The role of interactions between Prorocentrum minimum and Heterosigma akashiwo in bloom formation

We examined the growth and interactions between the bloom-forming flagellates Prorocentrum minimum and Heterosigma akashiwo using bi-algal culture experiments. When both species were inoculated at high cell densities, growth of H. akashiwo was inhibited by P. minimum. In other combinations of inoculation densities, the species first reaching the stationary phase substantially suppressed maximum cell densities of the other species, but the growth inhibition effect of P. minimum was stronger than that of H. akashiwo. We used a mathematical model to simulate growth and interactions of P. minimum and H. akashiwo in bi-algal cultures. The model indicated that P. minimum always out-competed H. akashiwo over time. Additional experiments showed that crude extracts from P. minimum and H. akashiwo cultures did not affect the growth of either species, but both strongly inhibited the growth of the bloom-forming diatom Skeletonema costatum. Further experiments showed that it was unlikely that reactive oxygen species produced by H. akashiwo were responsible for the inhibition of P. minimum growth.     

东海原甲藻与中肋骨条藻的种间竞争数值模拟

研究探讨了两个零维箱式模型在东海典型赤潮藻东海原甲藻和中肋骨条藻竞争与演替研究中的应用。模型在采用不同接种密度下的单种培养实验数据进行参数校正后,被用来模拟不同N/P条件下单种培养实验以及两藻种共培养竞争实验,并以实验数据对其结果进行了验证。模拟结果表明,在单种培养条件下,模型能够较好地重现两种藻在不同N/P环境中的生长及对营养盐的利用;共培养实验的模拟结果显示,在所有初始细胞密度比例条件下,中肋骨条藻的最终密度均会超过东海原甲藻,且PO4的消耗主要源于中肋骨条藻的利用,与实验结果一致,表明模型能够很好地体现两种藻的竞争结果及对营养盐的竞争关系;由于模型不足以模拟除营养盐竞争以外的藻间相互作用,模拟结果未体现东海原甲藻细胞数迅速衰减这一现象,有待进一步研究。     

Competitive interactions between two allelopathic algal species: Heterosigma akashiwo and Phaeodactylum tricornutum

ABSTRACT

Both Heterosigma akashiwo and Phaeodactylum tricornutum have been reported to produce allelochemicals capable of inhibiting the growth of co-occurring microalgae. Here, potential allelopathy between H. akashiwo and P. tricornutum was evaluated using bi-algal culture, cell-free culture filtrate and a no-contact co-culturing system in nutrient-replete media. Experiments were also conducted in the no-contact co-culturing system under nutrient-limited conditions. In nutrient-replete bi-algal culture, the growth of P. tricornutum and H. akashiwo each tended to be strongly suppressed when the other species was inoculated at high cell densities. A mathematical model was used to simulate the growth interactions of the two species in bi-algal culture and showed that P. tricornutum outcompeted H. akashiwo over time with different initial cell densities under nutrient-sufficient conditions, indicating that P. tricornutum was more potent in allelopathy. Heterosigma akashiwo growth was inhibited both in the P. tricornutum culture filtrate and no-contact co-culturing system. This confirmed that the extracellular allelopathic compounds released by P. tricornutum were one of the sources of the H. akashiwo growth inhibition. Nutrient deficiency did not increase the extent of allelopathic activity of allelochemicals.     

东海原甲藻与中肋骨条藻的种间竞争特征

对东海原甲藻和中肋骨条藻按照起始Chl-a比1∶5、1∶1和5∶1进行了f/2条件下的共培养实验,以探讨这两种藻的种间竞争特征。实验结果表明在共培养体系中,中肋骨条藻完全占优势,而东海原甲藻的生长受到明显的抑制。应用Lotka-Volterra种间竞争模型对共培养实验进行模拟的结果表明,东海原甲藻与中肋骨条藻的种间竞争结果与初始密度配比无关,中肋骨条藻总会竞争胜过东海原甲藻。为了探讨他感作用对东海原甲藻和中肋骨条藻种间竞争的影响,采用了中肋骨条藻的无藻细胞滤液来进行培养实验。实验结果显示,中肋骨条藻滤液对东海原甲藻及其本身的生长均无明显影响,这表明他感作用并非中肋骨条藻获得优势的主要竞争方式。     

