Preferential Plastid Retention by the Acquired Phototroph Mesodinium chamaeleon

The ciliate genus Mesodinium contains species that rely to varying degrees on photosynthetic machinery stolen from cryptophyte algal prey. Prey specificity appears to scales inversely with this reliance: The predominantly phototrophic M. major/rubrum species complex exhibits high prey specificity, while the heterotrophic lineages M. pulex and pupula are generalists. Here, we test the hypothesis that the recently described mixotroph M. chamaeleon, which is phylogenetically intermediate between M. major/rubrum and M. pulex/pupula, exhibits intermediate prey preferences. Using a series of feeding and starvation experiments, we demonstrate that M. chamaeleon grazes and retains plastids at rates which often exceed those observed in M. rubrum, and retains plastids from at least five genera of cryptophyte algae. Despite this relative generality, M. chamaeleon exhibits distinct prey preferences, with higher plastid retention, mixotrophic growth rates and efficiencies, and starvation tolerance when offered Storeatula major, a cryptophyte that M. rubrum does not appear to ingest. These results suggest that niche partitioning between the two acquired phototrophs may be mediated by prey identity. M. chamaeleon appears to represent an intermediate step in the transition to strict reliance on acquired phototrophy, indicating that prey specificity may evolve alongside degree of phototrophy.     

Use of Highly Specific Molecular Markers Reveals Positive Correlation between Abundances of Mesodinium cf. major and Its Preferred Prey,Teleaulax amphioxeia,During Red Water Blooms in the Columbia River Estuary

In a previous study, Teleaulax amphioxeia—the preferred prey of Mesodinium in the Columbia River estuary—were undetectable within intense annual blooms, suggesting blooms are prey‐limited or prey are acquired outside of bloom patches. We used a novel molecular approach specifically targeting the prey (i.e., Unique Sequence Element [USE] within the ribosomal RNA 28S D2 regions of T. amphioxeia nucleus and nucleomorph) in estuarine water samples acquired autonomously with an Environmental Sample Processor integrated within a monitoring network (ESP‐SATURN). This new approach allowed for both more specific detection of the prey and better constraint of sample variability. A positive correlation was observed between abundances of M. cf. major and T. amphioxeia during bloom periods. The correlation was stronger at depth (> 8.2 m) and weak or nonexistent in the surface, suggesting that predator–prey dynamics become uncoupled when stratification is strong. We confirmed exclusive selectivity for T. amphioxeia by M. cf. major and observed the incorporation of the prey nucleus into a 4‐nuclei complex, where it remained functionally active. The specific biomarker for T. amphioxeia was also recovered in M. cf. major samples from a Namibian coastal bloom, suggesting that a specific predator–prey relationship might be widespread between M. cf. major and T. amphioxeia.     

Growth and Chloroplast Replacement of the Benthic Mixotrophic Ciliate Mesodinium coatsi

While the ecophysiology of planktonic Mesodinium rubrum species complex has been relatively well studied, very little is known about that of benthic Mesodinium species. In this study, we examined the growth response of the benthic ciliate Mesodinium coatsi to different cryptophyte prey using an established culture of this species. M. coatsi was able to ingest all of the offered cryptophyte prey types, but not all cryptophytes supported its positive, sustained growth. While M. coatsi achieved sustained growth on all of the phycocyanin‐containing Chroomonas spp. it was offered, it showed different growth responses to the phycoerythrin‐containing cryptophytes Rhodomonas spp., Storeatula sp., and Teleaulax amphioxeia. M. coatsi was able to easily replace previously ingested prey chloroplasts with newly ingested ones within 4 d, irrespective of prey type, if cryptophyte prey were available. Once retained, the ingested prey chloroplasts seemed to be photosynthetically active. When fed, M. coatsi was capable of heterotrophic growth in darkness, but its growth was enhanced significantly in the light (14:10 h light:dark cycle), suggesting that photosynthesis by ingested prey chloroplast leads to a significant increase in the growth of M. coatsi. Our results expand the knowledge of autecology and ecophysiology of the benthic M. coatsi.     

