Red Tide Dinoflagellate Research Articles

The effects of the harmful alga Heterosigma akashiwo on cultures of Schmackeria inopinus (Copepoda, Calanoida)

Harmful effects of red tide dinoflagellates on copepods have gained particular attention in recent years. In this study, monocultures and mixed cultures of the harmful alga Heterosigma akashiwo and the non-harmful alga Isochrysis galbana were utilized as diets to investigate the ingestion, reproduction and recruitment of the copepod Schmackeria inopinus. When compared with a monoculture of I. galbana, a sole diet H. akashiwo apparently inhibited ingestion rates, proportions of gravid female (PGFs), egg production rates (EPRs), gross growth efficiencies (GGEs) and recruitment of S. inopinus. We found that the harmful effects of H. akashiwo on the abundance of S. inopinus were concentration- and time-dependent. S. inopinus exhibited significantly reduced ingestion rates, EPRs and GGEs when they were fed on food mixtures of H. akashiwo (0–75% of I. galbana) in comparison with the control diet (100% of I. galbana). The data on ingestion rates and EPRs in the diet mixtures suggest that H. akashiwo is toxic to S. inopinus, which might be attributable to the glycocalyx structures in the H. akashiwo cell surface. The results in this study indicate that a bloom of H. akashiwo could cause significant stress to S. inopinus.

Establishment of absolute stereostructure of falcarindiol, algicidal principle against Heterocapsa circularisquama from Notopterygii Rhizoma

Abstract Falcarindiol (1) was isolated as an algicidal principle against the harmful red tide dinoflagellate, Heterocapsa circularisquama, from Notopterygii Rhizoma through bioassay-guided separation. In order to determine the ambiguous absolute structure of this active principle, all three stereoisomers as well as falcarindiol (1) were synthesized. As a result of intensive analysis of their physicochemical properties, the configuration of 1 was revealed to be 3R,8S. On the other hand, (3S,8S)- and (3S,8R)-isomers were found to exhibit more potent algicidal activity than (3R,8S)-falcarindiol (1) isolated from Notopterygii Rhizoma. In addition, the diyne moiety of 1 was established as the crucial structural requirement for algicidal potency.

Synthesis of the ABC Ring Fragment of Brevisin, a New Dinoflagellate Polycyclic Ether

A polycyclic ether, brevisin was isolated from the red-tide dinoflagellate Karenia brevis. Its unique skeletal structure consists of two separate tricyclic ether assemblies connected by a methylene bridge. The ABC ring fragment of brevisin was synthesized via Suzuki―Miyaura cross coupling toward a total synthesis of brevisin.

Genome-Wide Screening and Identification of Factors Affecting the Biosynthesis of Prodigiosin by Hahella chejuensis , Using Escherichia coli as a Surrogate Host

A marine bacterium, Hahella chejuensis, recently has attracted attention due to its lytic activity against a red-tide dinoflagellate. The algicidal function originates from its red pigment, prodigiosin, which also exhibits immunosuppressive or anticancer activity. Genome sequencing and functional analysis revealed a gene set contained in the hap gene cluster that is responsible for the biosynthesis of prodigiosin. To screen for the factors affecting the prodigiosin biosynthesis, we constructed a plasmid library of the H. chejuensis genomic DNA, introduced it into Escherichia coli strains harboring the hap cluster, and observed changes in production of the red pigment. Among the screened clones, hapXY genes whose products constitute a two-component signal transduction system were elucidated as positive regulators of the pigment production. In addition, an Hfq-dependent, noncoding region located at one end of the hap cluster was confirmed to play roles in regulation. Identification of factors involved in the regulation of prodigiosin biosynthesis should help in understanding how the prodigiosin-biosynthetic pathway is organized and controlled and also aid in modulating the overexpression of prodigiosin in a heterologous host, such as E. coli, or in the natural producer, H. chejuensis.