Interactions between Prorocentrum donghaiense Lu and Scrippsiella trochoidea (Stein) Loeblich III under laboratory culture

Interactions between Prorocentrum donghaiense Lu and Scrippsiella trochoidea (Stein) Loeblich III, two species of causative bloom dinoflagellates in China, were investigated using bi-algal cultures under controlled laboratory conditions. The growth of P. donghaiense and S. trochoidea were significantly suppressed when the initial cell densities were set at 1.9 × 10⁴ cells mL⁻¹ or 1.9 × 10⁵ cells mL⁻¹ for P. donghaiense and 1.0 × 10⁴ cells mL⁻¹ for S. trochoidea when the initial size/density ratio was 1:1 or 10:1, respectively, but no out-competement was observed in either bi-algal culture by the end. The simultaneous assay on the culture filtrate showed that P. donghaiense filtrate prepared at a lower initial density (1.9 × 10⁴ cells mL⁻¹) stimulated the co-cultured S. trochoidea at a density of 1.0 × 10⁴ cells mL⁻¹, but filtrate at a higher density (1.9 × 10⁵ cells mL⁻¹) depressed its growth. Differently, the filtrate of S. trochoidea at a density of 1.0 × 10⁴ cells mL⁻¹ significantly suppressed the growth of P. donghaiense at a density of 1.9 × 10⁴ cells mL⁻¹, but had little stimulatory effect on P. donghaiense at a density of 1.9 × 10⁵ cells mL⁻¹compared to the control (P > 0.05). It is likely that these two species of microalgae interact with each other mainly by releasing allelochemical substance(s) into the culture medium, and a direct cell-to-cell contact was not necessary for their mutual interaction. We then quantify their interactions in the bi-algal culture by using a mathematical model. The estimated parameters from the model showed that the inhibition exerted by S. trochoidea on P. donghaiense was about 43 and 24 times stronger than the inhibitory effect that P. donghaiense exerted on S. trochoidea when the initial size/density were 1:1 and 10:1, respectively. S. trochoidea seemed to have a survival strategy that was superior to P. donghaiense in the bi-algal culture under controlled laboratory conditions. We also observed a closely positive relationship between the initial cell density and its effect on the co-cultured microalga by measuring the fluorenscence: filtrate prepared from higher initial cell density had stronger interference on the co-cultured microalga. Moreover, pre-treated under different temperature conditions (30 °C, 60 °C and 100 °C) would significantly changed the effect of culture filtrate on the co-cultured microalga. Result inferred that P. donghaiense or S. trochoidea would release allelochemicals into the bi-algal culture medium and the allelochemicals might be a mixture with temperature-sensitive components in it.     

Biochemical compositions of two dominant bloom- forming species isolated from the Yangtze River Estuary in response to different nutrient conditions

Variations of cellular total lipid, total carbohydrate and total protein content of two dominant bloom-forming species (Skeletonema costatum and Prorocentrum donghaiense) isolated from the Yangtze River Estuary were examined under six different nutrient conditions in batch cultures. Daily samples were collected to estimate the cell growth, nutrient concentration and three biochemical compositions content during 7 days for S. costatum and the same sampling procedure was done every other day during 10 days for P. donghaiense. Results showed that for S. costatum, cellular total lipid content increased under phosphorus (P) limitation, but not for nitrogen (N) limitation; cellular carbohydrate were accumulated under both N and P limitation; cellular total protein content of low nutrient concentration treatments were significantly lower than that of high nutrient concentration treatments. For P. donghaiense, both cellular total lipid content and total carbohydrate content were greatly elevated as a result of N and P exhaustion, but cellular total protein content had no significant changes under nutrient limitation. In addition, the capability of accumulation of three biochemical constituents of P. donghaiense was much stronger than that of S. costatum. Pearson correlation showed that for both species, the biochemical composition of three constituents (lipid, carbohydrate and protein) had no significant relationship with extracellular N concentration, but had positive correlation with extracellular and intracellular P concentration. The capability of two species to accumulate cellular total lipid and carbohydrate under nutrient limitation may help them accommodate the fluctuating nutrient condition of the Yangtze River Estuary. The different responses of two species of cellular biochemical compositions content under different nutrient conditions may provide some evidence to explain the temporal characteristic of blooms caused by two species in the Yangtze River Estuary.     

