SMORES: a simple microfluidic operating room for the examination and surgery of Stentor coeruleus.

Ciliates are powerful unicellular model organisms that have been used to elucidate fundamental biological processes. However, the high motility of ciliates presents a major challenge in studies using live-cell microscopy and microsurgery. While various immobilization methods have been developed, they are physiologically disruptive to the cell and incompatible with microscopy and/or microsurgery. Here, we describe a Simple Microfluidic Operating Room for the Examination and Surgery of Stentor coeruleus (SMORES). SMORES uses Quake valve-based microfluidics to trap, compress, and perform surgery on Stentor as our model ciliate. Compared with previous methods, immobilization by physical compression in SMORES is more effective and uniform. The mean velocity of compressed cells is 24 times less than that of uncompressed cells. The compression is minimally disruptive to the cell and is easily applied or removed using a 3D-printed pressure rig. We demonstrate cell immobilization for up to 2 h without sacrificing cell viability. SMORES is compatible with confocal microscopy and is capable of media exchange for pharmacokinetic studies. Finally, the modular design of SMORES allows laser ablation or mechanical dissection of a cell into many cell fragments at once. These capabilities are expected to enable biological studies previously impossible in ciliates and other motile species.

Microplastics drive community dynamics of periphytic protozoan fauna in marine environments.

The pollution of microplastics (MPs) to the marine environment has become a widespread focus of attention. To assess MP-induced ecotoxicity on marine ecosystems, periphytic protozoan communities were used as test organisms and exposed to five concentrations of MPs: 0, 1, 5, 25, and 125 mg l-1. Protozoan samples were collected using microscope slides from coastal waters of the Yellow Sea, northern China. A total of 13 protozoan species were identified and represented different tolerance to MP-induced ecotoxicity. Inhibition effects of MPs on the test protozoan communities were clearly shown in terms of both the species richness and individual abundance and followed linear relationships to MP concentrations. The community patterns were driven by MPs and significantly shifted at concentrations over 5 mg l-1. Our findings demonstrated that MPs may induce the community-level ecotoxic response of periphytic protozoan fauna and followed significant community dynamics. Thus, it is suggested that periphytic protozoan fauna may be used as useful community-based test model organisms for evaluating MP-induced ecotoxicity in marine environments.

Transcriptomic analysis of large yellow croaker (Larimichthys crocea) reveals the suppression of the inflammatory response from Cryptocaryon irritans infection.

Large yellow croaker (Larimichthys crocea) is the most productive marine fish in China. Cryptocaryon irritans is an extremely destructive parasite that causes great economic losses in large yellow croaker aquaculture industry. Therefore, it is very necessary to study the immune response of large yellow croaker in response to C. irritans infection. In this study, the transcriptomic profiles of large yellow croaker were sequenced and analyzed in the brain and head kidney at 72 h after C. irritans infection. Cytokines and chemokines related terms were significantly enriched based on the GO enrichment of down-regulated differentially expressed genes (DEGs) from the head kidney. Meanwhile, cytokine-cytokine receptor interaction was significantly enriched based on the KEGG enrichment of up-regulated DEGs from the brain and down-regulated DEGs from the head kidney, respectively. Moreover, the majority of inflammation-related DEGs were significantly up-regulated in the brain, but distinctly down-regulated in the head kidney. These results showed that the brain and head kidney might play different roles against C. irritans infection, and the inflammatory response of large yellow croaker may be restrained during C. irritans infection. Taken together, the transcriptomic analyses will be helpful to more comprehensively understand the immune mechanism of teleost against C. irritans infection, and provide a theoretical basis for the prevention and treatment of Cryptosporidiosis.

Does prey availability influence the detection of Dinophysis spp. by the imaging FlowCytobot?