Contrasting spring and summer phytoplankton dynamics in the nearshore Southern California Bight

We analyzed a 9‐yr record of ocean color data from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) together with a 4‐yr record of satellite‐derived sea surface temperature (SST) measurements to investigate potential mechanisms influencing nearshore phytoplankton variability in the Southern California Bight. We also compared in situ physicochemical conditions at four shoreline stations during a year with an extensive nearshore dinoflagellate red tide (2005) to those of the following year when there was no extended red tide (2006). SeaWiFS data indicated that in spring, chlorophyll a (Chl a) and nitrate (NO3−) were inversely correlated to SSTs, consistent with an upwelling‐dominated system, but in summer Chl a and NO3− were unrelated to SST. Summer dissolved inorganic nitrogen concentrations were not well correlated between alongshore sampling stations, but showed a significant effect of tide stage (low vs. high). Mean summer nearshore Chl a was positively, linearly related to rainfall in the previous winter. The relationship between summer Chl a and winter precipitation could be due either to direct freshwater inputs during the winter rainy season or to delayed input via groundwater discharge. A first‐order analysis of nutrient delivery from both surface runoff and groundwater discharge showed that they could sustain summer blooms of up to 7540 and 1700 km2, respectively—areas on the order of the study site. However, precipitation‐induced physical changes to the nearshore water column cannot be conclusively ruled out as a potential causative mechanism.

Allelopathic effects of Alexandrium tamarense on other algae: evidence from mixed growth experiments

The effect of 2 strains (Alex2 and Alex5) of the marine red tide dinoflagellate Alexan- drium tamarense on 10 other planktonic algal target species common in temperate waters was stud- ied in mixed growth experiments under nutrient-rich conditions. In a comparative approach, the 2 strains of A. tamarense, similar in their cellular paralytic shellfish toxin (PST) content, were selected because of their fundamentally different lytic potencies. The Alex2 strain clearly affected all target algae while the Alex5 strain had no negative effect on the growth of any of the target species during the study period, even though cell concentrations of Alex5 became very high (2 × 10 4 cells ml -1 ). As both strains contained comparable amounts of PST, this confirmed previous suggestions that so far unidentified compounds are causing the negative effects on other algae. Sensitivity of the tested algae to Alex2 differed considerably. The growth of some species was affected at very low Alex2 cell concentrations (<10 2 cells ml -1 ), while the growth of other algae was not affected until cell concentra- tions exceeded 10 3 cells ml -1 . While a complete dieoff was the ultimate fate for almost all target spe- cies when grown in mixed culture with Alex2, Scrippsiella trochoidea formed temporary cysts, the number of which remained constant during the course of the experiment. The pH in the mixed cul- tures increased as the cultures grew dense. This had a substantial effect on Alex5 in the mixed cul- tures, in which Alex5 eventually died off because the target species have a higher tolerance to high pH. pH values did not determine the outcome of the experiments with Alex2 because the adverse effects of Alex2 on the growth of the other algae was evident before pH values became too high. Lytic extracellular compounds, which are produced by the large majority of A. tamarense strains tested so far, clearly have the potential to benefit this dinoflagellate by reducing competitor growth rates.

Effects of the Florida red tide dinoflagellate, Karenia brevis, on oxidative stress and metamorphosis of larvae of the coral Porites astreoides