Inhibitory effect of Salicornia europaea on the marine alga Skeletonema costatum

Exploiting the negative biochemical interference between plants and algal species has been suggested as a method to control harmful algal blooms. In this work, we investigated the inhibitory effect of the salt marsh halophyte Salicornia europaea against the marine alga Skeletonema costatum. S. europaea suppressed the growth of S. costatum in a nutrient-sufficient co-culture system, indicating that the inhibition of algal growth was because of the phytotoxic effect of S. europaea, rather than nutrient competition. We tested aqueous and organic extracts from S. europaea roots against S. costatum. The organic extracts inhibited growth and affected the cell size and chlorophyll a content of S. costatum in a dose-dependent manner. Among the three tested organic extracts, the methanol extract had the greatest effects on S. costatum, followed by butanol extract, and then the chloroform extract. Two flavonoids, rutin and quercetin-3-??-D-glucoside, were identified in the methanol extract by high performance liquid chromatography. The concentration of rutin was much higher than that of quercetin-3-??-D-glucoside. In an algal bioassay, rutin inhibited the growth of S. costatum and the inhibitory effect increased with increasing rutin concentration and with decreasing initial algal density. Therefore, we concluded that S. europaea negatively affects the growth of S. costatum, and that rutin, a metabolite of S. europaea, may play a role in this inhibitory effect.     

Studies on N-limited growth of diatoms in dialysis culture

N-limited growth of Skeletonema costatum (Grev.) Cleve in dialysis culture has been studied. The division rate of exponentially growing cells was independent of the nitrate concentration in the growth medium in the range from 886 down to 0.25 μM N-salt, while no growth beyond one division took place in cultures to which no nitrogen salt was added. The half saturation constant, K₃, for growth must, therefore, be in the range 0–0.13 μM, provided the growth-nutrient relationship is hyperbolic for S. costatum.Contrary to growth rate, cellular chlorophyll and protein were markedly reduced in media poor in nitrogen salts. A dialysis culture chamber was used to demonstrate that the measurement of half saturation constants for S. costatum was influenced by stirring, the stirred culture growing almost twice as fast as the unstirred control under identical conditions. The ability of diatoms to grow rapidly at low nitrogen levels was used to remove nutrients from sewage enriched media. Removal efficiencies corresponding to 80 and 90 % were obtained for nitrate and ammonia, respectively, using the diatom Phaeodactylum tricornutum Bohlin. It was found that both this diatom and S. costatum as well as Thalassiosira pseudonana Hust (Hasle) tolerated ammonia up to at least 450 μM with no deleterious effects on growth rate.     

Comparative Studies on the Allelopathic Effects of Ulva pertusa Kjellml, Corllina pilulifera Postl et Ruprl, and Sargassum thunbergii mertl O. Kuntze on Skeletonema costatum （Grev.）Cleve

In the present study, we evaluated the allelopathic effects of three macroalgae, namely Ulva pertusa Kjellml, Corallina pilufifera Postl et Ruprl, and Sargassum thunbergii Mertl O. Kuntze, on the growth of the microalga Skeletonema costaturn （Grev.） Creve using culture systems in which the algae coexisted. The effects of the macroalgal culture medium filtrate on S. costatum were also investigated. Moreover, isolated co-culture systems were built to confirm the existence of allelochemicals and preclude growth inhibition by direct contact. The coexistence assay data demonstrated that the growth of S. costaturn was strongly inhibited when fresh tissues, dry powder and aqueous extracts were used; the allelochemicals were lethal to S. costatum at relatively higher concentrations. The effects of the macroalgal culture medium filtrate on the microalga showed both species specificity and complexity. The inhibitory effect of fresh macroalgal tissue and culture medium filtrate on the microalga was due to the alleochemicals released by the macroalgae. The results of the present study show that the allelopathic effects of macroalgae on the microalga are complex. The present study could shed light onto the basis of the interaction between macro- and microalgae.     

