Allelopathic effects of the dinophyte Prorocentrum minimum on the growth of the bacillariophyte Skeletonema costatum

We investigated growth interactions between the dinophyte Prorocentrum minimum and the bacillariophyte Skeletonema costatum using bi-algal cultures under axenic conditions. When low cell densities of P. minimum and high cell densities of S. costatum were inoculated into the same medium, growth of P. minimum was suppressed. Other inoculum combinations resulted in reduced S. costatum maximum cell densities. A mathematical model was used to simulate growth and interactions of P. minimum and S. costatum in bi-algal cultures. The model indicated that P. minimum always outcompeted S. costatum over time. Enriched filtrate from low-density P. minimum cultures significantly stimulated S. costatum growth, but enriched filtrate from high-density P. minimum cultures notably inhibited the growth of S. costatum. Growth of P. minimum was not affected by enriched filtrate from cultures of P. minimum at any density. Filtrates of P. minimum cultures were fractionated by ultrafiltration (molecular weight cutoff >3000 Da), and retentate that included polysaccharide(s) significantly inhibited the growth of S. costatum.     

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。     

Virus-Specific Responses of Heterosigma akashiwo to Infection

We used flow cytometry to examine the process of cell death in the bloom-forming alga Heterosigma akashiwo during infection by a double-stranded DNA virus (OIs1) and a single-stranded RNA virus (H. akashiwo RNA virus [HaRNAV]). These viruses were isolated from the same geographic area and infect the same strain of H. akashiwo. By use of the live/dead stains fluorescein diacetate and SYTOX green as indicators of cellular physiology, cells infected with OIs1 showed signs of infection earlier than HaRNAV-infected cultures (6 to 17 h versus 23 to 29 h). Intracellular esterase activity was lost prior to increased membrane permeability during infection with OIs1, while the opposite was seen with HaRNAV-infected cultures. In addition, OIs1-infected cells accumulated in the cultures while HaRNAV-infected cells rapidly disintegrated. Progeny OIs1 viruses consisted of large and small morphotypes with estimated latent periods of 11 and 17 h, respectively, and about 1,100 and 16,000 viruses produced per cell, respectively. In contrast, HaRNAV produced about 21,000 viruses per cell and had a latent period of 29 h. This study reveals that the characteristics of viral infection in algae are virus dependent and therefore are variable among viruses infecting the same species. This is an important consideration for ecosystem modeling exercises; calculations based on in situ measurements of algal physiology must be sensitive to the diverse responses of algae to viral infection.     

Using quantitative real-time PCR to study competition and community dynamics among Delaware Inland Bays harmful algae in field and laboratory studies

The Delaware Inland Bays (DIB) have experienced harmful algal blooms of dinoflagellates and raphidophytes in recent years. We used quantitative polymerase chain reaction (QPCR) techniques to investigate the community dynamics of three DIB dinoflagellates (Karlodinium veneficum, Gyrodinium instriatum, and Prorocentrum minimum) and one raphidophyte (Heterosigma akashiwo) at a single site in the DIB (IR-32) in summer 2006 relative to salinity, temperature and nutrient concentrations. We also carried out complementary laboratory culture studies. New primers and probes were developed and validated for the 18S rRNA genes in the three dinoflagellates. K. veneficum, H. akashiwo, and G. instriatum were present in almost all samples throughout the summer of 2006. In contrast, P. minimum was undetectable in late June through September, when temperatures ranged from 20 to 30 °C (average 25.7 °C). Dissolved nutrients ranged from 0.1 to 2.8 μM PO₄³⁻ (median = 0.3 μM), 0.7–30.2 μM NO_x (median = 12.9 μM), and 0–19.4 μM NH₄⁺ (median = 0.7 μM). Dissolved N:P ratios covered a wide range from 2.6 to 177, with a median of 40. There was considerable variability in occurrence of the four species versus nutrients, but in general P. minimum and H. akashiwo were most abundant at higher (>40) N:P ratios and dissolved nitrogen concentrations, while K. veneficum and G. instriatum were most abundant at low dissolved N:P ratios (<20) and dissolved nitrogen concentrations < 10 μM. The semi-continuous laboratory competition experiment used mixed cultures of K. veneficum, P. minimum, and H. akashiwo grown at dissolved N:P ratios of 5, 16, and 25. At an N:P of 16 and 25 P. minimum was the dominant alga at the end of the experiment, even at a temperature that was much higher than that at which this alga was found to bloom in the field (27 °C). P. minimum and H. akashiwo had highest densities in the N:P of 25. K. veneficum grew equally well at all three N:P ratios, and was co-dominant at times at an N:P of 5. H. akashiwo had the lowest densities of the three algae in the laboratory experiment. Laboratory and field results showed both interesting similarities and significant differences in the influences of important environmental factors on competition between these harmful algal species, suggesting the need for more work to fully understand HAB dynamics in the DIB.     

