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Labyrinthula spp. are saprobic, marine protists that also act as opportunistic pathogens and are the causative agents of seagrass wasting disease (SWD). Despite the threat of local- and large-scale SWD outbreaks, there are currently gaps in our understanding of the drivers of SWD, particularly surrounding Labyrinthula spp. virulence and ecology. Given these uncertainties, we investigated the Labyrinthula genus from a novel genomic perspective by presenting the first draft genome and predicted proteome of a pathogenic isolate Labyrinthula SR_Ha_C, generated from a hybrid assembly of Nanopore and Illumina sequences. Phylogenetic and cross-phyla comparisons revealed insights into the evolutionary history of Stramenopiles. Genome annotation showed evidence of glideosome-type machinery and an apicoplast protein typically found in protist pathogens and parasites. Proteins involved in Labyrinthula SR_Ha_C’s actin-myosin mode of transport, as well as carbohydrate degradation were also prevalent. Further, CAZyme functional predictions revealed a repertoire of enzymes involved in breakdown of cell-wall and carbohydrate storage compounds common to seagrasses. The relatively low number of CAZymes annotated from the genome of Labyrinthula SR_Ha_C compared to other Labyrinthula species may reflect the conservative annotation parameters, a specialized substrate affinity and the scarcity of characterized protist enzymes. Inherently, there is high probability for finding both unique and novel enzymes from Labyrinthula spp. This study provides resources for further exploration of Labyrinthula spp. ecology and evolution, and will hopefully be the catalyst for new hypothesis-driven SWD research revealing more details of molecular interactions between the Labyrinthula genus and its host substrate.

Seagrasses create foundational habitats in coastal ecosystems. One contributing factor to their global decline is disease, primarily caused by parasites in the genus Labyrinthula. To explore the relationship between seagrass and Labyrinthula spp. diversity in coastal waters, we examined the diversity and microhabitat association of Labyrinthula spp. in 2 inlets on Florida's Atlantic Coast, the Indian River Lagoon (IRL) and Banana River. We used amplicon-based high throughput sequencing with 2 newly designed primers to amplify Labyrinthula spp. from 5 seagrass species, water, and sediments to determine their spatial distribution and microhabitat associations. The SSU primer set identified 12 Labyrinthula zero-radius operational taxonomic units (ZOTUs), corresponding to at least 8 putative species. The ITS1 primer set identified 2 ZOTUs, corresponding to at least 2 putative species. Based on our phylogenetic analyses, which include sequences from previous studies that assigned seagrass-related pathogenicity to Labyrinthula clades, all but one of the ZOTUs that we recovered with the SSU primers were from non-pathogenic species, while the 2 ZOTUs recovered with the ITS1 primers were from pathogenic species. Some of the ZOTUs were widespread across the sampling sites and microhabitats (e.g. SSU ZOTU_10), and most were present in more than one site. Our results demonstrate that targeted metabarcoding is a useful tool for examining the relationships between seagrass and Labyrinthula diversity in coastal waters.

Seagrass meadows provide important ecological services to the marine environment but are declining worldwide. Although eelgrass meadows in the north Pacific are thought to be relatively healthy, few studies have assessed the presence of known disease pathogens in these meadows. In a pilot study to test the efficacy of the methods and to provide foundational disease biodiversity data in the north Pacific, we leveraged metabarcoding of environmental DNA extracted from water, sediment, and eelgrass tissue samples collected from five widely distributed eelgrass meadows in Alaska and one in Japan and uncovered wide prevalence of two classes of pathogenic organisms – Labyrinthula zosterae and other associated strains of Labyrinthula, and the Phytophthora/Halophytophthora blight species complex – known to have caused decline in eelgrass (Zostera marina) elsewhere in the species’ global distribution. Although the distribution of these disease organisms is not well understood in the north Pacific, we uncovered the presence of at least one eelgrass pathogen at every locality sampled.