Effects of ultraviolet-B stressed diatom food on the reproductive output in Mediterranean Calanus helgolandicus (Crustacea; Copepoda)

Recent predictions anticipate a solar ultraviolet-B increase of 5-10% over temperate latitudes within the next 15 years. As Mediterranean waters are clear, enabling deep UVB penetration, we used controlled experiments to test the effects of equivalent levels of UVB on trophic plankton interactions. We grew the diatom Skeletonema costatum in the laboratory under daily 4 h UVB exposure in doses of 10% higher than today in contrast to control cultures (no UVB). UVB-exposed algae showed modification in cell structure, volume increase and delay in cell division, eventually leading to lysis and cell death after five days of daily 4 h exposure. Both control and UVB-stressed S. costatum cultures were used as food for wild Calanus helgolandicus females collected in the NW Mediterranean. Those fed on control diatoms produced three times more eggs, healthier offspring with fewer lethal naupliar deformities than females fed on UVB-exposed diatoms. Our results provide the first experimental evidence of indirect UVB effects on reproductive output through trophic response in marine plankton. The sensitivity of the S. costatum strain is discussed as well as the reduced reproductive output of the female copepods. We suggest that the low egg numbers and high naupliar deformity ratios are possibly a consequence of UVB-induced deficiency of essential components in the UVB-stressed maternal diet.     

Increase in the production of allelopathic substances by Prymnesium parvum cells grown under N- or P-deficient conditions

The haptophyte Prymnesium parvum is known to produce a set of highly potent exotoxins, commonly called prymnesins. These toxins have been shown to have several biological activities, including ichthyotoxic, neurotoxic, cytotoxic, hepatotoxic and hemolytic activity towards a range of marine organisms. In addition, recent studies have shown that the toxicity of P. parvum is enhanced when the cells are grown under N- or P-deficient conditions. In this study, the influence of prymnesium toxins on the growth of other phytoplankton species was investigated by addition of cell-free filtrate of P. parvum cultures grown under nutrient-deficient (N or P) or non-deficient conditions. Addition of cell-free filtrate from P. parvum cultures grown under N or P limitation inhibited the growth of Thalassiosira weissflogii, Prorocentrum minimum and Rhodomonas cf. baltica. In contrast, a strain of Prymnesium patelliferum known to produce prymnesium toxins was not negatively affected under any conditions. Furthermore, addition of filtrates from nutrient-sufficient P. parvum cultures did not negatively influence the growth of any of the tested species. These findings suggest that prymnesium toxins may play an allelopathic role, and that the production of allelopathic substances is regulated by the availability of nutrients.     

Growth and competition of several harmful dinoflagellates under different nutrient and light conditions

Three near-shore type harmful dinoflagellates, Prorocentrum minimum, Prorocentrum donghaiense and Karlodinium veneficum, and one off-shore dinoflagellate, Karenia brevis, were grown in laboratory monocultures and mixed batch cultures. The dinoflagellate cultures were grown on treatments of two ambient nitrogen (N):phosphorus (P) ratios; two N substrates (nitrate and urea) and two light intensities. The microalgae Rhodomonas and Synechococcus were also added in separate treatments to the mixed culture treatments as potential food sources. All tested species grew well on both N substrates. In mixed culture, P. minimum outgrew K. veneficum, and P. donghaiense outgrew K. brevis in most treatments reaching higher growth rates and higher biomass. However, when a third algae, Rhodomonas, was added, the growth of P. minimum was inhibited relative to that of K. veneficum. In contrast, when grown with K. brevis, the growth rate of P. donghaiense was not significantly affected by the addition of potential prey. K. brevis had a longer growth phase, and kept growing after P. donghaiense reached stationary phase, suggesting better adaptation of K. brevis to low inorganic nutrient conditions. The growth of K. brevis was also significantly limited in the low light treatment. K. veneficum overgrew P. minimum in the presence of Rhodomonas, a potential nutrient source. The growth rates of both K. brevis and P. donghaiense were reduced with the presence of Synechococcus. In addition to nutrient competition, mixotrophy and allelopathy were likely mechanisms in determining the dominant species.     