Florida red tides, caused by the dinoflagellate Karenia brevis, produce a series of neurotoxins, brevetoxins, which have historically had a negative impact on coastal communities along the South Florida coast. While much work has examined the effects of K. brevis blooms or exposure to brevetoxins on a diverse array of marine organisms, there have been no reports studying the impact of K. brevis on coral physiology. We provide evidence that short-term exposure of naturally occurring concentrations of K. brevis and their associated toxins can induce oxidative stress in the coral larvae of Porites astreoides. Larvae of P. astreoides were exposed to aliquots of intact K. brevis cells at naturally occurring concentrations (6 × 10 5 – 4 × 10 6 cells L −1) or cellular lysates (containing 5.4–15.0 μg L −1 brevetoxin) for 20 h and a variety of larval physiological biomarkers were measured. In the presence of bloom scale concentrations of K. brevis larval respiration of P. astreoides was inhibited. This was accompanied with an increase in lipid hydroperoxide content and catalase activity indicating oxidative damage and a subsequent antioxidant response were occurring. However, when larvae were exposed to intact cells of K. brevis or their sonicated, lysed exudates stress indices including zooxanthellae expulsion (bleaching), larval protein carbonylation, and superoxide dismutase activity were not significantly altered. Even though there was evidence for short-term oxidative stress in larvae that were exposed to K. brevis there was no change in survival or settlement rates of larvae of P. astreoides.

Feeding by Phototrophic Red‐Tide Dinoflagellates on the Ubiquitous Marine Diatom Skeletonema costatum

We investigated feeding by phototrophic red-tide dinoflagellates on the ubiquitous diatom Skeletonema costatum to explore whether dinoflagellates are able to feed on S. costatum, inside the protoplasm of target dinoflagellate cells observed under compound microscope, confocal microscope, epifluorescence microscope, and transmission electron microscope (TEM) after adding living and fluorescently labeled S. costatum (FLSc). To explore effects of dinoflagellate predator size on ingestion rates of S. costatum, we measured ingestion rates of seven dinoflagellates at a single prey concentration. In addition, we measured ingestion rates of the common phototrophic dinoflagellates Prorocentrum micans and Gonyaulax polygramma on S. costatum as a function of prey concentration. We calculated grazing coefficients by combining field data on abundances of P. micans and G. polygramma on co-occurring S. costatum with laboratory data on ingestion rates obtained in the present study. All phototrophic dinoflagellate predators tested (i.e. Akashiwo sanguinea, Amphidinium carterae, Alexandrium catenella, Alexandrium tamarense, Cochlodinium polykrikoides, G. polygramma, Gymnodinium catenatum, Gymnodinium impudicum, Heterocapsa rotundata, Heterocapsa triquetra, Lingulodinium polyedrum, Prorocentrum donghaiense, P. micans, Prorocentrum minimum, Prorocentrum triestinum, and Scrippsiella trochoidea) were able to ingest S. costatum. When mean prey concentrations were 170-260 ng C/ml (i.e. 6,500-10,000 cells/ml), the ingestion rates of G. polygramma, H. rotundata, H. triquetra, L. polyedrum, P. donghaiense, P. micans, and P. triestinum on S. costatum (0.007-0.081 ng C/dinoflagellate/d [0.2-3.0 cells/dinoflagellate/d]) were positively correlated with predator size. With increasing mean prey concentration of ca 1-3,440 ng C/ml (40-132,200 cells/ml), the ingestion rates of P. micans and G. polygramma on S. costatum continuously increased. At the given prey concentrations, the maximum ingestion rates of P. micans and G. polygramma on S. costatum (0.344-0.345 ng C/grazer/d; 13 cells/grazer/d) were almost the same. The maximum clearance rates of P. micans and G. polygramma on S. costatum were 0.165 and 0.020 microl/grazer/h, respectively. The calculated grazing coefficients of P. micans and G. polygramma on co-occurring S. costatum were up to 0.100 and 0.222 h, respectively (i.e. up to 10% and 20% of S. costatum populations were removed by P. micans and G. polygramma populations in 1 h, respectively). Our results suggest that P. micans and G. polygramma sometimes have a considerable grazing impact on populations of S. costatum.