Seagrass wasting disease, caused by protists of the genus Labyrinthula, is an important stressor of the dominant macrophyte in Florida Bay (FB), United States, Thalassia testudinum. FB exhibits countervailing gradients in plant morphology and resource availability. A synoptic picture of the Thalassia-Labyrinthula relationship was obtained by assessing the activity of four immune biomarkers in conjunction with pathogen prevalence and load [via quantitative PCR (qPCR)] at 15 sites across FB. We found downregulated immune status paired with moderate pathogen load among larger-bodied host phenotypes in western FB and upregulated immunity for smaller-bodied phenotypes in eastern FB. Among the highest immune response sites, a distinct inshore-offshore loading pattern was observed, where coastal basins exposed to freshwater runoff and riverine inputs had the highest pathogen loads, while adjacent offshore locations had the lowest. To explain this, we propose a simple, conceptual model that defines a framework for testable hypotheses based on recent advances in resistance-tolerance theory. We suggest that resource availability has the potential to drive not only plant size, but also tolerance to pathogen load by reducing investment in immunity. Where resources are more scarce, plants may adopt a resistance strategy, upregulating immunity; however, when physiologically challenged, this strategy appears to fail, resulting in high pathogen load. While evidence remains correlative, we argue that hyposalinity stress, at one or more temporal scales, may represent one of many potential drivers of disease dynamics in FB. Together, these data highlight the complexity of the wasting disease pathosystem and raise questions about how climate change and ongoing Everglades restoration might impact this foundational seagrass species.

The temperate seagrass species eelgrass Zostera marina can be infected by the wasting disease pathogen Labyrinthula zosterae, which is believed to have killed about 90% of the seagrass in the Atlantic Ocean in the 1930s. It is not known why this opportunistic pathogen sometimes becomes virulent, but the recurrent outbreaks may be due to a weakening of the Z. marina plants from adverse environmental changes. This study investigated the individual and interactive effects of multiple extrinsic factors (temperature, light, and tissue damage) on the host-pathogen interaction between Z. marina and L. zosterae in a fully crossed infection experiment. The degree of infection was measured as both lesion coverage and L. zosterae cell concentration. We also investigated if the treatment factors affect the chemical defense of the host, measured as the inhibitory capacity of seagrass extracts in bioassays with L. zosterae. Finally, gene expression of a set of targeted genes was quantified in order to investigate how the treatments change Z. marina’s response to infection. Light had a pronounced effect on L. zosterae infection measured as lesion coverage, where reduced light conditions increased lesions by 35%. The response to light on L. zosterae cell concentration was more complex and showed significant interaction with the temperature treatment. Cell concentration was also significantly affected by physical damage, where damage surprisingly resulted in a reduced cell concentration of the pathogen. No treatment factor caused detectable decrease in the inhibitory capacity of the seagrass extracts. There were several interactive effects between L. zosterae infection and the treatment factors on Z. marina growth, and on the expression of genes associated with immune defense, phenol synthesis and primary metabolism, showing that the molecular reaction toward L. zosterae infection depends on prevailing environmental conditions. Our study shows that individual or interactive effects of light, temperature and tissue damage can affect multiple aspects of host-pathogen interactions in seagrasses. These results highlight the complexity of marine host-pathogen systems, showing that more multi-factorial investigations are needed to gain a better understanding of disease in marine plants under different environmental conditions.

Labyrinthula spp. are saprobic, marine protists that also act as opportunistic pathogens and are the causative agents of seagrass wasting disease (SWD). Despite the threat of local- and large-scale SWD outbreaks, there are currently gaps in our understanding of the drivers of SWD, particularly surrounding Labyrinthula virulence and ecology. Given these uncertainties, we investigated Labyrinthula from a novel genomic perspective by presenting the first draft genome and predicted proteome of a pathogenic isolate of Labyrinthula SR_Ha_C, generated from a hybrid assembly of Nanopore and Illumina sequences. Phylogenetic and cross-phyla comparisons revealed insights into the evolutionary history of Stramenopiles. Genome annotation showed evidence of glideosome-type machinery and an apicoplast protein typically found in protist pathogens and parasites. Proteins involved in Labyrinthula’s actin-myosin mode of transport, as well as carbohydrate degradation were also prevalent. Further, CAZyme functional predictions revealed a repertoire of enzymes involved in breakdown of cell-wall and carbohydrate storage compounds common to seagrasses. The relatively low number of CAZymes annotated from the genome of Labyrinthula SR_Ha_C compared to other Labyrinthulea species may reflect the conservative annotation parameters, a specialised substrate affinity and the scarcity of characterised protist enzymes. Inherently, there is high probability for finding both unique and novel enzymes from Labyrinthula spp. This study provides resources for further exploration of Labyrinthula ecology and evolution, and will hopefully be the catalyst for new hypothesis-driven SWD research revealing more details of molecular interactions between Labyrinthula species and its host substrate.

