantzschii but also cyanobacteria M. aeruginosa, dramatically. Algicidal substance might be located at the compartment of cytoplasmic membrane. SIGNIFICANCE AND IMPACT OF THE STUDY: Taken together, our results indicate that P. putida HYK0203-SK02 may be a potential bio-agent for future use in controlling freshwater diatomic blooms.     

A bacterium that inhibits the growth of Pfiesteria piscicida and other dinoflagellates

Toxic dinoflagellate blooms have increased in estuaries of the east coast of the United States in recent years, and the discovery of Pfiesteria piscicida has brought renewed attention to the problem of harmful algal blooms (HAB) in general. Many bacteria and viruses have been isolated that have algicidal or algistatic effects on phytoplankton, including HAB species. Twenty-two bacterial isolates from the Delaware Inland Bays were screened for algicidal activity. One isolate (Shewanella IRI-160) had a growth-inhibiting effect on all three dinoflagellate species tested, including P. piscicida (potentially toxic zoospores), Prorocentrum minimum, and Gyrodinium uncatenum. This bacterium did not have a negative effect on the growth of any of the other four common estuarine non-dinoflagellate species tested, and in fact had a slight stimulatory effect on a diatom, a prasinophyte, a cryptophyte, and a raphidophyte. Shewanella IRI-160 is the first non-microzooplankton example of a microbe with the ability to control and inhibit the growth of P. piscicida, suggesting that bacteria in the natural environment could play a role in controlling the growth and abundance of P. piscicida and other dinoflagellates. Such bacteria could also potentially be used as management tools to prevent the proliferation of potentially harmful dinoflagellates in estuaries and coastal waters.     

Enhancement of algicidal activity by immobilization of algicidal bacteria antagonistic to Stephanodiscus hantzschii (Bacillariophyceae)

AIMS: Enhancement of algicidal activity by immobilization of algicidal bacteria antagonistic to Stephanodiscus hantzschii. METHODS AND RESULTS: In laboratory studies, A diatom-lysing bacterium, Pseudomonas fluorescens HYK0210-SK09 showed strong algicidal activity against S. hantzschii, but a natural mesocosm study revealed that this bacterium failed to fully control natural blooms of Stephanodiscus at the low water temperatures that favour these blooms. Here, we sought to develop an effective immobilization strategy for enhancing the algicidal activity of HYK0210-SK09 in the natural setting. Bacterium HYK0210-SK09 was immobilized with various carriers including agar, alginate, polyurethane and cellulose sponge. The bacterial cells immobilized with cellulose sponge (CIS) induced more rapid and complete lysis of S. hantzschii than other carriers, and had a higher packing ability than polyurethane. Furthermore, CIS-immobilized cells showed higher lysis of S. hantzschii at the same concentrations as that of free cells (< or =1 x 10(7) cells ml(-1)), and had especially strong algicidal activity at the low temperatures (<10 degrees C). Based on these laboratory studies, we assessed the possible application of HYK0210-SK09 cells in the field by performing a mesocosm study during the winter season. The CIS-immobilized cells with species-specific activity towards the genera Stephanodiscus showed extremely high algicidal activity (up to 95%) against a bloom of Stephanodiscus hantzschii even at low water temperatures, because of high cell packing and subsequent cell protection against low temperatures and predators, whereas free cells showed negligible algicidal activities under these conditions. CONCLUSION: Immobilizing cells of HYK0210-SK09 in CIS foam, rather than in the other matrices tested, could achieve more efficient control of Stephanodiscus blooms and showed a significant algicidal activity on in vitro and in vivo blooms, even at low water temperature. SIGNIFICANCE AND IMPACT OF THE STUDY: Collectively, these results indicate that CIS of algicidal bacteria may form an important strategy for effective management of Stephanodiscus blooms at low water temperatures.     