The fate of cryptophyte cell organelles in the ciliate Mesodinium cf. rubrum subjected to starvation

Mesodinium rubrum Lohmann is a mixotrophic ciliate and one of the best studied species exhibiting acquired phototrophy. To investigate the fate of cryptophyte organelles in the ciliate subjected to starvation, we conducted ultrastructural studies of a Korean strain of M. cf. rubrum during a 10 week starvation experiments. Ingested cells of the cryptophyte Teleaulax amphioxeia were first enveloped by ciliate membrane, and then prey organelles, including ejectisomes, flagella, basal bodies and flagellar roots, were digested. Over time, prey nuclei protruded into the cytoplasm of the ciliate, their size and volume increased, and their number decreased, suggesting that the cryptophyte nuclei likely fused with each other in the ciliate cytoplasm. At 4 weeks of starvation, M. cf. rubrum cells without cryptophyte nuclei started to appear. At 10 weeks of starvation, only two M. cf. rubrum cells still possessing a cryptophyte nucleus had relatively intact chloroplast-mitochondria complexes (CMCs), while M. cf. rubrum cells without cryptophyte nuclei had a few damaged CMCs. This is the first ultrastructural study demonstrating that cryptophyte nuclei undergo a dramatic change inside M. cf. rubrum in terms of size, shape, and number following their acquisition.     

Combined Effects of Temperature,Irradiance, and pH on Teleaulax amphioxeia (Cryptophyceae) Physiology and Feeding Ratio For Its Predator Mesodinium rubrum (Ciliophora)1

The cryptophyte Teleaulax amphioxeia is a source of plastids for the ciliate Mesodinium rubrum and both organisms are members of the trophic chain of several species of Dinophysis. It is important to better understand the ecology of organisms at the first trophic levels before assessing the impact of principal factors of global change on Dinophysis spp. Therefore, combined effects of temperature, irradiance, and pH on growth rate, photosynthetic activity, and pigment content of a temperate strain of T. amphioxeia were studied using a full factorial design (central composite design 2³*) in 17 individually controlled bioreactors. The derived model predicted an optimal growth rate of T. amphioxeia at a light intensity of 400 μmol photons · m⁻² · s⁻¹, more acidic pH (7.6) than the current average and a temperature of 17.6°C. An interaction between temperature and irradiance on growth was also found, while pH did not have any significant effect. Subsequently, to investigate potential impacts of prey quality and quantity on the physiology of the predator, M. rubrum was fed two separate prey: predator ratios with cultures of T. amphioxeia previously acclimated at two different light intensities (100 and 400 μmol photons · m⁻² s⁻¹). M. rubrum growth appeared to be significantly dependent on prey quantity while effect of prey quality was not observed. This multi-parametric study indicated a high potential for a significant increase of T. amphioxeia in future climate conditions but to what extent this would lead to increased occurrences of Mesodinium spp. and Dinophysis spp. should be further investigated.     

Acquired phototrophy in Mesodinium and Dinophysis – A review of cellular organization,prey selectivity,nutrient uptake and bioenergetics

Acquired phototrophy, i.e. the use of chloroplasts from ingested prey, can be found among some species of dinoflagellates and ciliates. The best studied examples of this phenomenon in these groups are within the ciliate genus Mesodinium and the dinoflagellate genus Dinophysis, both ecologically important genera with a worldwide distribution. Mesodinium species differ considerably in their carbon metabolism. Some species rely almost exclusively on food uptake, while other species rely mostly on photosynthesis. In Mesodinium with acquired phototrophy, a number of prey organelles in addition to chloroplasts may be retained, and the host ciliate has considerable control over the acquired chloroplasts; Mesodinium rubrum is capable of dividing its acquired chloroplasts and can also photoacclimate. In Dinophysis spp., the contents of ciliate prey are sucked out, but only the chloroplasts are retained from the ingested prey. Some chloroplast house-keeping genes have been found in the nucleus of Dinophysis and some preliminary evidence suggests that Dinophysis may be capable for photoacclimation. Both genera have been claimed to take up inorganic nutrients, including NO₃⁻, indicating that processes beyond photosynthesis have been acquired. M. rubrum seems to depend upon prey species within the Teleaulax/Plagioselmis/Geminigera clade of marine cryptophytes. Up until now, Dinophysis species have only been maintained cultured on M. rubrum as food, but other ciliates may also be ingested. Dinophysis spp. and M. rubrum are obligate mixotrophs, depending upon both prey and light for sustained growth. However, while M. rubrum only needs to ingest 1–2% of its carbon demand per day to attain maximum growth, Dinophysis spp. need to obtain about half of their carbon demand from ingestion for maximum growth. Both Mesodinium and Dinophysis spp. can survive for months in the light without food. The potential role for modeling in exploring the complex balance of phototrophy and phago-heterotrophy, and its ecological implications for the mixotroph and their prey, is discussed.     