Algicidal activity against Skeletonema costatum by marine bacteria isolated from a high frequency harmful algal blooms area in southern Chinese coast

Four marine bacterial strains P1, P5, N5 and N21 were isolated from the surface water and sediment of Mirs Bay in southern Chinese coast using the liquid infection method with 48-well plates. These bacteria were all shown to have algicidal activities against Skeletonema costatum. Based on morphological observations, biochemical tests and homology comparisons by 16S rDNA sequences, the isolated strains P1, P5, N5 and N21 were identified as Halobacillus sp., Muricauda sp., Kangiella sp. and Roseivirga sp., respectively. Our results showed that bacterial strain P1 killed S. costatum by release of heat labile algicide, while strains P5, N5 and N21 killed them directly. The algicidal processes of four bacterial strains were different. Strains P1, N5 and N21 disrupted the chain structure and S. costatum appeared as single cells, in which the cellular components were aggregated and the individual cells were inflated and finally lysed, while strain P5 decomposed the algal chains directly. We also showed that the algicidal activities of the bacterial strains were concentration-dependent. More specifically, 10?% (v/v) of bacteria in algae showed the strongest algicidal activities, as all S. costatum cells were killed by strains N5 and N21 within 72?h and by strains P1 and P5 within 96?h. 5?% of bacteria in algae also showed significant algicidal activities, as all S. costatum were killed by strains N5, P5 and N21 within 72, 96 and 120?h, respectively, whereas at this concentration, only 73.4?% of S. costatum cells exposed to strain P1 were killed within 120?h. At the concentration of 1?% bacteria in algae, the number of S. costatum cells continued to increase and the growth rate of algae upon exposure to strain N5 was significantly inhibited.     

强壮前沟藻化感物质分析

微藻化感作用是一种极其复杂的生理、生态学现象。选取强壮前沟藻指数生长初期Ⅰ和平台生长初期Ⅱ两个阶段的滤液对中肋骨条藻、海洋原甲藻、锥状斯氏藻及球等鞭金藻生长的影响进行了研究,并萃取了阶段Ⅱ的粗提物,抑藻检测表明其具有"杀藻"效应,通过GC/MS分析该粗提物中具有潜在化感作用的物质种类。研究发现强壮前沟藻两个生长阶段的滤液对中肋骨条藻均产生强烈致死效应(phaseⅠ:F=15.18475,P=0.00298<0.05;phaseⅡ:F=6.24559,P=0.03149<0.05);锥状斯氏藻在强壮前沟藻滤液中生长,实验结束时两个阶段中的细胞密度分别是对照组的79.3%和68.9%;海洋原甲藻在强壮前沟藻生长阶段Ⅱ滤液实验的最后3d,其生长受到显著抑制(F=4.84438,P=0.04925<0.05);而等鞭金藻在强壮前沟藻两个生长阶段滤液中被抑制现象不明显(P>0.05)。强壮前沟藻滤液实验表明,强壮前沟藻能够向微环境中分泌代谢产物来抑制中肋骨条藻和海洋原甲藻的生长,并且这种抑制效应具有种类特殊对应性。上述实验结果还表明,强壮前沟藻生长阶段Ⅱ的滤液具有的生长抑制作用较为明显。采用乙酸乙酯萃取强壮前沟藻生长阶段Ⅱ滤液中的代谢产物,检测发现其代谢粗提物具有溶藻效应,GC/MS分析结果表明粗提物中存在4种可能产生化感抑制作用的物质,其中二丁基羟基甲苯(Butylated Hydroxytoluene BHT)被认为具有抗滤过性病原体和抗微生物活性。     

Larval development of Octomeris sulcata Nilsson-Cantell (Cirripedia: Thoracica: Chthamalidae) from Japan and Korea