Biological control of Stephanodiscus hantzschii (Bacillariophyceae) blooms in a field mesocosm by the immobilized algicidal bacterium Pseudomonas fluorescens HYK0210-SK09

The aim of the study was to determine the activity of an immobilized algicidal bacterium, Pseudomonas fluorescens HYK0210-SK09 (SK09), against blooms of the diatom Stephanodiscus hantzschii and to characterize its effect in an ecosystem. To this end, SK09 cells that had been immobilized using an activated carbon polyvinyl alcohol sponge (APVAS) were tested in a field mesocosm in the Nakdong River, South Korea. Immobilized SK09 cells showed species-specific activity toward S. hantzschii throughout the study, whereby up to 72 % of the population of this species was killed. The APVAS carrier absorbed nutrients in the mesocosms effectively, which reduced the free concentrations of nutrients. Other phytoplankton species, such as Cryptomonas ovata, did not show any fluctuation in abundance. The abundance of heterotrophic protists, such as heterotrophic flagellates and ciliates, increased significantly owing to the utilization of SK09 as a food source. The high level of algicidal activity of SK09 against S. hantzschii was sustained by the release of these algicidal cells from the carrier. Thus, the immobilization of algicidal bacteria has various advantages, which include high packing ability, enhancement of bacterial growth, protection from bacterial predators, effective nutrient removal, and improved algicidal activity. The present study demonstrates that immobilized SK09 is an effective biocontrol agent for natural S. hantzschii blooms. Taken together, the findings of this study clearly demonstrate that SK09 cells immobilized in APVAS can improve the water quality in mesocosm ecosystems without producing any ecological disturbance.     

60 Identification of diatom‐lytic bacterium sk‐02 and its capability to degrade stephanodiscus hantzschii

In order to control harmful algal blooms, many biological approaches have been tried. Specially, there have recently been discussions concerning the roles of bacteria in algal bloom dynamics. Then, algicidal bacteria are expected as an agent considerate for harmful algal blooms control. Development of these organisms as biological control agents involves isolation from environmental samples. With the aim of develop eco‐technology controlling water blooms in fresh waters, we isolated the diatom‐lysing bacteria from the sediments of Lake Seokchon and Pal¡¯tang River‐Reservoir. A soft agar‐overlay technique was used to isolate the diatom lytic bacteria. The SK‐02 showed a diatom lytic activity against Stephanodiscus hantzschii. Taxonomic identification including 16S rDNA base sequencing, and phylogenetic analysis indicated that the isolate SK‐02 had a 99.20% homology in its 16S rDNA base sequence with Pseudomonas putida. The nature of these diatom‐lying components is still under investigation. These results suggest that the indigenous bacteria isolated from the sediments may have a potential in the application and development of eco‐technology controlling harmful water blooms in the fresh water environments.     

Isolation,identification, and algicidal activity of marine bacteria against <Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis>

The aim of this study was to isolate and identify algicidal bacteria against the dinoflagellate Cochlodinium polykrikoides, and to determine the algicidal activity and algicidal range. During the declining period of C. polykrikoides blooms, seven algicidal bacteria were isolated. The algicidal bacteria against C. polykrikoides were enumerated using the most probable number (MPN) method. The number of algicidal bacteria was high (3.7 × 10³ mL⁻¹). Algicidal bacteria were identified on the basis of biochemical and chemotaxonomic characteristics, and analysis of 16S rDNA sequences. Seven algicidal bacteria isolated in this study belonged to the genera Bacillus, Dietzia, Janibacter, and Micrococcus. The most algicidal bacterium, designated Micrococcus luteus SY-13, is assumed to produce secondary metabolites. When 5% culture filtrate of this strain was applied to C. polykrikoides cultures, over 90% of C. polykrikoides cells were destroyed within 6 h. M. luteus SY-13 showed significant algicidal activities against C. polykrikoides and a wide algicidal range against various harmful algal bloom (HAB) species. Taken together, our results suggest that M. luteus SY-13 could be a candidate for controlling HABs.     