© 2017 Marine disease ecology is a growing field of research, particularly for host organisms negatively impacted by a changing climate and anthropogenic activities. A decrease in health and increase in susceptibility to disease has been hypothesised as the mechanism behind wide-spread seagrass die-offs related to wasting disease in the past. However, seagrass wasting disease and the causative pathogen, Labyrinthula, have been vastly understudied in the southern hemisphere. Our aim was to build on the current knowledge of Australian Labyrinthula descriptions and phylogeny, while also providing a first look at wasting disease ecology in Australia. Five seagrass species along a 750 km stretch of coastline in southeastern Australia were sampled. The resulting 38 Labyrinthula isolates represented a diversity of morphotypes and five haplotypes of varying phylogenetic clade positions and virulence. The haplotypes clustered with previously-described phylogenetic clades containing isolates from Asia, USA and Europe. Pathogenicity tests confirmed, for the first time, the presence of at least two pathogenic haplotypes in Australia. While historically there have been no reports of wasting disease-related seagrass habitat loss, the presence of pathogenic Labyrinthula highlights the need for disease monitoring and research to understand seagrass wasting disease ecology in Australia.

Zostera marina L. plants have been seriously impacted by wasting disease along the Atlantic coasts of North America and Europe since the 1930s (Muehlstein 1989). Sudden declines in the population sizes of Zostera marina affect primary and secondary producers of different trophic levels in blue carbon ecosystems (Gleason et al. 2013). Muehlstein et al. (1991) first identified Labyrinthula zosterae (Labyrinthulomycetes) as the pathogen causing wasting disease in Zostera marina. However, there have been no reports of wasting disease pathogens affecting seagrass in Korea. In this study, we collected leaves of Z. marina showing symptoms of wasting disease in the southern region of South Korea (Dongdaeman, Namhae, Gyeongnam Province) during field monitoring (from April to September 2013). The pathogens of wasting disease, Labyrinthula zosterae has been isolated from the infected leaves of Z. marina and established as a culture strain (Supplementary Figure 1). Samples of Z. marina and L. zosterae were deposited at the Fisheries Seed and Breeding Research Institute (previous Seaweed Research Center, National Institute of Fisheries Science, South Korea). Microscopic examination of the infected leaf tissues revealed fusiform or spindle-shaped vegetative Labyrinthula cells (4-5 × 15-20 μm). These were similar in size and shape to those previously described for Labyrinthula species. The fusiform cells were cultured in 1% serum seawater agar medium, and they formed colonies and showed gliding motility along a network of hyaline slime filaments. To validate the pathogenicity, re-inoculation tests by L. zosterae were performed with the isolated strains in accordance with Koch's postulates. Healthy leaves of Z. marina collected from the field were used in the re-inoculation tests and were cultured at 15°C under white fluorescent irradiation of approximately 20 μmol·photons·m-2·s-1 and a 12:12-h light:dark cycle (Supplementary Figure 1). Labyrinthula zosterae re-isolated from artificially infected leaves of Z. marina was confirmed by DNA sequence similarity analysis. Total genomic DNA from the infected leaf cells and the culture strains was extracted using the QIAamp DNA Stool Mini Kit (Qiagen, Germany). Internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA were determined to identify Labyrinthula species. L. zosterae-specific primers (Lz2forward (5'- CTAAGACTAAACGAGGCGAAAGCCTAC-3') and Lz2reverse (5'-AGGTTTACAAAACACACTCGTCCACA-3') in Bergmann et al. (2011)) were used to confirm the infection of L. zosterae in the leaves from the field samples and the re-inoculation test samples. Next, PCR products were cloned using a pLUG-Prime® TA-cloning Vector (iNtRON Biotechnology, Korea) and commercially sequenced (SolGent, Korea). The ITS sequence of Korean L. zosterae (accession number MW357748) showed high sequence similarity (99.3-100%) with that of L. zosterae deposited in GenBank (National Center for Biotechnology Information) from BLAST searches. These findings confirm that this is the first report of L. zosterae as the causal pathogen of wasting disease in Z. marina in Korea.

