Marine bacteria antagonistic to the harmful algal bloom species Alexandrium tamarense (Dinophyceae)

As part of efforts to enhance the strategies employed to manage and mitigate algal blooms and their adverse effects, algicidal bacteria have shown promise as potential suppressors of these events. Nine strains of bacteria algicidal against the toxic dinoflagellate, Alexandrium tamarense, were isolated from the East Sea area, China. Sequence analysis of 16S rDNA showed that all the algicidal bacteria belonged to the γ-proteobacteria subclass and the genera Pseudoalteromonas (strain SP31 and SP44), Alteromonas (strain DH12 and DH46), Idiomarina (strain SP96), Vibrio (strain DH47 and DH51) and Halomonas (strain DH74 and DH77). To assess the algicidal mode of these algicidal bacteria, bacterial cells and the filtrate from bacterial cultures were inoculated into A. tamarense cultures, and fluorescein diacetate vital stain was applied to monitor the growth of the algal cells. The results showed that all the algicidal bacteria exhibited algicidal activity through an indirect attack since algicidal activity was only detected in cell free supernatants but not the bacterial cells. This is the first report of bacteria from the genus Idiomarina showing algicidal activity to the toxic dinoflagellate A. tamarense and these findings would increase our knowledge of bacterial–algal interactions and the role of bacteria during the population dynamics of HABs.     

Genome sequence of the algicidal bacterium Kordia algicida OT-1

Kordia algicida OT-1 is an algicidal bacterium against the bloom-forming microalgae. The genome sequence of K. algicida revealed a number of interesting features, including the degradation of macromolecules, the biosynthesis of carotenoid pigment and secondary metabolites, and the capacity for gliding motility, which might facilitate the understanding of algicidal mechanisms.     

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.     

Growth and release of extracellular organic compounds by benthic diatoms depend on interactions with bacteria

Phototrophic epilithic biofilms harbour a distinct assemblage of heterotrophic bacteria, cyanobacteria and photoautotrophic algae. Secretion of extracellular polymeric substances (EPS) by these organisms and the physicochemical properties of the EPS are important factors for the development of the biofilms. We have isolated representative diatom and bacteria strains from epilithic biofilms of Lake Constance. By pairwise co-cultivating these strains we found that diatom growth and EPS secretion by diatoms may depend on the presence of individual bacteria. Similar results were obtained after addition of spent bacterial medium to diatom cultures, suggesting that soluble substances from bacteria have an impact on diatom physiology. While searching for putative bacterial signal substances, we found that concentrations of various dissolved free amino acids (DFAA) within the diatom cultures changed drastically during co-cultivation with bacteria. Further, the secretion of extracellular carbohydrates and proteins can be influenced by bacteria or their extracellular substances. We have performed mass spectrometric peptide mapping to identify proteins which are secreted when co-cultivating the diatom Phaeodactylum tricornutum Bohlin and Escherichia coli. The identified proteins are possibly involved in signalling, extracellular carbohydrate modification and uptake, protein and amino acid modification, and cell/cell aggregation of diatom and bacteria strains. Our data indicate that diatom-bacteria biofilms might be regulated by a complex network of chemical factors involving EPS, amino acid monomers and other substances. Thus interactions with bacteria can be considered as one of the main factors driving biofilm formation by benthic diatoms.     

Wheat Bran Enhances the Cytotoxicity of Immobilized Alcaligenes aquatilis F8 against Microcystis aeruginosa