Producing mechanism of an algicidal compound against red tide phytoplankton in a marine bacterium γ-proteobacterium

Strain MS-02-063, γ-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 μg/ml of erythromycin or 3.13 μg/ml of chloramphenicol, while minimal inhibitory concentrations for cell growth of erythromycin and chloramphenicol against this bacterium were >100 and 25 μg/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 β-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.     

Pseudomonas fluorescens HYK0210-SK09 offers species-specific biological control of winter algal blooms caused by freshwater diatom Stephanodiscus hantzschii

Aims:  The study of an algicidal activity and mechanism of the isolated Pseudomonas fluorescens HYK0210-SK09 (SK09) against a winter bloomed harmful diatom, Stephanodiscus hantzschii.
Methods and Results:  SK09 was isolated from the Paldang reservoir, Korea and used to biological control of S. hantzschii . The inoculation of SK09 at the final density of 5 × 10⁶ cells ml⁻¹ caused degradation of >90% of S. hantzschii cells within 5 days. The algal cell lysis was achieved by a direct attack of the bacteria to the diatom cells, and the algicidal compound was located in the cytoplasm of the cell. As SK09 did not suppress Microcystis aeruginosa , Anabaena cylindrica , Coelastrum astroideum or Cyclotella meneghiniana , it appeared to attack S. hantzschii in a species-specific manner. Testing in an indoor mesocosms confirmed that SK09 effectively reduced S. hantzschii cells by 88% within 9 days.
Conclusions:  This bacterium is useful in regulating blooms of S. hantzschii . However, it should be studied in the future that their impact in shaping phytoplankton community and their activity in natural environments.
Significance and Impact of the Study:  The bacterium enabled us to develop a new strategy, to understand the interaction for anthropogenic control of harmful algal blooms in nature.     

A novel marine bacterium algicidal to the toxic dinoflagellate Alexandrium tamarense

Aims: This work is aiming at investigating algicidal characterization of a bacterium isolate DHQ25 against harmful alga Alexandrium tamarense. Methods and Results: 16S rDNA sequence analysis showed that the most probable affiliation of DHQ25 belongs to the γ‐proteobacteria subclass and the genus Vibrio. Bacterial isolate DHQ25 showed algicidal activity through an indirect attack. Xenic culture of A. tamarense was susceptible to the culture filtrate of DHQ25 by algicidal activity assay. Algicidal process demonstrated that the alga cell lysed and cellular substances released under the visual field of microscope. DHQ25 was a challenge controller of A. tamarense by the above characterizations of algicidal activity assay and algicidal process. Conclusion: Interactions between bacteria and harmful algal bloom (HAB) species proved to be an important factor regulating the population of these algae. Significance and Impact of Study: This is the first report of a Vibrio sp. bacterium algicidal to the toxic dinoflagellate A. tamarense. The findings increase our knowledge of the role of bacteria in algal–bacterial interaction.     

Characterization of an algicidal bacterium Brevundimonas J4 and chemical defense of Synechococcus sp. BN60 against bacterium J4