PLASTID DYNAMICS DURING SURVIVAL OF DINOPHYSIS CAUDATA WITHOUT ITS CILIATE PREY1

To survive, the marine dinoflagellate Dinophysis caudata Saville‐Kent must feed on the plastidic ciliate Myrionecta rubra (=Mesodinium rubrum), itself a consumer of cryptophytes. Whether D. caudata has its own permanent chloroplasts or retains plastids from its ciliate prey, however, remains unresolved. Further, how long D. caudata plastids (or kleptoplastids) persist and remain photosynthetically active in the absence of prey remains unknown. We addressed those issues here, using the first established culture of D. caudata. Phylogenetic analyses of the plastid 16S rRNA and psbA gene sequences directly from the three organisms (D. caudata, M. rubra, and a cryptophyte) revealed that the sequences of both genes from the three organisms are almost identical to each other, supporting that the plastids of D. caudata are kleptoplastids. A 3‐month starvation experiment revealed that D. caudata can remain photosynthetically active for ～2 months when not supplied with prey. D. caudata cells starved for more than 2 months continued to keep the plastid 16S rRNA gene but lost the photosynthesis‐related genes (i.e., psaA and psbA genes). When the prey was available again, however, D. caudata cells starved for more than 2 months were able to reacquire plastids and slowly resumed photosynthetic activity. Taken all together, the results indicate that the nature of the relationship between D. caudata and its plastids is not that of permanent cellular acquisitions. D. caudata is an intriguing protist that would represent an interesting evolutionary adaptation with regard to photosynthesis as well as help us to better understand plastid evolution in eukaryotes.     

DOES DINOPHYSIS CAUDATA (DINOPHYCEAE) HAVE PERMANENT PLASTIDS?1

The marine photosynthetic dinoflagellates Dinophysis Ehrenb. species are obligate mixotrophs that require both light and the ciliate prey Myrionecta rubra (= Mesodinium rubrum) for long‐term survival. Despite rapid progress on the study of Dinophysis using laboratory cultures, however, whether it has its own permanent plastids or kleptoplastids (i.e., stolen plastids from its ciliate prey) is not fully resolved. Here, we addressed this issue using established cultures of D. caudata Saville‐Kent strain DC‐LOHABE01 and cross‐feeding/starvation experiments encompassing the prey M. rubra strain MR‐MAL01 cultures grown on two different cryptophytes (strains CR‐MAL01 and CR‐MAL11). To follow the fate of prey plastids, psbA gene as a tracer was amplified from individually isolated D. caudata cells, and the PCR products were digested with a restriction enzyme, SfaNI. The RFLP pattern of the PCR products digested by SfaNI revealed that D. caudata continued to keep CR‐MAL01–type plastids, while it lost CR‐MAL11–type plastids with increasing starvation time. Our results suggest that Dinophysis treats in different ways plastids taken up from different cryptophytes via its ciliate prey M. rubra. Alternatively, D. caudata may already have its own CR‐MAL01–type permanent plastid, with two types of plastids (CR‐MAL01 and CR‐MAL11) obtained from M. rubra being lost within 1 month. This result highlights the need to identify more accurately the origin of plastids in newly isolated photosynthetic Dinophysis species to resolve the issue of plastid permanence.     

