Flagellate Oxyrrhis Marina Research Articles

Negative frequency-dependent selection is intensified at higher population densities in protist populations.

Natural populations of free-living protists often exhibit high-levels of intraspecific diversity, yet this is puzzling as classic evolutionary theory predicts dominance by genotypes with high fitness, particularly in large populations where selection is efficient. Here, we test whether negative frequency-dependent selection (NFDS) plays a role in the maintenance of diversity in the marine flagellate Oxyrrhis marina using competition experiments between multiple pairs of strains. We observed strain-specific responses to frequency and density, but an overall signature of NFDS that was intensified at higher population densities. Because our strains were not selected a priori on the basis of particular traits expected to exhibit NFDS, these data represent a relatively unbiased estimate of the role for NFDS in maintaining diversity in protist populations. These findings could help to explain how bloom-forming plankton, which periodically achieve exceptionally high population densities, maintain substantial intraspecific diversity.

A rapid and cost‐effective quantitative microsatellite genotyping protocol to estimate intraspecific competition in protist microcosm experiments

Summary High levels of intraspecific variation are commonly observed in natural microbial populations, yet the consequences of this variation for ecological and evolutionary processes remain poorly understood. Protists are excellent experimental models for investigating fundamental and applied questions in ecology and evolution, but studying intraspecific variation remains a challenge due to a lack of molecular resources to aid in quantifying and distinguishing strains during experiments. Here we present a molecular method, quantitative microsatellite genotyping, to accurately quantify strain‐specific frequencies from microcosm experiments of the marine flagellate Oxyrrhis marina, both between many pairs of strains and between strains in a multistrain mixture. We find that for pairs of strains, the method is effective for relative frequencies as low as 0·02 and with around 99% accuracy. The method is able to quantify four strains reasonably well, though less accurate than for pairs (range 92–97% accuracy). This makes accessible a cheap and easy‐to‐implement method for quantifying strain (or allele) frequencies and is suitable for use in a broad range of single‐celled eukaryotes (protists) where copy number should correlate well with number of individuals (i.e. cells). This opens up the possibility of examining the role of intraspecific variation using experimental protist microcosms.

Intraspecific selectivity, compensatory feeding and flexible homeostasis in the phagotrophic flagellate Oxyrrhis marina: three ways to handle food quality fluctuations

The phagotrophic flagellate Oxyrrhis marina shows a strong stoichiometric plasticity when fed differently grown Rhodomonas salina. We tested whether differently pre-conditioned O. marina displayed selective feeding behaviour from a mixture of nitrogen and phosphorus depleted R. salina. We observed selective feeding of O. marina, always selecting phosphorus rich R. salina independent of the pre-conditioning of the protists. In a second experiment, O. marina was again pre-conditioned either with nitrogen- or phosphorus-depleted R. salina and was refed with either of the differently limited R. salina in single food treatments (not in a mixture). The phagotrophic flagellate displayed compensatory feeding which means that O. marina feed more on the food source which they were not given before. Due to its stoichiometric plasticity, O. marina might handle bad quality food by following the stoichiometry of its prey and additionally by active selective feeding towards P-rich algae to enhance growth. Post-ingestion selection might as well be an important feature which means that ingested elements in excess are quickly excreted and scarce elements are ingested through accelerated food uptake.

High Genetic Diversity and Fine-Scale Spatial Structure in the Marine Flagellate Oxyrrhis marina (Dinophyceae) Uncovered by Microsatellite Loci

Free-living marine protists are often assumed to be broadly distributed and genetically homogeneous on large spatial scales. However, an increasing application of highly polymorphic genetic markers (e.g., microsatellites) has provided evidence for high genetic diversity and population structuring on small spatial scales in many free-living protists. Here we characterise a panel of new microsatellite markers for the common marine flagellate Oxyrrhis marina. Nine microsatellite loci were used to assess genotypic diversity at two spatial scales by genotyping 200 isolates of O. marina from 6 broad geographic regions around Great Britain and Ireland; in one region, a single 2 km shore line was sampled intensively to assess fine-scale genetic diversity. Microsatellite loci resolved between 1–6 and 7–23 distinct alleles per region in the least and most variable loci respectively, with corresponding variation in expected heterozygosities (He) of 0.00–0.30 and 0.81–0.93. Across the dataset, genotypic diversity was high with 183 genotypes detected from 200 isolates. Bayesian analysis of population structure supported two model populations. One population was distributed across all sampled regions; the other was confined to the intensively sampled shore, and thus two distinct populations co-occurred at this site. Whilst model-based analysis inferred a single UK-wide population, pairwise regional FST values indicated weak to moderate population sub-division (0.01–0.12), but no clear correlation between spatial and genetic distance was evident. Data presented in this study highlight extensive genetic diversity for O. marina; however, it remains a substantial challenge to uncover the mechanisms that drive genetic diversity in free-living microorganisms.