Degradation of Cochlodinium polykrikoides using photocatalytic reactor with TiO2-coated alumina

Cochlodinium polykrikoides (C. polykrikoides) is one of the most harmful red tide dinoflagellates due to its great economic damage and compromising recreational opportunity and public health. Titanium dioxide (TiO2) is known to be used as a photocatalyst for the control of the aquatic invasive algae under natural and artificial light. The purpose of this study was to design a highly efficient continuous photocatalytic reactor with TiO2-coated alumina for the demonstrated efficient degradation of C. polykrikoides. TiO2 photocatalyst beads were prepared by sol-gel dip-coating method using titanium tetra iso-propoxide on alumina. After 40 min of ultraviolet illumination time, the reduction of C. polykrikoides cell number was more than 80%. The degree of degradation of C. polykrikoides increased in a time-dependent manner in this novel reactor. The degradation begins with photocatalytic action by the oxidative species of TiO2 on the protective cell structures of C. polykrikoides.

Benthic Herbivores are not Deterred by Brevetoxins Produced by the Red Tide Dinoflagellate Karenia Brevis

Gulf of Mexico blooms of the dinoflagellate Karenia brevis produce neurotoxic cyclic polyethers called brevetoxins. During and after a red tide bloom in southwestern Florida, K. brevis cells lyse and release brevetoxins, which then sink to the benthos and coat the surfaces of seagrasses and their epiphytes. We tested the possibility that these brevetoxin-laden foods alter the feeding behavior and fitness of a common benthic herbivore within Floridean seagrass beds, the amphipod Ampithoe longimana. We demonstrated that coating foods with K. brevis extracts that contain brevetoxins at post-bloom concentrations (1 microg g(-1) drymass) does not alter the feeding rates of Florida nor North Carolina populations of A. longimana, although a slight deterrent effect was found at eight and ten-fold greater concentrations. During a series of feeding choice assays, A. longimana tended not to be deterred by foods coated with K. brevis extracts nor with the purified brevetoxins PbTx-2 and PbTx-3. Florida juveniles isolated with either extract-coated or control foods for 10 days did not differ in survivorship nor growth. A similar lack of feeding response to brevetoxin-laden foods also was exhibited by two other generalist herbivores of the southeastern United States, the amphipod A. valida and the urchin Arbacia punctulata. Given that benthic mesograzers constitute a significant portion of the diet for the juvenile stage of many nearshore fishes, we hypothesize that the ability of some mesograzers to feed on and retain brevetoxins in their bodies indicates that mesograzers may represent an important route of vertical transmission of brevetoxins through higher trophic levels within Gulf of Mexico estuaries.

Effect of fully characterized unsteady flow on population growth of the dinoflagellate Lingulodinium polyedrum

Dinoflagellate population growth is inhibited by fluid motion, which is typically characterized by some average flow property, regardless if the flow is steady or unsteady. This study compares the effect of fully characterized steady and unsteady flow on net population growth of the red tide dinoflagellate Lingulodinium polyedrum. The unsteady flow fields were generated using oscillatory laminar Couette flow and characterized analytically to provide complete knowledge of the fluid shear exposure over space and time throughout the chamber. Experimental conditions were selected so all cells experienced a similar shear exposure regardless of their position within the chamber. Unsteady flow with maximum shears of 6.4 s−1 and 6.7 s−1 and an average absolute shear of 4 s−1, comparable with levels found at the ocean surface on a windy day, resulted in higher levels of growth inhibition than for steady Couette flow with shears of 4 and 8 s−1. Over the parameter space studied, growth inhibition increased with increasing treatment duration (5–120 min) but was insensitive to oscillation period (60–600 s) or whether the unsteady flow changed in direction. These results indicate that over the parameter space studied, unsteady flow is more inhibitory to net growth than steady flow, for the same average flow conditions, and demonstrate that flow characterization on the basis only of average flow properties is inadequate for comparing population growth in unsteady and steady flows.