The Effect of Starvation on Plastid Number and Photosynthetic Performance in the Kleptoplastidic Dinoflagellate Amylax triacantha

The dinoflagellate Amylax triacantha is known to retain plastids of cryptophyte origin by engulfing the mixotrophic ciliate Mesodinium rubrum, itself a consumer of cryptophytes. However, there is no information on the fate of the prey's organelles and the photosynthetic performance of the newly retained plastids in A. triacantha. In this study, we conducted a starvation experiment to observe the intracellular organization of the prey's organelles and temporal changes in the photosynthetic efficiency of acquired plastids in A. triacantha. The ultrastructural observations revealed that while the chloroplast‐mitochondria complexes and nucleus of cryptophyte were retained by A. triacantha, other ciliate organelles were digested in food vacuoles. Acquired plastids were retained in A. triacantha for about 1 mo and showed photosynthetic activities for about 18 d when measured by a pulse‐amplitude modulation fluorometer.     

DINOPHYSIS CAUDATA (DINOPHYCEAE) SEQUESTERS AND RETAINS PLASTIDS FROM THE MIXOTROPHIC CILIATE PREY MESODINIUM RUBRUM1

“Phototrophic”Dinophysis Ehrenberg species are well known to have chloroplasts of a cryptophyte origin, more specifically of the cryptophyte genus complex Teleaulax/Geminigera. Nonetheless, whether chloroplasts of “phototrophic”Dinophysis are permanent plastids or periodically derived kleptoplastids (stolen chloroplasts) has not been confirmed. Indeed, molecular sequence data and ultrastructural data lead to contradictory interpretations about the status of Dinophysis plastids. Here, we used established cultures of D. caudata strain DC‐LOHABE01 and M. rubrum strain MR‐MAL01 to address the status of Dinophysis plastids. Our approach was to experimentally generate D. caudata with “green” plastids and then follow the ingestion and fate of “reddish‐brown” prey plastids using light microscopy, time‐lapse videography, and single‐cell TEM. Our results for D. caudata resolve the apparent discrepancy between morphological and molecular data by showing that plastids acquired when feeding on M. rubrum are structurally modified and retained as stellate compound chloroplasts characteristic of Dinophysis species.     

Fate of green plastids in Dinophysis caudata following ingestion of the benthic ciliate Mesodinium coatsi: Ultrastructure and psbA gene

Phototrophic Dinophysis species are known to acquire plastids of the cryptophyte Teleaulax amphioxeia through feeding on the ciliate Mesodinium rubrum or M. cf. rubrum. In addition, several molecular studies have detected plastid encoding genes of various algal taxa within field populations of Dinophysis species. The trophic pathway by which Dinophysis species acquire plastids from algae other than the cryptophyte genus Teleaulax, however, is unknown. In this study, we examined the fate of prey organelles and plastid genes obtained by Dinophysis caudata through ingestion of Mesodinium coatsi, a benthic ciliate that retains green plastids of Chroomonas sp. Transmission electron microscopy and molecular analysis revealed relatively rapid digestion of prey-derived plastids. Following digestion of M. coatsi, however, photodamaged D. caudata cells having olive-green rather than reddish-brown plastids were able to recover some of their original reddish-brown pigmentation. Results further suggest that plastid genes of various algal taxa detected in field populations of Dinophysis species may reflect prey diversity rather than sequestration of multiple plastid types. Ingestion and digestion of prey other than M. rubrum or M. cf. rubrum may also provide nutritional requirements needed to repair and perhaps maintain sequestered T. amphioxeia plastids.     