Comparative studies on the allelopathic effects of two different strains of Ulva pertusa on Heterosigma akashiwo and Alexandrium tamarense

Allelopathic effects of fresh tissue and dry powder of a nonsexual and a sexual strain of the macroalga Ulva pertusa on the growth of the microalgae Heterosigma akashiwo and Alexandium tamarense were evaluated using long-term coexistence culture systems in which several concentrations of macroalga fresh tissue and dry powder were used. The effects of macroalga culture medium filtrate on the two HAB algae were also investigated. Moreover, isolation co-culture systems were built to confirm the existence of allelochemicals and preclude the growth inhibition by direct contact. Short-term algicidal effect assays of macroalgae on H. akashiwo were carried out to measure the rate of algal cell lysis. The results of the coexistence assays showed that the growth of H. akashiwo and A. tamarense was strongly inhibited by fresh tissue and by dry powder of both strains of U. pertusa. The allelochemicals were lethal to H. akashiwo at relatively higher concentrations. The macroalga culture medium filtrate exhibited no apparent growth inhibitory effect on the two HAB algae under initial or semicontinuous filtrate addition, which suggested that continuous release of small quantities of rapidly degradable allelochemicals from the fresh tissue of both strains of U. pertusa was essential to effectively inhibit the growth of H. akashiwo and A. tamarense.     

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。     

Ecological niche partitioning of the invasive dinoflagellate Prorocentrum minimum and its native congeners in the Baltic Sea

This study analyses three decades of the peculiar bloom-formation history of the potentially toxic invasive planktonic dinoflagellates Prorocentrum minimum (Pavillard) Schiller in the SW Baltic Sea. We tested a research hypothesis that the unexpectedly long delay (nearly two decades) in population development of P. minimum prior to its first bloom was caused by competition with one or several closely related native dinoflagellate species due to ecological niche partitioning which hampered the spread and bloom-forming potential of the invader. We applied the ecological niche concept to a large, long-term phytoplankton database and analysed the invasion history and population dynamics of P. minimum in the SW Baltic Sea coastal waters using the data on phytoplankton composition, abundance and biomass. The ecological niche dimensions of P. minimum and its congener P. balticum were identified as the optimum environmental conditions for the species during the bloom events based on water temperature, salinity, pH, concentration of nutrients (PO₄³⁻; total phosphorus, TP; total nitrogen, TN; SiO₄⁴⁻), TN/TP-ratio and habitat type. The data on spatial distribution and ecological niche dimensions of P. minimum have contributed to the development of the “protistan species maximum concept”. High microplankton diversity at critical salinities in the Baltic Sea may be considered as a possible reason for the significant niche overlap and strong competitive interactions among congeners leading to prolonged delay in population growth of P. minimum preceding its first bloom in the highly variable brackishwater environment.     

Alga-lytic activity of Pseudomonas fluorescens against the red tide causing marine alga Heterosigma akashiwo (Raphidophyceae)

A bacterial strain, HAK-13, exhibited strongest activity against Heterosigma akashiwo and was capable of controlling this bloom forming phytoplankton. Based on 16S rDNA sequences and biochemical and morphological characteristics, the strain HAK-13 was determined to be Pseudomonas fluorescens on the basis of 99.9% similarity with reference strains in the DNA databases. The growth of H. akashiwo was strongly suppressed by HAK-13 in all growth phases, with the strongest alga-lytic activity noted against harmful bloom-forming species in the exponential stage (6–22 days). Host range tests showed that HAK-13 also significantly inhibited the growth of Alexandrium tamarense and Cochlodinium polykrikoides but could not destroy Gymnodinium catenatum. P. fluorescens HAK-13 indirectly attacked H. akashiwo by alga-lytic substances that might be located at the compartment of cytoplasmic membrane of the bacterium at a level of 45.86 units/mg of specific activity. The results indicated that P. fluorescens HAK-13 caused cell lysis and death of H. akashiwo, A. tamarense, and C. polykrikoides dramatically and Prorocentrum dentatum slightly. Therefore, P. fluorescens HAK-13 has potential for use as a selective biocontrol of harmful algal blooms.     

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.