Embryos obtained from gravid adults of the chthamalid barnacle Octomeris sulcata Nilsson-Cantell from Japan and Korea were cultured through six naupliar stages to the cyprid and juvenile barnacle stage in laboratory conditions, fed either the diatom Skeletonema costatum (Grev.) Cleve or the dinoflagellate Prorocentrum minimum (Pavillard) Schiller. The nauplii were planktotrophic and, depending on diet, reached the cyprid stage 9 or 17 days after hatching in individual cultures at 22 °C with 24 h illumination. The survival rate was higher and the duration of the naupliar stages was shorter when fed P. minimum rather than S. costatum. This is probably due to the presence of feathered setae on the antennae. Feathered or plumose setae in nauplii of different cirripede taxa are apparently linked to the type of phytoplankton in the seas when these taxa first evolved.The larval stages of O. sulcata are described, and morphological differences between larvae reared from Japanese andKorean adults are compared. The polygonal cephalic shield and unilobed labrum, a pair of posterior shield spines after naupliar stage IV, feathered setae and a hispid seta on the coxa of the antenna, a cuspidate seta on the mandible, and the gnathobase of the antenna are important in distinguishing the nauplii of this species from other species, including Chthamalidae.     

Phosphate Stress in Cultures and Field Populations of the Dinoflagellate Prorocentrum minimum Detected by a Single-Cell Alkaline Phosphatase Assay

Alkaline phosphatase activity is a common marker of phosphate stress in many phytoplankton, but it has been difficult to attribute alkaline phosphatase activity to specific organisms or groups of phytoplankton in the field with traditional biochemical procedures. A new alkaline phosphatase substrate, ELF-97 (enzyme-labeled fluorescence), shows promise in this regard. When a phosphate group is cleaved from the ELF-97 reagent, the remaining molecule precipitates near the site of enzyme activity, thus fluorescently tagging cells with alkaline phosphatase activity. We characterized ELF-97 labeling in axenic cultures of a common dinoflagellate, Prorocentrum minimum, in order to understand ELF-97 labeling dynamics when phosphate nutrition varies. Enzyme activity, as detected by ELF-97 labeling, appears to be induced in late-log- or early-stationary-phase cultures if cells are grown in low-phosphate media and is lost when phosphate-stressed cells are refed with phosphate. ELF-97 appears to label an inducible intracellular alkaline phosphatase in P. minimum based on confocal microscopy studies. This may limit the use of this reagent to organisms that lack high levels of constitutive intracellular phosphatases. After laboratory cultures were characterized, ELF-97 was used to assay field populations of P. minimum in Narragansett Bay during two 1-week periods, and 12 to 100% of the P. minimum cells were labeled. The level of cell labeling was reduced by 3 days of incubation with added inorganic phosphate. Our results indicate that ELF-97 is an excellent new tool for monitoring phytoplankton phosphate stress in the environment when the data are supported by appropriate laboratory studies.     

Natural chelators in sea water: Detoxification of Zn2+ by brown algal polyphenols

We have examined growth responses of several species of marine phytoplankton, cultured with and without heavy metal stress, to supplements of polymeric polyphenols from the brown algae Ascophyllum nodosum (L.) Le Jol. and Fucus vesiculosus L. In the absence of additional heavy metals. supplements of up to 4000 μg. 1 of polyphenols had a small effect on initial growth rates for three of these microalgae and had no effect on maximum cell densities for four species. One very common, heavy metal-sensitive diatom, Skeletonema costatum (Grev.) Cl., showed significantly increasing maximum cell density in the cultures, with increasing addition of polyphenols to the medium. The toxicity of Zn²⁺ (0.5–2.0 mg·1 ¹) to cultures of the diatom Phaeodactylum tricornutum Bohlin was relieved by supplements (100–200 μg·1 ¹) of the brown algal polyphenols. Exudation of these polyphenols from brown seaweeds may contribute to the natural chelating capacity of inshore sea water.     

Prorocentrum minimum blooms: potential impacts on dissolved oxygen and Chesapeake Bay oyster settlement and growth