Ultrastructure of the Oral Apparatus of Mesodinium rubrum from Korea

Mesodinium rubrum Lohmann is a photosynthetic marine ciliate that has functional chloroplasts of cryptophyte origin. Little is known about the oral ultrastructure of M. rubrum compared with several reports on the sequestration of nuclei and plastids from prey organisms, such as Geminigera cryophila and Teleaulax species. Here, we describe the fine structure of the oral apparatus of a M. rubrum strain from Gomso Bay, Korea. The cytopharynx was cone‐shaped and supported by 20–22 ribbons of triplet microtubules. At the anterior end of the cytopharynx, an annulus anchored small cylinders composed of 11 microtubules. The small cylinders were spaced at regular intervals, each reinforced by one set of the triplet microtubules. At the opening of the cytostome, larger 14‐membered microtubular cylinders were set adjacent to the small, 11‐membered microtubular cylinders, each pair surrounded by separate membranes, however, only the large cylinders extended into the oral tentacles. There were 20–22 oral tentacles each having one to five extrusomes at its tip. At the anterior end of the oral apparatus, microtubular bands supporting the cytostome curved posteriad, extending beneath the cell cortex to the kinetosomes of the somatic cirri. The microtubular bands were connected by striated fibers and originated from kinetosomes anchored by fibers. Each cirrus consisted of eight cilia associated with 16 kinetosomes. The ultrastructure of M. rubrum from Korea provides information useful for taxonomic characterization of the genus Mesodinium and relevant to developing a better understanding of the acquisition of foreign organelles through phagocytosis by M. rubrum.     

Ultrastructure and Molecular Phylogeny of Mesodinium coatsi sp. nov., a Benthic Marine Ciliate

Mesodinium is a globally distributed ciliate genus forming frequent and recurrent blooms in diverse marine habitats. Here, we describe a new marine species, Mesodinium coatsi n. sp., originally isolated from interstitial water of surface sand samples collected at Mohang Beach, Korea. The species was maintained under a mixotrophic growth condition for longer than 1 yr by providing a cryptomonad, Chroomonas sp., as the sole prey. Cell morphology and subcellular structure were examined by light microscopy, scanning, and transmission electron microscopy, and molecular phylogeny was inferred from nuclear‐encoded 18S rDNA sequence data. Like other Mesodinium species, M. coatsi consisted of two hemispheres separated by two types of kinetids, and had tentacles located at the oral end of the cell. Several food vacuoles were observed in the cytoplasm, and partially digested prey cells sometimes existed in food vacuoles. Kinetids and the associated accessory structures were quite similar to those previously reported, but M. coatsi was differentiated from other marine Mesodinium species by ultrastructural characters of the dikinetids, polykinetids, and tentacles. We also provided a detailed illustration of infraciliature. Molecular phylogeny revealed that M. coatsi and Mesodinium chamaeleon were closely related to each other.     

Karlotoxin mediates grazing by Oxyrrhis marina on strains of Karlodinium veneficum

Karlodinium veneficum is a common member of temperate, coastal phytoplankton assemblages that occasionally forms blooms associated with fish kills. Here, we tested the hypothesis that the cytotoxic and ichthyotoxic compounds produced by K. veneficum, karlotoxins, can have anti-grazing properties against the heterotrophic dinoflagellate, Oxyrrhis marina. The sterol composition of O. marina (>80% cholesterol) renders it sensitive to karlotoxin, and does not vary substantially when fed different algal diets even for prey that are resistant to karlotoxin. At in situ bloom concentrations (10⁴–10⁵ K. veneficum ml⁻¹), grazing rates (cells ingested per Oxyrrhis h⁻¹) on toxic K. veneficum strain CCMP 2064 were 55% that observed on the non-toxic K. veneficum strain MD5. At lower prey concentrations typical of in situ non-bloom levels (<10³ cells ml⁻¹), grazing rates (cells ingested per Oxyrrhis h⁻¹) on toxic K. veneficum strain CCMP 2064 were 70–80% of rates on non-toxic strain MD5. Growth of O. marina was significantly suppressed when fed the toxic strain of K. veneficum. Experiments with mixed prey cultures, where non-toxic strain MD5 was fluorescently stained, showed that the presence of toxic strain CCMP 2064 inhibited grazing of O. marina on the co-occurring non-toxic strain MD5. Exogenous addition of a sub-lethal dose (100 ng ml⁻¹) of purified karlotoxin inhibited grazing of O. marina by approximately 50% on the non-toxic K. veneficum strain MD5 or the cryptophyte S. major. These results identify karlotoxin as an anti-grazing compound for those grazers with appropriate sterol composition (i.e., desmethyl sterols). This strategy is likely to be an important mechanism whereby growth of K. veneficum is uncoupled from losses due to grazing, allowing it to form ichthyotoxic blooms in situ.     