As part of efforts to enhance the strategies explored to eliminate the adverse impacts of cyanobacterial blooms, we isolated an algicidal bacterium, J4, from Lake Taihu. Analysis of 16S rDNA sequence revealed that strain J4 belonged to the genus Brevundimonas. Bacterium J4 exhibited algicidal activity mainly through excretion of extracellular algicidal compounds that were further extracted with methanol and purified by silica gel chromatography and high performance liquid chromatography (HPLC). The compounds showed thermal stability, strong polarity and water solubility in J4 cultures. Study on the algicidal activity of J4 against two dominant cyanobacterial bloom-forming species in Lake Taihu showed that J4 exhibited lower algicidal rate against Synechococcus sp. BN60 (48.6%, t = 6 days) than against Microcystis aeruginosa 9110 (91.8%, t = 6 days). Additionally, rapid reduction in cell density of J4 was observed in co-cultures of Synechococcus sp. BN60 and bacterium J4 but not observed in co-cultures of M. aeruginosa 9110 and bacterium J4 during algicidal process, which was the main reason why the algicidal rate of J4 against BN60 was lower than against 9110. The reduction in cell density of J4 resulted from inducible production of antimicrobial-like compound secreted by Synechococcus sp. BN60 in co-cultures of Synechococcus sp. BN60 and bacterium J4, which reflected a kind of chemical defense from cyanobacteria (BN60) against algicidal bacteria (J4). However, M. aeruginosa 9110 had no chemical defense against J4, suggesting that whether cyanobacterial chemical defense occurs or not between cyanobacteria and algicidal bacteria depends on specific cyanobacteria–algicidal bacteria pairs. These results show that not only one-sided algicidal effect but also two-sided reciprocal inhibition interactions exist between algicidal bacteria and cyanobacteria, indicating the complexity of cyanobacteria–algicidal bacteria interactions in Lake Taihu and the need to take the cyanobacterial defensive responses into consideration when assessing potential use of algicidal bacteria.     

The association of algicidal bacteria and raphidophyte blooms in South Carolina brackish detention ponds

Over the past 5 years, raphidophyte blooms have been frequently observed along the South Carolina coastal zone. During the 2002, 2003, and 2004 sampling seasons, we investigated temporal fluctuations of algicidal bacteria abundance against raphidophycean flagellates (Heterosigma akashiwo, Chattonella subsalsa, and Fibrocapsa japonica) using the microplate most probable number (MPN) method in three Kiawah Island brackish stormwater detention ponds (K1, K2, and K75). Local axenic isolates of H. akashiwo, C. subsalsa, and F. japonica were obtained and their susceptibility to algicidal bacteria tested. A total of 195 algicidal bacterial strains were isolated from raphidophyte blooms in the study ponds, and 6 of them were identified at the genus level, and the taxonomic specificity of their algicidal activity was tested against local (pond) and nonlocal isolates of raphidophytes (3 species, 10 total strains). In the ponds, a consistent association was found between raphidophyte bloom development and an increase in bacteria algicidal to the bloom species. In 12 of 15 cases, bloom decline followed the increase in algicidal bacteria to maximum abundances. Although variability was found in the taxonomic specificity of the algicidal bacteria effect (i.e. the number of raphidophyte species affected by a particular bacteria strain) and raphidophyte susceptibility (i.e. the number bacteria strains affecting a particular raphidophyte species), a toxic effect was always found when strains of a raphidophyte species were exposed to algicidal bacteria isolated from a bloom caused by that same species. The results suggest that algicidal bacteria may be an important limiting factor in raphidophyte bloom sustenance and can promote bloom decline in brackish lagoonal eutrophic estuaries.     

Testing addition of <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> HYK0210-SK09 to mitigate blooms of the diatom <Emphasis Type="Italic">Stephanodiscus hantzschii</Emphasis> in small- and large-scale mesocosms

An algicidal bacterium Pseudomonas fluorescens HYK0210-SK09 (SK09) was applied to a natural bloom of Stephanodiscus hantzschii using a small-scale mesocosm (SM) and a large-scale mesocosm (LM) to clarify the algicidal effects and evaluate the response of the planktonic community and environments. When SK09 cells were inoculated at a final concentration of 5 × 10⁶ cells mL⁻¹, the abundance of S. hantzschii decreased significantly by 95% in SM and 85% in LM. The microcosm in the laboratory revealed that the abundance of Pseudomonas increased rapidly after inoculation with a corresponding decrease in the S. hantzschii population. Nutrient concentrations increased following the decline in the diatom cells. The abundances of nondominant species such as Chlamydomonas, Cryptomonas, and Navicula increased slightly with increased nutrient concentrations. The abundance of heterotrophic protists also increased significantly due to utilization of SK09 as food. The present study demonstrates that SK09 is an effective biocontrol agent for natural S. hantzschii bloom, and grazing pressure plays a crucial role in the successful application of algicidal bacteria to natural environments.     