The Imaging FlowCytobot (IFCB) is a field-deployable imaging-in-flow cytometer that is increasingly being used to monitor harmful algae. The IFCB acquires images of suspended particles based on their chlorophyll-a fluorescence and/or the amount of light they scatter (side scattering). The present study hypothesized that fluorescence-based image acquisition would undercount Dinophysis spp., a genus of non-constitutive mixotrophs, when prey is limited. This is because Dinophysis spp. acquire plastids via ingestion of their ciliate prey Mesodinium spp., and lose photosynthetic capacity and autofluorescence in the absence of prey. Even small blooms of Dinophysis spp. can be highly toxic and result in diarrhetic shellfish poisoning (DSP), highlighting the importance of accurately detecting low abundances. To explore this, laboratory experiments were conducted to determine optimal IFCB settings for a fed culture of Dinophysis acuminata, and an existing time series of IFCB observations collected in Puget Sound (Washington, U.S.A) was used to compare Dinophysis spp. abundance estimates from samples triggered via side scattering versus fluorescence in relation to Mesodinium spp. abundance. This study introduces a quantitative approach for optimizing the detection of target harmful algae which can be repeated across multiple IFCBs and demonstrates the effects of IFCB calibration on Dinophysis spp. detection. The laboratory experiments showed that IFCB settings for fluorescence-based image acquisition need to be fairly sensitive to accurately detect D. acuminata cells. A poorly calibrated IFCB can miss a significant proportion of D. acuminata abundance whatever the method used to trigger the image acquisition. Field results demonstrated that the physiological status of Dinophysis spp. can influence their detection by the IFCB when triggering on fluorescence. This was observed during a 7-day period when the IFCB failed to detect Dinophysis spp. cells when triggering on fluorescence while cells were still detected using the side scattering triggering method as well as observed by microscopy. During this period, Mesodinium spp. was not detected, IFCB-derived autofluorescence level of individual cells of Dinophysis spp. was low, and less than 50 % of Dinophysis spp. cells exhibited autofluorescence under the microscope. Together, this indicates that the unique feeding ecology of Dinophysis spp. may affect their detection by the IFCB when cells are starved.

Transcriptomic Variation of Amphiprion Percula (Lacepède, 1802) in Response to Infection with Cryptocaryon Irritans Brown, 1951.

Cryptocaryon irritans (Brown 1951) frequently infect the Pomacentridae fishes causing severe economic losses. However, the anti-C. irritans' molecular mechanism in these fishes remains largely unknown. To address this issue, we conducted RNA-Seq for C. irrtians-infected gills of the clownfish Amphiprion percula (Lacepède 1802) at the early (day 1) and late (day 3) stages of infection. A total of 1655 differentially expressed genes (DEGs) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEGs showed a vast genetic variation related to the following aspects: ECM-receptor interaction, P13K-Akt signalling, cytokine-cytokine receptor interaction, and endocytosis. During the early phase of infection, key genes involved in ATP production, energy homeostasis, and stress control were abruptly increased. In the late phase, however, acute response molecules of the peripheral nervous system (synaptic transmission and local immunity), metabolic system triggering glycogen synthesis, energy maintenance, and osmoregulation were found to be critical. The highest number of upregulated genes (URGs) recovered during the early phase was included under the 'biological process' category, which primarily functions as response to stimuli, signalling, and biological regulation. In the late phase, most of the URGs were related to gene regulation and immune system processes under 'molecular function' category. The immune-related URGs of early infection include major histocompatibility complex (MHC) class-II molecules apparently triggering CD4+ T-cell-activated Th responses, and that of late infection include MHC class-1 molecules for the possible culmination of CD8+ T-cell triggered cytotoxicity. The high level of genic single nucleotide polymorphisms (SNPs) identified during the late phase of infection is likely to influence their susceptibility to secondary infection. In summary, the identified DEGs and their related metabolic and immune-related pathways and the SNPs may provide new insights into coordinating the immunological events and improving resistance in Pomacentridae fishes against C. irritans.

Tandem mass tag (TMT) quantitative proteomics and phosphoproteomic of Takifugu rubripes infected with Cryptocaryon irritans.

In this study, we identified the differentially expressed proteins in gills stimulated by infected ciliates and analyzed the immune mechanisms of T. rubripes infected with the ciliate Cryptocaryon irritans. Through liquid chromatography analysis, a total of 144 proteins were identified with significant differences, of which 58 were upregulated and 86 were downregulated. Among phosphorylated proteins, we identified a total of 167 significantly different phosphorylated proteins, of which 44 were upregulated, 123 were downregulated, 60 were upregulated, and 208 were downregulated. We analyzed the data of proteomics and Phosphorylated proteome quantification protein omics to finally identify three phosphorylated proteins (RPS27, eNOS and CaM) and two phosphorylated protein kinases(CaMKII and MAPK1) as potential biomarkers for T. rubripes immune responses. We finally identified three phosphorylated proteins (RPS27, eNOS and CaM) and two phosphorylated protein kinases (CaMKII and MAPK1) as potential biomarkers of immune response of T. rubripes. Our research findings provide new insights into the immune mechanism of T. rubripes, which may serve as an effective indicator of C. irritans infection in T. rubripes.

Vibrio harveyi co-infected with Cryptocaryon irritans to orange-spotted groupers Epinephelus coioides.