Algicidal bacteria offer a promising option for killing cyanobacteria. Therefore, a new Alcaligenes aquatilis strain F8 was isolated to control Microcystis aeruginosa in this study. The algicidal activity of strain F8 was dependent on the cell density of M. aeruginosa, and the maximal algicidal rate of the free bacterium reached 88.45% within 72 h. With a view to its application to the control of M. aeruginosa in the natural environment, strain F8 was immobilized in sodium alginate beads, but immobilization of the strain decreased its algicidal rate compared to that of the free bacterium. However, addition of wheat bran to the sodium alginate matrix used to immobilize strain F8 not only eliminated the adverse effects of immobilization on the bacteria but also resulted in an 8.83% higher algicidal rate of the immobilized than free bacteria. Exclusion and recovery methods were used to identify key ingredients of wheat bran and gain insight into the mechanism underlying the observed enhancement of algicidal activity. This analysis indicated that certain factors in wheat bran, including vitamins B₁, B₂, B₉, and E were responsible for promoting bacterial growth and thereby improving the algicidal rate of immobilized strain F8. Our findings indicate that wheat bran is able to improve the algicidal efficiency of A. aquatilis strain F8 for killing M. aeruginosa and is a good source of not only carbon and nitrogen but also vitamins for bacteria.     

Algicidal bacteria in the sea and their impact on algal blooms

Over the past two decades, many reports have revealed the existence of bacteria capable of killing phytoplankton. These algicidal bacteria sometimes increase in abundance concurrently with the decline of algal blooms, suggesting that they may affect algal bloom dynamics. Here, we synthesize the existing knowledge on algicidal bacteria interactions with marine eukaryotic microalgae. We discuss the effectiveness of the current methods to characterize the algicidal phenotype in an ecosystem context. We briefly consider the literature on the phylogenetic identification of algicidal bacteria, their interaction with their algal prey, the characterization of algicidal molecules, and the enumeration of algicidal bacteria during algal blooms. We conclude that, due to limitations of current methods, the evidence for algicidal bacteria causing algal bloom decline is circumstantial. New methods and an ecosystem approach are needed to test hypotheses on the impact of algicidal bacteria in algal bloom dynamics. This will require enlarging the scope of inquiry from its current focus on the potential utility of algicidal bacteria in the control of harmful algal blooms. We suggest conceptualizing bacterial algicidy within the general problem of bacterial regulation of algal community structure in the ocean.     

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.     

Isolation and algicidal characterization of <Emphasis Type="Italic">Bowmanella denitrificans</Emphasis> S088 against <Emphasis Type="Italic">Chlorella vulgaris</Emphasis>

One strain of algicidal bacterium, named as S088, was isolated from the intestine of healthy sea cucumbers (Stichopus horrens) in the South China Sea. Based on the analysis of its biochemical characteristics and 16S rDNA gene sequence, S088 was identified as Bowmanella denitrificans. Importantly, the algicidal activity of S088 on Chlorella vulgaris was characterized in this study. The initial densities of bacterial and algal cell showed strong influence on the removal rates of chlorophyll a. When the strain S088 was cultured under a complete darkness condition at 30 °C, its algicidal activity reached the highest level. Furthermore, it was found that the filtered supernatant from bacterial cultures had full algicidal activity, suggesting that the secreted compounds from S088 are involved in the observed algicidal action of S088. Moreover, the algicidal compounds were heat tolerant and had no cytotoxicity against fish cells, indicating that S088 would have a promising application as a safe probiotics for S. horrens. Finally, this is the first report about the algicidal activities in B. denitrificans.     

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.     

Bacteroides intermedius binds fibrinogen.

The binding of Bacteroides intermedius VPI 8944 to human fibrinogen has been characterized. The binding is time dependent, at least partially reversible, saturable, and specific. On an average, a maximum of 3,500 fibrinogen molecules bind per bacterial cell, with a dissociation constant of 1.7 X 10(-11) M. These bacteria also exhibit a fibrinogenolytic activity which can be partially inhibited by protease inhibitors. Bacteria release fibrinogenolytic activity into the surrounding medium without loss of binding activity, but more pronounced fibrinogen breakdown occurs when 125I-labeled fibrinogen is associated with the bacteria, suggesting that fibrinogen is degraded at the cell surface. Fibrinogen binding by B. intermedius might represent a mechanism of bacterial tissue adherence.     