Comparative analysis of two algicidal bacteria active against the red tide dinoflagellate Karenia brevis

The red tide dinoflagellate Karenia brevis blooms annually along the eastern Gulf of Mexico, USA, and is often linked to significant economic losses through massive fish kills, shellfish harvest closures, and the potential threat to humans of neurotoxic shellfish poisonings as well as exposure to aerosolized toxin. As part of an effort to enhance the strategies employed to manage and mitigate these events and their adverse effects, several approaches are being investigated for controlling blooms. Previous studies have established the presence of algicidal bacteria lethal to K. brevis in these waters, and we aim to characterize bacterial–algal interactions, evaluate their role as natural regulators of K. brevis blooms, and ultimately assess possible management applications. Herein, the algicidal activity of a newly isolated Cytophaga/Flavobacterium/Bacteroidetes (CFB)-bacterium, strain S03, and a previously described CFB-bacterium, strain 41-DBG2, was evaluated against various harmful algal bloom (HAB) and non-HAB species (23 total), including multiple clones of K. brevis, to evaluate algal target specificity. Strains S03 and 41-DBG2, which employ direct and indirect modes of algicidal lysis, respectively, killed 20% and 40% of the bacteria-containing isolates tested. Interestingly, no bacteria-free algal cultures were resistant to algicidal attack, whereas susceptibility varied occasionally among bacteria-containing isolates of a single algal taxon originating from either the same or different geographic location. The dynamics of K. brevis culture death appeared to differ according to whether the algicidal bacterium did or did not require direct contact with algal cells, with the former most rapidly affecting K. brevis morphology and causing cell lysis. Both bacterial strains promoted the formation of a small number of cyst-like structures in the K. brevis cultures, possibly analogous to temporary cysts formed by other dinoflagellates exposed to certain types of stress. Results were also consistent with earlier work demonstrating that bacterial assemblages from certain cultures can confer resistance to attack by algicidal bacteria, again indicating the complexity and importance of microbial interactions, and the need to consider carefully the potential for using such bacteria in management activities.     

Microbial community interactions and population dynamics of an algicidal bacterium active against Karenia brevis (Dinophyceae)

The population dynamics of Cytophaga strain 41-DBG2, a bacterium algicidal to the harmful algal bloom (HAB) dinoflagellate Karenia brevis, were investigated in laboratory experiments using fluorescent in-situ hybridization (FISH) and denaturing gradient gel electrophoresis (DGGE). Following its introduction into non-axenic K. brevis cultures at concentrations of 10³ or 10⁵ bacterial cells per milliliter, 41-DBG2 increased to 10⁶ cells per milliliter before initiation of its algicidal activity. Such threshold concentrations were not achieved when starting algal cell numbers were relatively low (10³ cells per milliliter), suggesting that the growth of this bacterium may require high levels of dissolved organic matter (DOM) excreted by the algae. It remains to be determined whether this threshold concentration is required to trigger an algicidal response by 41-DBG2 or, alternatively, is the point at which the bacterium accumulates to an effective killing concentration. The ambient microbial community associated with these algal cultures, as determined by DGGE profiles, did not change until after K. brevis cells were in the process of lysing, indicating a response to the rapid input of algal-derived organic matter. Resistance to algicidal attack exhibited by several K. brevis clones was found to result from the inhibition of 41-DBG2 growth in the presence of currently unculturable bacteria associated with those clones. These bacteria apparently prevented 41-DBG2 from reaching the threshold concentration required for initiation of algicidal activity. Remarkably, resistance and susceptibility to the algicidal activity of 41-DBG2 could be transferred between K. brevis clones with the exchange of their respective unattached bacterial communities, which included several dominant phylotypes belonging to the α-proteobacteria, γ-proteobacteria, and Cytophaga–Flavobacterium–Bacteroides (CFB) groups. We hypothesize that CFB bacteria may be successfully competing with 41-DBG2 (also a member of the CFB) for nutrients, thereby inhibiting growth of the latter and indirectly providing resistance against algicidal attack. We conclude that if algicidal bacteria play a significant role in regulating HAB dynamics, as some authors have inferred, bacterial community interactions are crucial factors that must be taken into consideration in future studies.     