Active prey selection in two pelagic copepods feeding on potentially toxic and non-toxic dinoflagellates

Grazing on two red tide dinoflagellates, the potentially toxic Karenia mikimotoi and the non-toxic Gyrodinium instriatum, was examined in two species of marine copepods, Pseudocalanus elongatus and Temora longicornis. Both copepods cleared K. mikimotoi at rates that were a little lower but comparable to those at which they cleared the slightly larger G. instriatum when the two dinoflagellates were offered separately. However, when feeding on mixtures of the two prey species, the clearance rates on K. mikimotoi were substantially reduced in both copepods while their clearances of G. instiatum remained unaltered, suggesting active prey selection. Video observations of individual prey capture and feeding events showed prey rejection frequencies (caught and then released cells) that did not differ between mixed and mono-specific diets. This suggests that the selection between prey cells occurs prior to capture and that it is based on remote characterization of the cells.

Isotopic evidence for dead fish maintenance of Florida red tides, with implications for coastal fisheries over both source regions of the West Florida shelf and within downstream waters of the South Atlantic Bight

Toxic Florida red tides of the dinoflagellate Karenia brevis have downstream consequences of 500–1000 km spatial extent. Fish stocks, shellfish beds, and harmful algal blooms of similar species occupy the same continental shelf waters of the southeastern United States, amounting to economic losses of more than 25 million dollars in some years. Under the aegis of the Center for Prediction of Red tides, we are now developing coupled biophysical models of the conditions that lead to red tides and impacted coastal fisheries, from the Florida Panhandle to Cape Hatteras. Here, a nitrogen isotope budget of the coastal food web of the West Florida shelf (WFS) and the downstream South Atlantic Bight (SAB) reaffirms that diazotrophs are the initial nutrient source for onset of red tides and now identifies clupeid fish as the major recycled nutrient source for their maintenance. The recent isotope budget of WFS and SAB coastal waters during 1998–2001 indicates that since prehistoric times of Timacua Indian settlements along the Georgia coast during 1075, ∼50% of the nutrients required for large red tides of >1 μg chl l −1 of K. brevis have been derived from nitrogen-fixers, with the other half from decomposing dead sardines and herrings. During 2001, >90% of the harvest of WFS clupeids was by large ichthyotoxic red tides of >10 μg chl l −1 of K. brevis, rather than by fishermen. After onset of the usual red tides in summer of 2006 and 2007, the simulated subsequent fall exports of Florida red tides in September 2007 to North Carolina shelf waters replicate observations of just ∼1 μg chl l −1 on the WFS that year. In contrast, the earlier red tides of >10 μg chl l −1 left behind off West Florida during 2006, with less physical export, are instead 10-fold larger than those of 2007. Earlier, 55 fish kills were associated with these coastal red tides during September 2006, between Tampa and Naples. Yet, only six fish kills were reported there in September 2007. With little export of red tides and their fish prey during the former year, the computed larger nutrient-sated, fish-fed growth rates of the model’s dinoflagellates also replicate satellite-observed daily increments of K. brevis during fall maintenance in 2006, compared to simulated smaller fish-starved growth rates of decanted red tides during fall 2007. During the last few decades, K. brevis has remained a “prudent predator” of some clupeids, i.e. Spanish sardine, whereas humans have now overfished other Florida stocks of both thread herring and Atlantic shad. Thus, future operational forecasts of the land falls and durations of Florida red tides, from Louisiana to North Carolina, as well as prudent management of regional fisheries of the southeastern United States, require consideration of negelected fish losses, at intermediate trophic levels, to algal predators. Some clupeids are harvested by K. brevis, but these fish are separately supported by a longer parallel diatom-based food chain of calanoid copepods, feeding the zooplanktivores and thence other piscivore fish predators, while intersecting the shorter food chain of just diazotrophs and red tide dinoflagellates, poorly grazed in turn by harpactacoid copepods. The distinct phytoplankton functional groups, different herbivores, as well as zoophagous and piscivore fishes, must all be formulated as explicit state variables of the next set of complex ecological models, cued by satellite data and driven by nested circulation models, within an ecosystem-based management paradigm of commercial and sport harvests of biotic marine resources at higher trophic levels of the WFS and SAB.