Labyrinthulomycetes are mostly fungus-like heterotrophic protists that absorb nutrients in an osmotrophic or phagotrophic manner. Members of order Labyrinthulida produce unique membrane-bound ectoplasmic networks for movement and feeding. Among the various types of labyrinthulids' food substrates, diatoms play an important role due to their ubiquitous distribution and abundant biomass. We isolated and cultivated new diatom consuming Labyrinthulida strains from shallow coastal marine sediments. We described Labyrinthula diatomea n. sp. that differs from all known labyrinthulids in both molecular and morphological features. We provided strain delimitation within the genus Labyrinthula based on ITS sequences via haplotype network construction and compared it with previous phylogenetic surveys.

Species of the genera Labyrinthula, Schizochytrium, Thraustochytrium and related organisms have recently attracted attention in biotechnology, and here is a short note on how to classify these rather special organisms.

Recent trends suggest that marine disease outbreaks caused by opportunistic pathogens are increasing in frequency and severity. One such malady is seagrass wasting disease, caused by pathogens in the genus Labyrinthula. It is suspected that pathogenicity is intimately linked to the ability of the host to initiate defense responses; however, supportive evidence is lacking. To address this, we developed two techniques, including 1) a new qPCR-based pathogen detection method, and 2) an immune profiling panel via four host-biomarker assays (measuring peroxidase, exochitinase, polyphenol oxidase, and lysozyme activities). These techniques were then used to experimentally investigate the impact of environmental stressors (namely, elevated temperature and salinity) on host immunity and how immune status might affect susceptibility to Labyrinthula infection. In the first experiment, we subjected individual turtlegrass (Thalassia testudinum) shoots to short-term (7 d) abiotic stressors alone. In a second experiment, the same abiotic stressor conditions were followed by pathogen exposure (7 additional d), simulating a scenario where we attempt to isolate the impact of environmental stressors on the host seagrass species by removing the stressor as the pathogen is introduced. The qPCR assay successfully quantified the abundance of Labyrinthula spp. cells from both pure cultures and seagrass tissues across a broad range of predominately pathogenic strains, with high sensitivity. Immune enzyme assays revealed that all four biomarkers were constitutively active in turtlegrass individuals, but specific activities were largely unaffected by the chosen abiotic stressor conditions. We also identified positive correlations between pathogen load and two biomarkers (peroxidase, exochitinase), regardless of abiotic stress treatment, further demonstrating the potential utility of these biomarkers in future applications.

This chapter describes infections of cephalopods by both the Labyrinthulomycetes (formerly fungi, now protists) and organisms still classified as fungi, with information on how to diagnose them. Both types of infection are rare, but those by Labyrinthulomycetes in captive cephalopod populations can increase with time and may last for three or more years, raising concerns about such ubiquitous organisms for cephalopod culture.

Recently, a simple method for the extraction of lipids from wet biomass using liquefied dimethyl ether (DME) without drying, cell disruption, or heating was proposed. Here, the versatility of this method was evaluated for labyrinthula Aurantiochytrium limacinum (A. limacinum). The liquefied DME was passed through the extractor that filled by A. limacinum at different time intervals. The extraction of lipids from A. limacinum of moisture-rich microorganism was successfully achieved, the yield of lipid was 46.1 wt% of the dry weight of the sample. In comparison, the yields of lipid were 21.3 wt%, 43.6 wt% and 50.7 wt% when supercritical carbon dioxide (SCCO 2 ), hexane-Soxhlet and Bligh-Dyer (BD) extraction methods were applied as extractants, respectively. However, the drying and cell-disruption process were required in SCCO 2 , hexane-Soxhlet, and BD extraction methods.

Seagrass meadows form ecologically and economically valuable coastal habitat on every continental margin except the Antarctic, but their areal extent is declining by approximately 2–5 % per year. Seagrass wasting disease is a contributing factor in these declines, with the protist Labyrinthula identified as the etiologic agent. To help elucidate the role of Labyrinthula spp. in global seagrass declines, we surveyed roughly one fourth of all seagrass species to identify Labyrinthula diversity at the strain and/or species level, combining results from culturing methods and two common nuclear DNA markers: the ITS and 18S regions of the ribosomal RNA gene complex. After assaying a subset of the resulting isolates (of which 170 were newly sequenced), we produced a cladogenic context for putative seagrass-pathogenic versus non-pathogenic Labyrinthula while also defining host and geographic ranges. Assays also suggest that pathogenicity is consistently high (when present; and, even when comparing susceptibility of US East- versus West Coast Zostera marina hosts) while virulence is variable, that some isolate-host combinations have the potential for host cross-infection, and that several modes of transmission can be effective. Taken together, these data provide additional means for delimiting putative species of Labyrinthula, suggesting at least five seagrass-pathogenic and perhaps ten or more non-pathogenic marine “species”, yielding a working definition for ecologists and epidemiologists attempting to reconcile the sundry data related to seagrass wasting disease.