Selective feeding and foreign plastid retention in an Antarctic dinoflagellate

The peridinin‐containing plastid found in most photosynthetic dinoflagellates is thought to have been replaced in a few lineages by plastids of chlorophyte, diatom, or haptophyte origin. Other distinct lineages of phagotrophic dinoflagellates retain functional plastids obtained from algal prey for different durations and with varying source species specificity. 18S rRNA gene sequence analyses have placed a novel gymnodinoid dinoflagellate isolated from the Ross Sea (RSD) in the Kareniaceae, a family of dinoflagellates with permanent plastids of haptophyte origin. In contrast to other species in this family, the RSD contains kleptoplastids sequestered from its prey, Phaeocystis antarctica. Culture experiments were employed to determine whether the RSD fed selectively on P. antarctica when offered in combination with another polar haptophyte or cryptophyte species, and whether the RSD, isolated from its prey and starved, would take up plastids from P. antarctica or from other polar haptophyte or cryptophyte species. Evidence was obtained for selective feeding on P. antarctica, plastid uptake from P. antarctica, and increased RSD growth in the presence of P. antarctica. The presence of a peduncle‐like structure in the RSD suggests that kleptoplasts are obtained by myzocytosis. RSD cells incubated without P. antarctica were capable of survival for at least 29.5 months. This remarkable longevity of the RSD's kleptoplasts and its species specificity for prey and plastid source is consistent with its prolonged co‐evolution with P. antarctica. It may also reflect the presence of a plastid protein import mechanism and genes transferred to the dinokaryon from a lost permanent haptophyte plastid.     

A Dinoflagellate Amylax triacantha with Plastids of the Cryptophyte Origin: Phylogeny,Feeding Mechanism,and Growth and Grazing Responses

The gonyaulacalean dinoflagellates Amylax spp. were recently found to contain plastids of the cryptophyte origin, more specifically of Teleaulax amphioxeia. However, not only how the dinoflagellates get the plastids of the cryptophyte origin is unknown but also their ecophysiology, including growth and feeding responses as functions of both light and prey concentration, remain unknown. Here, we report the establishment of Amylax triacantha in culture, its feeding mechanism, and its growth rate using the ciliate prey Mesodinium rubrum (= Myrionecta rubra) in light and dark, and growth and grazing responses to prey concentration and light intensity. The strain established in culture in this study was assigned to A. triacantha, based on morphological characteristics (particularly, a prominent apical horn and three antapical spines) and nuclear SSU and LSU rDNA sequences. Amylax triacantha grew well in laboratory culture when supplied with the marine mixotrophic ciliate M. rubrum as prey, reaching densities of over 7.5 × 10³ cells/ml. Amylax triacantha captured its prey using a tow filament, and then ingested the whole prey by direct engulfment through the sulcus. The dinoflagellate was able to grow heterotrophically in the dark, but the growth rate was approximately two times lower than in the light. Although mixotrophic growth rates of A. triacantha increased sharply with mean prey concentrations, with maximum growth rate being 0.68/d, phototrophic growth (i.e. growth in the absence of prey) was ?0.08/d. The maximum ingestion rate was 2.54 ng C/Amylax/d (5.9 cells/Amylax/d). Growth rate also increased with increasing light intensity, but the effect was evident only when prey was supplied. Increased growth with increasing light intensity was accompanied by a corresponding increase in ingestion. In mixed cultures of two predators, A. triacantha and Dinophysis acuminata, with M. rubrum as prey, A. triacantha outgrew D. acuminata due to its approximately three times higher growth rate, suggesting that it can outcompete D. acuminata. Our results would help better understand the ecophysiology of dinoflagellates retaining foreign plastids.     