Collection, isolation and culturing strategies for Oxyrrhis marina

The heterotrophic marine flagellate Oxyrrhis marina is a popular and tractable model organism, being common in the intertidal environment and relatively straight forward to maintain in the laboratory. In this report, based on our experience of collecting and culturing ∼400 isolates, we provide a quantification of how “easy” O. marina is to locate and isolate from the environment and subsequently maintain in culture. In addition, we provide a brief review of the literature pertaining to O. marina culturing, stressing in particular the broad physiological tolerance and a wide diet range of this organism. Finally, in the light of the increasing interest in the genetics and genomics of O. marina, and heterotrophic protists more generally, we highlight existing strategies for culturing heterotrophs for “-omic” applications and summarize our specific culturing approaches for the genomic and proteomic studies of O. marina. In this context, we outline a simplified version of Droop's medium for axenic culturing and offer details on the use of heat-killed Escherichia coli as a simple and robust food source for culturing O. marina in the absence of other eukaryotes and live prokaryotes.

Contributions of Oxyrrhis marina to molecular biology, genomics and organelle evolution of dinoflagellates

The flagellate Oxyrrhis marina has been the subject of numerous studies addressing diverse aspects of protist biology including feeding, motility, ecology and cell biology. In spite of the rich body of information that has been built around this organism, the molecular biology of 0. marina has remained virtually unstudied until very recently. Studying the molecular biology and genomics of 0. marina is not only important due to its role as a model organism and practical accessibility; current evidence shows that it occupies a basal position within the dinoflagellate lineage, making it an ideal starting point to reconstruct the evolution of several interesting characters in this diverse group and its sister group Apicomplexa. Among these features, dinoflagellates have very divergent mitochondrial genomes, a complex history of plastid evolution and unique features involving the nuclear genome and transcription. Here we review and discuss how recent findings from 0. marina are contributing to shed light on these and other aspects of the evolutionary history of two important eukaryotic lineages, the apicomplexans and dinoflagellates.

Dietary-induced responses in the phagotrophic flagellate Oxyrrhis marina

Primary producers may be limited by different nutrients as well as by light availability, which in turn affects their quality as food for higher trophic levels. Typically, algae with high C:N and/or C:P ratios are low-quality food for consumers. Heterotrophic protists are important grazers on these autotrophes, but despite their importance as grazers, knowledge of food quality effects on heterotrophic protists is sparse. In the present study, we examined how differently grown Rhodomonas salina (nutrient replete, N-limited and P-limited) affected the phagotrophic flagellate Oxyrrhis marina. The functional response of O. marina (based on ingested biovolume) did not show significant differences between food sources, thus food uptake was independent of food quality. O. marina was weakly homoeostatic which means that its C:N:P ratio still reflected the elemental composition of its food to some extent. Food quality had a significantly negative effect on the numerical response of O. marina. Whereas N-limited R. salina and nutrient replete R. salina resulted in similar growth rates, P-limited algae had a significantly negative effect on the specific growth rate of O. marina. Hence, the lack of elemental phosphorus of O. marina feeding on P-limited algae caused a reduction in growth. Thus, despite their weaker homoeostasis, heterotrophic protists are also affected by high C:P food in a similar way to crustacean zooplankton.

Do temperature–food interactions matter? Responses of production and its components in the model heterotrophic flagellate Oxyrrhis marina

The consequence of interactions between temperature and food concentration for pro- tistan population dynamics and estimates of aquatic productivity are relatively unknown, primarily because we lack adequate parameters for models. Here, using the heterotrophic flagellate Oxyrrhis marina Dujardin, we demonstrate the importance of considering temperature and food concentration in combination, to determine the responses of grazing rate, specific growth rate, cell volume, specific production and yield. Specific growth rate and cell volume responded in different ways to tempera- ture-food concentrations: prey concentration had greatest positive effects on specific growth rate with increasing temperature, and prey concentration had greatest positive effects on cell volume with decreasing temperature. The effect of these contrasting interactions on specific production (=specific growth rate × cell carbon) was a greater response to prey concentration at intermediate temperatures. We also observed that the threshold food concentration for growth increased with increasing temper- ature, but yield showed no clear thermal response. By applying iterative curve-fitting to data obtained from multiple temperature-food concentration combinations, we produced phenomenolog- ical models of grazing rate, specific growth rate, and cell volume. We then compared predictions from a simple predator-prey simulation model that applied either our derived equations or a single expo- nential (Q10) relationship to the specific growth and ingestion responses at 20°C. Considerable differ- ences in predator and prey abundance were obtained between the 2 models. Our results demonstrate the potentially complex effects of food and temperature in combination on production parameters, and we argue that these should be considered in aquatic ecosystem simulation models.