Slime nets, Labyrinthula sp., were found associated with several seagrass species, e.g., Zannichellia palustris, Zostera marina and Myriophyllum sibiricum, and on the alga Vaucheria sp. in autumn and winter samples from western Aland (N Baltic Sea, salinity 4–6 PSU). Labyrinthula survived for many months and increased in dark and cool conditions among decaying plants, indicating a non-parasitic mode of life. Labyrinthula colonies from natural samples were studied in small chambers where the organism, even in cool water, settled and made new slimeways, filopods and lamellipods on the bottom glass. Labyrinthula was preyed upon by several amoeba species. Slime nets have probably been overlooked and they are seldom reported from the Baltic Sea.

Seagrasses are ecosystem engineers of essential marine habitat. Their populations are rapidly declining worldwide. One potential cause of seagrass population declines is wasting disease, which is caused by opportunistic pathogens in the genus Labyrinthula. While infection with these pathogens is common in seagrasses, theory suggests that disease only occurs when environmental stressors cause immunosuppression of the host. Recent evidence suggests that host factors may also contribute to disease caused by opportunistic pathogens. In order to quantify patterns of disease, identify risk factors, and investigate responses to infection, we surveyed shoot density, shoot length, epiphyte load, production of plant defenses (phenols), and wasting disease prevalence in eelgrass Zostera marina across 11 sites in the central Salish Sea (Washington state, USA), a region where both wasting disease and eelgrass declines have been documented. Wasting disease was diagnosed by the presence of necrotic lesions, and Labyrinthula cells were identified with histology. Disease prevalence among sites varied from 6 to 79%. The probability of a shoot being diseased was higher in longer shoots, in patches of higher shoot density, and in shoots with higher levels of biofouling from epiphytes. Phenolic concentration was higher in diseased leaves. We hypothesize that this results from the induction of phenols during infection. Additional research is needed to evaluate whether phenols are an adaptive defense against Labyrinthula infection. The high site-level variation in disease prevalence emphasizes the potential for wasting disease to be causing some of the observed decline in eelgrass beds.

© Inter-Research 2018. Marine heterotrophic protists of the Labyrinthulomycota are of interest for their biotechnological (e.g. thraustochytrid production of lipids) and ecological (e.g. wasting disease and rapid blight by pathogens of the genus Labyrinthula) applications; culture-based laboratory studies are a central technique of this research. However, maintaining such microorganism cultures can be labour- and cost-intensive, with a high risk of culture contamination and die-off over time. Deep-freeze storage, or cryopreservation, can be used to maintain culture back-ups, as well as to preserve the genetic and phenotypic properties of the microorganisms; however, this method has not been tested for the ubiquitous marine protists Labyrinthula spp. In this study, we trialled 12 cryopreservation protocols on 3 Labyrinthula sp. isolates of varying colony morphological traits. After 6 mo at -80°C storage, the DMSO and glycerol protocols were the most effective cryoprotectants compared to methanol (up to 90% success vs. 50% success, respectively). The addition of 30% horse serum to the cryoprotectant solution increased Labyrinthula sp. growth success by 20-30%. We expect that these protocols will provide extra security for culture-based studies, as well as opportunities for long-term research on key Labyrinthula sp. isolates.

Like both terrestrial plants and other benthic marine organisms, seagrasses host abundant and diverse communities of microorganisms. These microbes fundamentally influence seagrass physiology and health, while also regulating the biogeochemical dynamics of entire seagrass meadows. Discrete populations of bacteria, fungi, microalgae, archaea and viruses inhabit seagrass leaves, roots and rhizomes and the surrounding sediments. The plethora of ecological interactions taking place between seagrasses and this microbiome span the continuum of symbiotic relationships from mutualism to parasitism. Indeed, the metabolic activities of some seagrass associated microbes, such as diazotrophic and sulphur oxidizing bacteria, govern the local chemical environment in ways that facilitate seagrass survival. On the other hand, pathogens, such as the protozoan parasite Labyrinthula cause disease outbreaks that can lead to mass seagrass die offs. While the role of the seagrass microbiome in defining the success of seagrass habitats is becoming increasingly apparent, there is still much to be learnt. For instance, the development of an understanding of how seagrass associated microbes may buffer or augment the negative impacts of growing environmental pressures will be valuable for informing decisions regarding the management and conservation of threatened seagrass habitats. In this chapter we will synthesise the current state of knowledge on the microbiology of seagrasses, with a goal of conveying the often overlooked importance of the seagrass microbiome in governing seagrass health and the biogeochemical stability of seagrass ecosystems.