PHOTOACCLIMATION IN THE PHOTOTROPHIC MARINE CILIATE MESODINIUM RUBRUM (CILIOPHORA)1

Mesodinium rubrum (=Myrionecta rubra), a marine ciliate, acquires plastids, mitochondria, and nuclei from cryptophyte algae. Using a strain of M. rubrum isolated from McMurdo Sound, Antarctica, we investigated the photoacclimation potential of this trophically unique organism at a range of low irradiance levels. The compensation growth irradiance for M. rubrum was 0.5 μmol quanta · m⁻² · s⁻¹, and growth rate saturated at ∼20 μmol quanta · m⁻² · s⁻¹. The strain displayed trends in photosynthetic efficiency and pigment content characteristic of marine phototrophs. Maximum chl a–specific photosynthetic rates were an order of magnitude slower than temperate strains, while growth rates were half as large, suggesting that a thermal limit to enzyme kinetics produces a fundamental limit to cell function. M. rubrum acclimates to light‐ and temperature‐limited polar conditions and closely regulates photosynthesis in its cryptophyte organelles. By acquiring and maintaining physiologically viable, plastic plastids, M. rubrum establishes a selective advantage over purely heterotrophic ciliates but reduces competition with other phototrophs by exploiting a very low‐light niche.     

Inducible Mixotrophy in the Dinoflagellate Prorocentrum minimum

Prorocentrum minimum is a neritic dinoflagellate that forms seasonal blooms and red tides in estuarine ecosystems. While known to be mixotrophic, previous attempts to document feeding on algal prey have yielded low grazing rates. In this study, growth and ingestion rates of P. minimum were measured as a function of nitrogen (‐N) and phosphorous (‐P) starvation. A P. minimum isolate from Chesapeake Bay was found to ingest cryptophyte prey when in stationary phase and when starved of N or P. Prorocentrum minimum ingested two strains of Teleaulax amphioxeia at higher rates than six other cryptophyte species. In all cases ‐P treatments resulted in the highest grazing. Ingestion rates of ‐P cells on T. amphioxeia saturated at ~5 prey per predator per day, while ingestion by ‐N cells saturated at 1 prey per predator per day. In the presence of prey, ‐P treated cells reached a maximum mixotrophic growth rate (μ_max) of 0.5 d^?1, while ‐N cells had a μ_max of 0.18 d^?1. Calculations of ingested C, N, and P due to feeding on T. amphioxeia revealed that phagotrophy can be an important source of all three elements. While P. minimum is a proficient phototroph, inducible phagotrophy is an important nutritional source for this dinoflagellate.     

Prey diversity of Polistes rothneyi koreanus in different landscapes using DNA barcoding

For the present study, we investigated the prey of Polistes rothneyi koreanus, which is the most common social wasp in South Korea, and the relationship between prey diversity and vegetation cover around their nests. Prey was collected over two 6‐hr sampling from seven nests between July and mid‐August 2015 in Daegu, Gyeongsan and Gunwi in South Korea. To analyse the prey spectrum, we identified species using DNA barcodes; to analyse vegetation cover, we used the normalized difference vegetation index in a 200 m radius around the nests. A total of 338 prey samples were collected, and eight orders, 24 families, and 65 species were identified, demonstrating a much broader prey spectrum than those previously recorded for Polistes spp. Lepidoptera were the most prevalent, with 158 samples and 47 species. Nest 7, located in a rural area, had the highest numbers of samples and species per worker (5.2 and 1.9, respectively). Lepidoptera accounted for over half of the prey spectrum for all nests, and the families Noctuidae and Geometridae accounted for 60% of all Lepidoptera. Tenodera sinensis (Mantodea) and Gabala argentata (Noctuidae) were the most ubiquitous species, collected at five locations. Six species and some genera of prey are designated as pests in South Korea, indicating that P. r. koreanus also has a beneficial role in pest control. A higher vegetation cover was associated with significantly higher prey species diversity (R² = .4597, p < .1) and abundance (R² = .5986, p < .05), indicating that vegetation cover is an important factor for maintaining colonies. Therefore, the recent increase in green spaces in South Korean cities is probably a major contributor to the increased density of social wasps.