Macronutrients requirements of the dinoflagellate Protoceratium reticulatum

The basal L1 medium was found to be unsatisfactory for culturing the red tide dinoflagellate Protoceratium reticulatum at a high growth rate and biomass yield. The L1 medium enhanced with phosphate to a total concentration of 217 μM supported the highest attainable growth rate and biomass yield. Once the phosphate concentration exceeded 6× L1, phosphate inhibited the dinoflagellate growth and negatively affected cell viability. At the optimal phosphate concentration of 217 μM, an increase in nitrate concentration over the range of 882–8824 μM, did not affect cell growth and yield. Nitrate did not inhibit growth at any of the concentrations used. Clearly, the basal nitrate level in L1 is sufficient for effectively culturing P. reticulatum. At the ranges of phosphate and nitrate concentrations tested, cell volume was not sensitive to the concentration of nutrients but the concentration of phosphate affected both the specific cell number and cell volume growth rates. Elevated levels of nutrients supported their intracellular accumulation. Cell-specific production of yessotoxin was not influenced by concentration of phosphate in the culture medium, but elevated (>1764 μM) nitrate concentration did enhance the yessotoxin level. Phosphate concentration that maximized biomass yield also maximized volumetric production of yessotoxin in the culture broth.

The Toxic Dinoflagellate Karenia brevis Encodes Novel Type I-like Polyketide Synthases Containing Discrete Catalytic Domains

Karenia brevis is the Florida red tide dinoflagellate responsible for detrimental effects on human and environmental health through the production of brevetoxins. Brevetoxins are thought to be synthesized by a polyketide synthase (PKS) complex, but the gene cluster for this PKS has yet to be identified. Here, eight PKS transcripts were identified in K. brevis by high throughput cDNA library screening. Full length sequences were obtained through 3' and 5' RACE, which demonstrated the presence of polyadenylation, 3'-UTRs, and an identical dinoflagellate-specific spliced leader sequence at the 5' end of PKS transcripts. Six transcripts encoded for individual ketosynthase (KS) domains, one ketoreductase (KR), and one transcript encoded both acyl carrier protein (ACP) and KS domains. Transcript lengths ranged from 1875 to 3397 nucleotides, based on sequence analysis, and were confirmed by northern blotting. Baysian phylogenetic analysis of the K. brevis KS domains placed them well within the protist type I PKS clade. Thus although most similar to type I modular PKSs, the presence of individual catalytic domains on separate transcripts suggests a protein structure more similar to type II PKSs, in which each catalytic domain resides on an individual protein. These results identify an unprecedented PKS structure in a toxic dinoflagellate.

Loss of waterborne brevetoxins from exposure to phytoplankton competitors

We tested whether interactions among phytoplankton competitors affect toxin dynamics involving the red tide dinoflagellate Karenia brevis, whose brevetoxins incapacitate and kill coastal wildlife. The addition of a live diatom, Skeletonema costatum, led to decreased concentrations of brevetoxin B (PbTx-2) associated with K. brevis cells in co-culturing experiments and with two of three natural bloom samples containing K. brevis. Similar decreases in PbTx-2 concentration, but not PbTx-3 concentration, occurred when a mixture of brevetoxins (without live K. brevis cells) was exposed to S. costatum, indicating that S. costatum metabolizes waterborne PbTx-2. Liquid chromatography–mass spectrometry (LC–MS) and ELISA analyses indicated that PbTx-2 is probably not transformed into other brevetoxins or into known brevetoxin metabolites, and instead is biotransformed by a previously unrecognized mechanism. Four different S. costatum strains from around the world caused similar loss of PbTx-2, suggesting that evolutionary experience with K. brevis is not a pre-requisite for the ability to metabolize PbTx-2. Additionally, phytoplankton-associated bacteria were found to play no role in the loss of PbTx-2, as bacteria-free S. costatum strains metabolized PbTx-2. Finally, loss of waterborne PbTx-2 caused by exposure to a dinoflagellate, a cryptophyte, and two additional diatom species indicates that this phenomenon is widespread among phytoplankton. Our results unexpectedly suggest that competing phytoplankton species present during K. brevis blooms, and possibly other red tides, could mediate bloom toxicity and therefore ecosystem-level consequences of red tides.