Labyrinthula terrestris, the causal agent of rapid blight disease in cool-season turfgrasses, is the first Labyrinthula species found to be associated with higher plants in terrestrial ecosystems. An initial study suggested a single common genotype associated with turfgrass in the US, but a later study revealed genetic diversity in this pathogen. In the present study, genetic diversity as well as differences in pathogenicity and morphology was determined in Labyrinthula spp. from turfgrasses in New Mexico and Arizona where rapid blight had not previously been identified. In total, 96 Labyrinthula isolates from New Mexico and four from Arizona were sequenced in 2011 and 2012. A phylogenetic approach based on 18S and ITS-LSU of ribosomal DNA was used for genotypic diversity, and pathogenicity and microscopy were used for phenotypic diversity. A maximum parsimony analysis revealed four genetic groups including two previously described Labyrinthula groups, Laby10 (L. terrestris) and Laby31. Laby10 was the most predominant with 70 % of isolates followed by Laby31 with 24 %. Two new groups, Laby879 and Laby2020, contained only five and one isolate, respectively. An association between genotypes and phenotypes was observed. Laby10 and Laby31 were highly virulent as well as larger in cell size (14.9 and 18.1 μm, respectively), while Laby879 was less virulent with an average cell size of 8.5 μm. Laby2020 was the smallest (0.8 μm) and non-pathogenic. Results showed a prevalence of pathogenic and non-pathogenic Labyrinthula spp. in turfgrasses in New Mexico and the presence of the Laby31 group in the US for the first time.

Thraustochytrids are marine protists belonging to the class Labyrinthula of the kingdom Chromista. This group of microbes is considered a promising alternative source of high-valued omega-3 oils, especially docosahexaenoic acid (DHA, C22:6 n 3), replacing fish oil which is the current major source of DHA and eicosapentaenoic acid (EPA, C20:5 n 3). Our new isolate, SW1, was considered to be an Aurantiochytrium based on its morphology, fatty acid profile and molecular phylogenetic analysis. Microscopic observations revealed that SW1 has high similarity to Aurantiochytrium limacinum ATCC MYA-1381 (SR21), possessing spherical vegetative cells which undergo repeated bipartition to form diads, tetrads, octads and zoosporangia that release motile zoospores. When cultivated in four different media, SW1 produced the highest biomass (13.17 g/L) and DHA (3.6 g/L) in a medium composed of glucose, sea salt, monosodium glutamate (MSG) and yeast extract. Visualization of lipid droplet development in SW1 using Sudan Black B dye revealed that lipid droplets enlarge to occupy almost the entire cell volume within 72 to 96 h of cultivation. This strain was also found to be able to utilize various saccharides as carbon source. The results of this study show that Aurantiochytrium sp. SW1 is a potential candidate to be developed as commercial microbial DHA producer.

a b s t r a c t Temperate and tropical seagrasses are susceptible to wasting disease outbreaks caused by pathogenic protists of the genus Labyrinthula. Even though there is an increasing awareness of the environmental conditions that influence the etiology of seagrass-Labyrinthula disease dynamics, the biochemical basis of seagrass defense responses, in particular chemical defenses, is still vastly understudied. Using an in vitro bioassay, we provide evidence that previously characterized phenolic and potentially novel, undescribed non-phenolic metabolites derived from Thalassia testudinum Banks ex Konig exhibit anti-labyrinthulid activity. All phenolic compounds tested displayed dose-dependent behavior and selected combinations interacted synergistically. The flavone glycoside thalassiolin B was roughly 20-100 times more active than any phenolic acid tested. Based upon values reported in the literature, it was calculated that infected specimens of T. testudinum contain natural concentrations of phenolic acids that are consistently greater than what is required to inhibit Labyrinthula growth. This suggests that while there may be an ample supply of phenolic-based derivatives available to inhibit Labyrinthula growth, they may not be readily bio-accessible. Using a bioactivity-guided approach, a semi-purified chemical fraction from T. testudinum was found