Dynamics of seagrass bed microbial communities in artificial Chattonella blooms: A laboratory microcosm study

The influence of algicidal and growth-inhibiting bacteria in a seagrass (Zostera marina) bed, and their capability of controlling blooms of the fish-killing raphidophyte flagellate, Chattonella antiqua, were examined in laboratory microcosm experiments. Bacterial communities in seawater collected from the seagrass bed and Z. marina biofilm suppressed artificial Chattonella blooms in the presence of their natural competitors and predators. Phylogenetic analysis suggest that considerable numbers of bacteria that suppress Chattonella, including algicidal or growth-inhibiting bacteria isolated from seagrass biofilm and seawater from the seagrass bed, are members of Proteobacteria that can decompose lignocellulosic compounds. A direct comparison of partial 16S rRNA gene sequences (500 bp) revealed that the growth-limiting bacterium (strain ZM101) isolated from Z. marina biofilm belonged to the genus Phaeobacter (Alphaproteobacteria) showed 100% similarity with strains of growth-limiting bacteria isolated from seawater of both the seagrass bed and nearshore region, suggesting that the origin of these growth-limiting bacteria are the seagrass biofilm or seawater surrounding the seagrass bed. This study demonstrates that Chattonella growth-limiting bacteria living on seagrass biofilm and in the adjacent seawater can suppress Chattonella blooms, suggesting the possibility of Chattonella bloom prevention through restoration, protection, or introduction of seagrass in coastal areas.     

ALGICIDAL BACTERIA ACTIVE AGAINST GYMNODINIUM BREVE (DINOPHYCEAE). I. BACTERIAL ISOLATION AND CHARACTERIZATION OF KILLING ACTIVITY1,3

Interactions between bacteria and species of harmful and/or toxic algae are potentially important factors affecting both the population dynamics and the toxicity of these algae. Recent reports of bacteria lethal to certain harmful algal bloom (HAB) species, coupled with a rapidly evolving interest in attempting to minimize the adverse effects of HABs through various prevention, control, and mitigation strategies, have focused attention on defining the role of algicidal bacteria in bloom termination. The aim of the present study was to determine whether algicidal bacteria active against Gymnodinium breve Davis, a dinoflagellate responsible for frequent and protracted red tides in the Gulf of Mexico, are present in the waters of the west Florida shelf. To date, we have isolated two bacterial strains from this region lethal to G. breve and have begun to characterize the algicidal activity of one of these strains, 41-DBG2. This bacterium, a yellow-pigmented, gram-negative rod, was isolated from waters containing no detectable G. breve cells, suggesting that such bacteria are part of the ambient microbial community and are not restricted to areas of high G. breve abundance. Strain 41-DBG2 produced a dissolved algicidal compound(s) that was released into the growth medium, and the algicide was effective against the four Gulf of Mexico G. breve isolates tested as well as a closely related HAB species that also occurs in this region, Gymnodinium mikimotoi Miyake et Kominami ex Oda. Nonetheless, data showing that a nontoxic isolate of Gymnodinium sanguineum Hirasaka from Florida Bay was not affected indicate that the algicidal activity of this bacterium does exhibit a degree of taxonomic specificity. Our efforts are currently being directed at resolving several critical issues, including the identity of the algicide(s), the mechanisms regulating its production and ability to discriminate between target algal species, and how the growth rate of 41-DBG2 is affected by the presence of G. breve cells. We have also proposed a conceptual model for interactions between algicidal bacteria and their target species to serve as a testable framework for ensuing field studies.     