On the role of food quality in grazing and development of life stages, and genetic change of body size during cultivation of pelagic copepods

> 100 consecutive generations of Temora longicomis and 70 of Pseudocalanus elongatus (Copepoda: Calanoida) have been bred in the laboratory using a suitable mixture of three food organisms. The food value of the large heterotrophic flagellate Oxyrrhis marina is demonstrated by observations on grazing on this flagellate by the different life stages of T. longicornis. Weight-specific ingestion increased with age from 35% in NII and III to an optimum in CV of 115% (males)and 170% (females)of body C · day −1. Body size in the adult stage and rate of development were reduced when T. longicornis was raised using only the small Isochrysis galbana as food, compared to the intermediate Rhodomonas sp. or the mixture of flagellates. P. elongatus developed equally fast with the three types of food, however, body size was significantly reduced with I. galbana as the sole food whereas lipid content of females decreased with smaller sizes of the food particles. I. galbana and Rhodomonas sp. must be the main food of young nauplii in the cultures whereas O. marina becomes so in older copepodite stages. During prolonged cultivation of four copepod species, body size increased or decreased, depending on the species. In T. longicornis, body size decreased genetically. This appeared from interbreeding experiments of males freshly collected from the sea and females cultured in the laboratory in which the original size of the offspring was restored. Large seasonal changes of body size of male parents did not influence the adult size of their progeny when cultivated at constant conditions, indicating that genotypic variance does not contribute significantly to seasonal changes of body size. It is hypothesized that changes of body size in the cultures might be an adaptation to the food offered.

A UNIQUE MITOTIC VARIATION IN THE MARINE DINOFLAGELLATE OXYRRHIS MARINA (PYRROPHYTA)1

ABSTRACTMitosis is described in the flagellate Oxyrrhis marina Dujardin and is compared in related genera. Dense plaques develop in the nuclear envelope at prophase and give rise to an intranuclear spindle. Some of the microtubules associate with the chromosomes while others extend across the nucleus. The basal bodies migrate toward the poles early in division and retain a position lateral to the nuclear poles throughout mitosis. Microtubules are not present between the nucleus and the basal bodies. The nucleolus is persistent and elongates throughout anaphase and telophase. Chromosomal separation is accomplished by sliding of non‐chromosomal microtubules and by elongation of the nuclear envelope rather than by shortening of the spindle microtubules. The nuclear envelope begins to constrict in the center early in anaphase. Continued constriction of the envelope and elongation of the nucleus leads to the formation of a dumbbell‐shaped nucleus by late telophase. Mitosis culminates by the constriction of the nucleus into two daughter nuclei. The taxonomic position of Oxyrrhis marina is discussed in light of these findings.

Cytological studies on trichocysts. VI. Fine structure and mode of function of trichocysts in the flagellateOxyrrhis marina and the ciliatePleuronema marinum

1. Der DinoflagellatOxyrrhis marina und der CiliatPleuronema marinum wurden hinsichtlich der Feinstruktur und des Funktionsmodus ihrer Trichocysten untersucht. 2. Die ruhende Trichocyste vonO. marina ist aus Schaft und Spitze aufgebaut. Während die Spitze eine tubuläre Innenstruktur aufweist, besitzt das Material des Schaftes eine längsorientierte Streifung von 80 Å mit einer Unterperiode von 40 Å. 3. Die ruhende Trichocyste vonP. marinum besitzt eine parakristalline Struktur mit einer Periode von 65–80 Å. 4. Die gestreckte Trichocyste vonO. marina ist durch eine Querstreifung mit einer Hauptperiode von 680 Å mit Unterperioden von 160–170 Å gekennzeichnet. Es werden als kleinste Strukturkomponenten des Schaftes Filamente mit einem Durchmesser von 10 Å nachgewiesen, die zur Trichocystenlängschse parallel verlaufen. 5. Die gestreckte Trichocyste vonP. marinum besitzt eine Querstreifung mit einer Periode von 560 Å. 10–15 Å messende Filamente, die in der Hauptrichtung parallel zur Trichocystenlängsachse verlaufen, bilden ein geordnetes Netzwerk, das den Strukturen der gestrecktenParamecium-Trichocyste ähnlich ist. 6. An Hand gehemmter Trichocystenstadien vonO. marina undP. marinum kann gezeigt werden, daß die Ausschleuderung der Trichocysten als Streckung zu verstehen ist, die durch den Übergang eines parakristallinen Zustandes in einen anderen parakristallinen Zustand gedeutet werden muß. 7. Die Befunde derOxyrrhis- undPleuronema-Trichocysten werden mit den anParamecium-Trichocysten erzielten Ergebnissen verglichen. Es zeigt sich, daß bei den drei Trichocysten gleichgeartete Verhältnisse insofern vorliegen, als auch in der Streckung derOxyrrhis- undPleuronema-Trichocysten ein Entfaltungsvorgang präformierter Strukturen gesehen werden muß.