Thraustochytrids are unicellular fungi-like (heterotrophic) marine protists that have long been considered to play an important role in the biogeochemical cycles of the coastal oceans. However, the significance of their ecological functions and their diversity in marine ecosystems remain largely unknown. In this report, we examined the spatial and temporal variations of planktonic thraustochytrids, their relationship with other environmental factors, and their diversity in the subtropical coastal waters of China. The abundance of planktonic thraustochytrids ranged from 2.56 × 105 to 17.57 × 105 cells L-1 with highest abundance detected in polluted coastal water in the Spring (March) season. The thraustochytrid biomass was greater than the bacterial biomass in most seawater samples, ranging from 32.29 to 359.51% that of bacterioplankton. The abundance of thraustochytrids appeared to be largely related to that of bacterioplankton and to chemical oxygen demand in water columns. High-throughput sequencing analyses revealed a total of 105 OTUs (97% similarity), which were members of genera Thraustochytrium, Aplanochytrium, Oblongichytrium, Ulkenia, Labyrinthula and undescribed novel phylotypes. Results of this study indicated unprecedented high diversity of labyrinthulomycetes as well as the presence of novel labyrinthulomycete and thraustochytrid lineages, and also provided new information on the significant role of thraustochytrids in microbial food webs in a coastal marine ecosystem.

Seagrass meadows suffered large-scale declines in the past century. The 'wasting disease', pathognomonically associated with Labyrinthula zosterae, reduced populations of Zostera marina on both sides of the North Atlantic in, and since, the 1930s, coinciding with intensive agricultural use of artificial fertilizers and herbicides. This study tests the long-standing hypothesis that nutrient enrichment and a herbicide increases vulnerability to pathogens. Z. marina shoots from the Thames Estuary grown in elevated nitrate concentrations had significantly higher rates of infection by L. zosterae than controls, but not by Aplanochytrium sp., another slime-mould like protist. Z. marina shoots grown in 2μg·l-1 Diuron solutions and infected separately by L. zosterae and Aplanochytrium sp. had significantly higher wasting indices than controls. The results identified Aplanochytrium sp. as another opportunistic pathogen causing a seagrass wasting-type disease and support the hypothesis that pollution by herbicides and nitrate increases the susceptibility of Z. marina to infections.

The phylum Labyrinthulomycota comprises diverse marine fungus-like protists that are an abundant and widespread component of the marine microbiota. Despite their ubiquity in marine ecosystems, relatively little is known about the ecology of any of the pathogenic species in the Labyrinthulomycota. Most are thought to exist as saprobes, but many species have been documented as pathogens of marine metazoans and metaphytes. The best studied labyrinthulomycotan pathogen in molluscs is Quahog Parasite Unknown (QPX), which causes mortality events in both wild and cultured hard clams

© 2014, Coastal and Estuarine Research Federation. Climate change is predicted to alter pathogen–host relationships and there is evidence of an increase in disease in both terrestrial and marine environments. Infection rates do not always increase linearly with temperature since thermal tolerance ranges of host and pathogens do not necessarily overlap and the host may benefit from thermal refugia of low or no disease pressure. Here, we demonstrate that climate warming may alleviate pathogen pressure in a climate-vulnerable Mediterranean seagrass, Posidonia oceanica. We tested the impact of warming on infection by Labyrinthula sp.—the causative pathogen of wasting disease—and the combined effect of elevated temperature and disease on photobiology. Infected and control shoots of P. oceanica were incubated at temperatures between 24 and 32 °C, encompassing maximum summer seawater temperatures projected for the Mediterranean Sea during the twenty-first century. Warming reduced the occurrence and severity of the disease and temperatures >28 °C inhibited cell division and growth of Labyrinthula. Photochemical efficiency was not significantly affected by short-term warming or by Labyrinthula infection. These results suggest that, unlike what has been predicted for the majority of pathogen–host relationships, warming may lead to a reduced risk of wasting disease in P. oceanica and relieve pathogen pressure from this species., This research was supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme (TEMSPATH, 254297: FP7-PEOPLE-2009-IEF) awarded to YS Olsen and the ESTRESX project funded by the Spanish Ministry of Economy and Competitivity (CTM2012-32603). M. Potouroglou was funded by a Short-Term Scientific Mission of the COST action ES0906 “Seagrass productivity: from genes to ecosystem management”