A marine bacterium producing protein with algicidal activity against Alexandrium tamarense

Interactions between bacteria and harmful algal bloom (HAB) species have been acknowledged as an important factor of regulating the population of these algae. In the study, two strains of algicidal bacteria, DHQ25 and DHY3, were screened out because of their probably secreting algicidal proteins against axenic Alexandrium tamarense. Molecular characterization classified them to the γ-proteobacteria subclass and to the genus Vibrio and Pseudoalteromonas, respectively. After centrifugation and ultrafiltration, chromatography of the cultural supernatants of DHQ25 revealed 8 peaks by HPLC. SDS-PAGE and Native PAGE determination showed that peak 7 to be a monoband peak. Both xenic and axenic culture of A. tamarense were susceptible to the purified protein (short for P7 below) indicated by algicidal activity assay. Observation of algicidal process demonstrated that algal cells were lysed and cellular substances were released under visual fields of microscope. P7 proved to be a challenge controller of A. tamarense by the above characterizations of algicidal activity assaying and algicidal process. This is the first report of a protein algicidal to the toxic dinoflagellate A. tamarense. The findings increase our knowledge of bacterial–algal interactions and the role of bacteria during controlling HABs.     

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.     

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.     

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.     

Isolation and characterization of algicidal bacteria from <Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> culture

In this study, we analyzed a bacterial community closely associated with Cochlodinium polykrikoides that caused harmful algal blooming in the sea. Filtration using a plankton mesh and percoll gradient centrifugation were performed to eliminate free-living bacteria. Attached bacteria were analyzed by culture-dependent and culture-independent methods. Five culturable bacterial strains were isolated and identified from the C. polykrikoides mixed bacterial community. The isolates belonged to α-Proteobacteria (Nautella sp., Sagittula sp., and Thalassobius sp.) and γ-Proteobacteria (Alteromonas sp. and Pseudoalteromonas sp.). All of the 5 isolates showed algicidal activity against C. polykrikoides and produced extracellular compounds responsible for algicidal properties after entering the stationary phase. The algicidal compounds produced by the 5 isolates were heat-stable and had molecular masses of less than 10,000 Da. Furthermore, the algicidal compounds were relatively specific for C. polykrikoides in terms of their algicidal activities. Culture-independent analysis of the bacterial community in association with C. polykrikoides was performed using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). On the basis of the PCR-DGGE profile, Sagittula sp. was identified as a dominant species in the bacterial community of C. polykrikoides.     

A robust plate assay for detection of extracellular microbial protease activity in metagenomic screens and pure cultures

A robust, efficient and cost-effective agar that utilises lactose free milk powder for identification of bacterial protease activity in pure cultures and metagenomic screens has been developed and tested on protease positive bacteria, selected strains and false protease positives isolated from a previously constructed metagenomic library.     

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.     

Stimulative and inhibitory effects of bacteria on the growth of microalgae

Several examples of stimulative and inhibitoryeffects of bacteria on microalgal growth areintroduced, and the importance of bacteria in algalmass culture is investigated. Diatoms are often usedas live food for planktonic larvae of sea urchin andbivalves. Monodispersed Chaetoceros ceratosporum hasbeen cultivated by using clean, high nutrient content,deep seawater (DSW). However, the growth rate and cellyield of diatoms fluctuated, to relatively largeextent, with the season that DSW was collected. Whensome bacterial strains isolated from DSW were added tothe culture, diatom growth was often stimulated and arelatively constant cell yield was obtained. Anotherdiatom species, C. gracilis, was also stimulated byadding some bacterial strains to cultures. Thepositive effect of bacteria on diatoms was observednot only for planktonic species, but also on attachedspecies. A benthic diatom, Nitzschia sp., wasstimulated by a bacterial film of Alcaligenes on thesurface of the substratum. On the other hand, a strainof Flavobacterium sp. isolated from natural seawaterduring the decline period of an algal bloom had a strongalgicidal effect on the red tide plankton,Gymnodinium mikimotoi. Recent reports demonstratethat many bacterial strains have significantalgicidal effects on many species of red tideplankton. These results indicate that bacterialeffects should be taken into account to obtain stablemass culture of food microalgae.     

Inputs of seabird guano alter microbial growth,community composition and the phytoplankton–bacterial interactions in a coastal system

Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guano stimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton–bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton–bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria.     

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.