Influence of microbial interactions on the susceptibility of Karenia spp. to algicidal bacteria

A bacterial strain (D38BY) belonging to the family Flavobacteriaceae and antagonistic towards an algicidal bacterium (strain S03; Flavobacteriaceae) was isolated from a culture of the red tide dinoflagellate Karenia brevis that had previously been characterized as resistant to attack by strain S03. This antagonistic bacterium increased the survival time of otherwise susceptible, bacteria- free K. brevis cultures in a concentration-dependent manner during exposure to the algicidal bac- terium. Experimental evidence indicated that direct contact was required in order for strain D38BY to inhibit the killing activity of algicidal strain S03. While further work is needed to determine its pre- cise mode of action, the antagonistic properties of strain D38BY provide further evidence that the resistance or susceptibility of certain algal taxa to algicidal attack can be more a function of inter- actions within the ambient microbial community than an intrinsic property of the alga.

Effects of harmful algal blooms on competitors: Allelopathic mechanisms of the red tide dinoflagellate Karenia brevis

Because competitive interactions may have led to adaptations enabling bloom‐forming phytoplankton to dominate pelagic communities, we explored the allelopathic effects of one red tide dinoflagellate, Karenia brevis, on competing phytoplankton species. Exposure to waterborne compounds from natural 9;A. brevis blooms resulted in growth inhibition or death for four of five co‐occurring species tested, whereas compounds exuded by 9A. brevis cultures suppressed three of these same competitors (the diatoms Asterionellopsis glacialis and Skeletonema costatum and the dinoflagellate Prorocentrum minimum) plus one additional species (the dinoflagellate Akashiwo cf. sanguinea) that was unaffected by bloom exudates. 9A. brevis exudates lowered photosynthetic efficiency and damaged cell membranes of competing phytoplankton, but had no effect on competitor esterase activity, nor did they limit competitor access to iron. Overall, during blooms, 9A. brevis exudes potent allelopathic compounds, competitors vary in their susceptibility to 9A. brevis allelopathy, and 9A. brevis may achieve nearly monospecific blooms by lowering the photosynthetic efficiency of competitor species and increasing competitor membrane permeability, eventually resulting in competitor growth suppression or death.

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.

The influence of upwelling, coastal currents and water temperature on the distribution of the red tide dinoflagellate, Noctiluca scintillans, along the east coast of Australia

Quasi-synoptic surveys along the east coast of Australia between 28 and 34°S show that the heterotrophic dinoflagellate, Noctiluca scintillans, occurs along this entire stretch of the coast. Areas of relatively high abundance of Noctiluca were observed downstream of regions predisposed to current-induced upwellings as a consequence of alongshore topographic variations. High-resolution temporal and spatial sampling of upwelling events showed that Noctiluca was abundant (up to 28 cells l−1) within mature upwelled waters. A high proportion (>80%) of fed Noctiluca cells (cells with prey in their vacuoles) was observed in the mature upwelled waters indicating that the observed increase in abundance of Noctiluca was associated with increased feeding activity. The absolute abundance of Noctiluca in upwelled waters was, however, found to vary from one upwelling location to another and between seasons. In particular, highest abundances of Noctiluca were recorded south of 31.5°S, where the East Australian Current (EAC) characteristically separates from the coast. The high abundances partly arise from southward advection and retention of the Noctiluca cells, and partly from upwelling inshore of the separated EAC driven by cross-shelf boundary layer fluxes. The temperature of the EAC was also found to influence absolute abundances. Surface water temperatures during our summer cruise were anomalously high due to a strong La Nina phase, and up to 4°C warmer than during our spring cruise. We found that the warmer surface water temperatures were associated with relatively lower average abundances of Noctiluca in the near shore zone.