Marine pathogens of the genus Labyrinthula have been identified as the cause of wasting disease in seagrass systems in both temperate and subtropical regions. An understanding of the association between environmental factors and the prevalence of wasting disease in seagrass meadows is important for elucidating plant–pathogen interactions in coastal environments. We conducted a survey of 7 turtle grass-dominated beds within the Florida Keys National Marine Sanctuary to assess the relationship between environmental and biological parameters on seagrass health. All sites contained Labyrinthula spp.; the most pathogenic strain was obtained from an anthropogenically impacted site. Leaf and total biomass, in addition to root/rhizome carbon content, canopy light and % light transmitted, all displayed strong negative correlations with a wasting index (WI). It was noted that many of the same environmental measurements that showed negative correlations with WI also displayed strong positive correlations with canopy light levels. These data suggest that light availability may be an important factor that has previously been understated in the seagrass disease literature yet warrants more attention with respect to turtle grass susceptibility to infection. Studies such as this are important because they identify gaps in our understanding of plant–pathogen interactions in subtropical marine ecosystems. Furthermore, the relationships identified in this study may offer insight into which factors are most useful in identifying “at-risk” meadows prior to the onset of larger scale die-off events.

Overwhelming evidence suggests that seagrass ecosystems are declining around the world. Pathogens from the genus Labyrinthula have repeatedly been found to cause disease in a variety of seagrass species. For example, the ‘wasting disease’ of Zostera marina has been attributed to Labyrinthula infection. Although poorly characterized taxonomically, species of Labyrinthula are very common in marine ecosystems, virulence of genotypes/phylotypes is known to be variable, and highly virulent species are able to cause ecologically significant diseases of protists, plants and animals. Here, the pathosystem model is applied to host–parasite relationships in seagrass ecosystems. Known physical and biological stressors of seagrass are reviewed. Finally, we make the case that it is time to expand research on this poorly studied microorganism in order to quantify the role of disease in seagrass populations world-wide.

As a result of anthropogenic influences and global climate change, emerging infectious marine diseases are thought to be increasingly more common and more severe than in the past. The aim of our investigation was to confirm the presence of Labyrinthula, the aetiological agent of the seagrass wasting disease, in Southeastern Australia and provide the first isolation and characterisation of this protist, in Australia. Colonies and individual cells were positively identified as Labyrinthula using published descriptions, diagrams, and photographs. Their identity was then confirmed using DNA barcoding of a region of the 18S rRNA gene. Species level identification of isolates was not possible as the taxonomy of the Labyrinthula is still poorly resolved. Still, a diversity of Labyrinthula was isolated from small sections of the southeast coast of Australia. The isolates were grouped into three haplotypes that are biogeographically restricted. These haplotypes are closely related to previously identified saprotrophic clades. The study highlights the need for further investigation into the global distribution of Labyrinthula, including phylogenetic pathogenicity and analysis of host-parasite interactions in response to stressors. Given the results of our analyses, it is prudent to continue research into disease and epidemic agents to better prepare researchers for potential future outbreaks.

Seagrasses worldwide are commonly infected by endophytic protists of the genus Labyrinthula. To date, the nature of interaction of endophyte and host is not well understood. In eelgrass (Zostera marina) Labyrinthula zosterae may become virulent (pathogenic) and lead to the loss of entire sea grass beds. One of the best known examples of any marine epidemic were outbreaks of the ´wasting disease´ on both sides of the Atlantic in the 1930s, but smaller infestations have been reported until recently. Up to now, detection of infection by Labyrinthula was based on the wasting index, i.e. the relative area of leaf lesions or microscopy, while genetic data are virtually absent. We characterized a ~1400 base pair portion of the 18S small subunit rDNA in L. zosterae isolates (N=41) from six northern European sites and one southern location (Adriatic Sea) in order to assess identity and potential diversity of endophytic protists. Because there are indications that low salinity impedes Labyrinthula growth, sampling sites included a wide range of salinities from 5-34 psu. A search against the non-redundant GENBANK data base revealed that most isolates are 99% similar to the only L. zosterae 18S sequence available from the data base at all but the Finish site (salinity values 5-7 psu). At the latter site, a different Labyrinthula species occurred, which was also found in fully marine Wadden Sea cultures. A third species was detected in Skagerrak, south-western Baltic and North Sea samples (20-25 psu). We conclude that L. zosterae is widespread among northern European eelgrass sites across wide ranges of salinity.