The cosmopolitan dinoflagellate Prorocentrum minimum is a recurrent bloom forming species in the Chesapeake Bay and its tributaries, generally observed at its highest levels in late spring and summer. Laboratory studies were conducted to assess potential bloom impacts on diel oxygen concentrations in shallow littoral zones as well as settlement success and post-set growth of the eastern oyster Crassostrea virginica. Using light–dark and dark cultures and periodic diel sub-sampling, bloom levels of P. minimum produced supersaturated oxygen levels at the end of each day while darkened cultures were typified by rapid decreases in dissolved oxygen (DO) (1.1–1.3 mg L⁻¹ h⁻¹) to hypoxic and anoxic levels within 4 days. These data suggest shallow, poorly flushed systems and the biota in them will experience rapid and large diel variations in oxygen, implying recurrent P. minimum blooms need be considered as short-term oxygen stressors for Bay oyster spat and other living resources. Direct effects of P. minimum impacts on oysters were not as expected or previously reported. In one experiment, pre-bloom isolates of P. minimum were grown and then exposed to polyvinyl chloride (PVC) settlement plates to see whether dinoflagellate preconditioning of the hard substrate might affect oyster sets. No differences were noted between set on the PVC with P. minimum exposure to set recorded with filtered seawater, Instant Ocean^®, or Isochrysis. In the second oyster experiment, spat on PVC plates were exposed to field collected P. minimum blooms and a commercial mixture of several other food types including Isochrysis. Oyster growth was significantly higher in P. minimum exposures than noted in the commercial mix. These results, compared to results with other isolates from the same region, indicate substantial positive impact from some of the P. minimum blooms of the area while others separated in space, time, or nutrient status could severely curtail oyster success through toxin production induced by nutrient limitation.     

Maternal and neonate diatom diets impair development and sex differentiation in the copepod Temora stylifera

Development and sex differentiation in the copepod Temora stylifera was studied in the presence of maternal and larval diets of the diatoms Thalassiosira rotula and Skeletonema marinoi, either provided alone or supplemented with the control dinoflagellate Prorocentrum minimum. Both diatoms had deleterious effects on growth compared to the control when used as pure diets, inducing very low or even zero survival from hatching to adulthood. This effect was deleted when the diet was supplemented with a good food (P. minimum) only in the case of T. rotula. By contrast, with a maternal or larval diet of S. marinoi, nauplii did not pass metamorphosis even when this alga was mixed with P. minimum. Arrested development was not due to lack of feeding since early and late nauplii (NII and NV) ingested all three algae at similar rates when used as single diets, and did not show any preference when the algae were offered as mixtures. Since mortality rates with a mixed diet of S. marinoi + P. minimum were similar or even higher than those obtained with a single diet of S. marinoi, we suggest that this diatom is more toxic than nutritionally deficient for T. stlyfera development. No males were produced in cohorts reared on pure diatom diets or with a mixture of S. marinoi + P. minimum, and intermediate male:female sex ratios were obtained with the mixed T. rotula + P. minimum diet. Possibly some diatoms produce compounds such as oxylipins or new molecules that alter sex differentiation in T. stylifera.     

Growth,death, and photobiology of dinoflagellates (Dinophyceae) under bacterial-algicide control

Naturally occurring allelopathic compounds, specific to some phytoplankton, may be a good source of bio-control agents against microalgae responsible for harmful algal blooms (HABs). Global expansion of HABs has invigorated research into different approaches to control these algae, including the search for naturally derived algicidal compounds. Here, we investigated the effects of a filtrate from the algicidal marine bacterium Shewanella sp. IRI-160 on photochemical function of four cultured dinoflagellates, Karlodinium veneficum, Gyrodinium instriatum, Prorocentrum minimum, and Alexandrium tamarense. The filtrate (designated IRI-160AA) contains bioactive compound(s), which were recently shown to inhibit growth of several dinoflagellate species. Results of this study show that all dinoflagellates but P. minimum exhibited photosystem II (PSII) inhibition, loss of photosynthetic electron transport, and varying degrees of cellular mortality. Exposure assays over 24 h showed that PSII inhibition and loss of cell membrane integrity occurred simultaneously in G. instriatum, but not in K. veneficum, where PSII activity declined prior to losing outer-membrane integrity. In addition, PSII inhibition and population growth inhibition were dose-dependent in K. veneficum, with an average EC-50 of 7.9 % (v/v) IRI-160AA. Application of IRI-160AA induced significantly higher PSII inhibition and cell mortality in K. veneficum subjected to continuous darkness as compared to cells maintained with 12:12 h light/dark cycles, while no such dark effect was noted for G. instriatum. The marked differences in the rate and impact of this algicide suggest that multiple cellular targets and different cascades of cellular dysfunction occur across these dinoflagellates.