The Algicidal Characteristics of One Algae-Lysing FDT5 Bacterium on Microcystis aeruginosa

One strain of algicidal bacterium which can inhibit Harmful algal blooms (HABs), FDT5, was isolated from activated sludge and found to have good algicidal effects on Microcystis aeruginosa. It was revealed that: The FDT5 was a Gram-negative bacterium and identified as Ochrobactrum sp.; The greater the initial bacterial cell density, the faster the degradation of chlorophyll a.; The algicidal efficiency was evaluated at the most favorable conditions which were a temperature of 30–35°C, a pH of 7.6 and complete darkness; The FDT5 strain lysed Microcystis aeruginosa not directly but by secreting metabolites which could withstand high temperatures and pressure.     

Dynamics of cyanophage-like particles and algicidal bacteria causing Microcystis aeruginosa mortality

The dynamics of cyanophage-like particles and algicidal bacteria that infect the bloom-forming cyanobacterium Microcystis aeruginosa was followed in a hyper-eutrophic pond from September 1998 to August 1999. The densities of M. aeruginosa ranged between 4.0 × 10⁵ and 1.9 × 10⁷ cells ml⁻¹, whereas those of algicidal bacteria were between 4.0 and 5.1 × 10² plaque-forming units (PFU) ml⁻¹ and those of cyanophage-like particles were between <5.0 × 10² and 7.1 × 10³ PFU ml⁻¹. A significant relationship was found between the densities of algicidal bacteria and M. aeruginosa (r = 0.81, n = 69, P < 0.001), suggesting that the dynamics of the algicidal bacteria may regulate the abundance of M. aeruginosa. Occasional peaks of density of cyanophage-like particles were detected in October, June, and August, when sharp declines in M. aeruginosa cell densities were also observed. The densities of cyanophage-like particles became undetectable when the abundance of M. aeruginosa was low, suggesting the density-dependent infection of M. aeruginosa by cyanophage-like particles. Thus, we suggest that infections of both algicidal bacteria and cyanophage-like particles are important biological agents that decompose blooms of M. aeruginosa in freshwater environments. Received: August 31, 2000 / Accepted: December 6, 2000     

Revealing the algicidal characteristics of Maribacter dokdonensis: An investigation into bacterial strain P4 isolated from Karenia mikimotoi bloom water

Harmful algal blooms (HABs) are a global environmental concern, causing significant economic losses in fisheries and posing risks to human health. Algicidal bacteria have been suggested as a potential solution to control HABs, but their algicidal efficacy is influenced by various factors. This study aimed to characterize a novel algicidal bacterium, Maribacter dokdonensis (P4), isolated from a Karenia mikimotoi (Hong Kong strain, KMHK) HAB and assess the impact of P4 and KMHK's doses, growth phase, and algicidal mode and the axenicity of KMHK on P4's algicidal effect. Our results demonstrated that the algicidal effect of P4 was dose-dependent, with the highest efficacy at a dose of 25% v/v. The study also determined that P4's algicidal effect was indirect, with the P4 culture and the supernatant, but not the bacterial cells, showing significant effects. The algicidal efficacy was higher when both P4 and KMHK were in the stationary phase. Furthermore, the P4 culture at the log phase could effectively kill KMHK cells at the stationary phase, with higher algicidal efficacy in the bacterial culture than that of the supernatant alone. Interestingly, P4's algicidal efficacy was significantly higher when co-culturing with xenic KMHK (~90% efficacy at day 1) than that with the axenic KMHK (~50% efficacy at day 1), suggesting the presence of other bacteria could regulate P4's algicidal effect. The bacterial strain P4 also exhibited remarkable algicidal efficacy on four other dinoflagellate species, particularly the armored species. These results provide valuable insights into the algicidal effect of M. dokdonensis on K. mikimotoi and on their interactions.