The orange-spotted grouper (Epinephelus coioides) is a high economic value aquacultural fish in China, however, it often suffers from the outbreak of parasitic ciliate Cryptocaryon irritans as well as bacterium Vibrio harveyi which bring great loss in grouper farming. In the present study, we established a high dose C. irritans local-infected model which caused the mortality of groupers which showed low vitality and histopathological analysis demonstrated inflammatory response and degeneration in infected skin, gill and liver. In addition, gene expression of inflammatory cytokines was detected to assist the estimate of inflammatory response. Furthermore, we also found that the activity of Na+/K+ ATPase in gill was decreased in groupers infected C. irritans and the concentration of Na+/Cl- in blood were varied. Base on the morbidity symptom occurring in noninfected organs, we hypothesized that the result of morbidity and mortality were due to secondary bacterial infection post parasitism of C. irritans. Moreover, four strains of bacteria were isolated from the infected site skin and liver of local-infected groupers which were identified as V. harveyi in accordance of phenotypic traits, biochemical characterization and molecular analysis of 16S rDNA genes, housekeeping genes (gyrB and cpn60) and species-specific gene Vhhp2. Regression tests of injecting the isolated strain V. harveyi has showed high pathogenicity to groupers. In conclusion, these findings provide the evidence of coinfections with C. irritans and V. harveyi in orange-spotted grouper.

Temperature-dependent resistance to starvation of three contrasting freshwater ciliates.

We investigated the temperature-dependent response to starvation of three contrasting freshwater ciliates (Ciliophora). The cyst-forming algivorous species Meseres corlissi and the bactivorous species Glaucomides bromelicola, which cannot form cysts, co-occur in the reservoirs (tanks) of tree bromeliads. The mixotrophic species Coleps spetai is common in many lakes. We hypothesized that the ciliates' different traits and life strategies would affect their survival rates and temperature sensitivity under food depleted conditions. We measured the decline of the ciliate populations in microcosm experiments at different temperatures for several days. We used an imaging flow cytometer to size the ciliates and documented their morphological and physiological changes in response to starvation. We found that the cyst-forming species had the highest mortality rates but may endure long-term starvation by encystment. The sympatric, non-encysting species suffered the lowest mortality rates and could survive for more than three weeks without food. The mixotrophic species had intermediate mortality rates but showed the highest phenotypic plasticity in response to starvation. A significant fraction of the C. spetai population appeared unaffected by starvation, suggesting that the endosymbionts provided some resources to the host cells. The mean mortality rate per day of all three species increased with temperature by 0.09 °C-1.

Integrated mRNA and miRNA expression analyses for Cryptocaryon irritans resistance in large yellow croaker (Larimichthys crocea).

Large yellow croaker (Larimichthys crocea) is one of the most important mariculture fish in China. However, cryptocaryonosis caused by Cryptocryon irritans infection has brought huge economic losses and threatened the healthy and sustainable development of L. crocea industry. Recently, a new C. irritans resistance strain of L. crocea (RS) has been bred using genomic selection technology in our laboratory work. However, the molecular mechanisms for C. irritans resistance of RS have not been fully understood. MicroRNAs (miRNAs) are endogenous small non-coding RNAs that are post-transcriptional regulators, and they play vital roles in immune process of bony fish. Identification of anti-C.irritans relevant miRNA signatures could, therefore, be of tremendous translational value. In the present study, integrated mRNA and miRNA expression analysis was used to explore C. irritans resistance mechanisms of the L. crocea. RS as well as a control strain (CS) of L. crocea, were artificially infected with C. irritans for 100 h, and their gill was collected at 0 h (pre-infection), 24 h (initial infection), and 72 h (peak infection) time points. The total RNA from gill tissues was extracted and used for transcriptome sequencing and small RNA sequencing. After sequencing, 23,172 known mRNAs and 289 known miRNAs were identified. The differential expression was analyzed in these mRNAs and mRNAs and the interactions of miRNA-mRNA pairs were constructed. KEGG pathway enrichment analyses showed that these putative target mRNAs of differentially expressed miRNAs (DEMs) were enriched in different immune-related pathways after C. irritans infection in RS and CS. Among them, necroptosis was the immune-related pathway that was only significantly enriched at two infection stages of RS group (RS-24 h/RS-0h and RS-72 h/RS-0h). Further investigation indicates that necroptosis may be activated by DEMs such as miR-133a-3p, miR-142a-3p and miR-135c, this promotes inflammation responses and pathogen elimination. These DEMs were selected as miRNAs that could potentially regulate the C. irritans resistance of L. crocea. Though these inferences need to be further verified, these findings will be helpful for the research of the molecular mechanism of C. irritans resistance of L. crocea and miRNA-assisted molecular breeding of aquatic animals.

Functional ecology of planktonic ciliates: Measuring mortality rates in response to starvation.

Population dynamics of aquatic ciliates are controlled "bottom-up" via food supply and "top-down" by grazing and parasitism. While intrinsic growth rates of ciliates under saturating food conditions have been studied in some detail, mortality rates induced by starvation have received little attention thus far. To this end, we examined the response of three algivorous freshwater ciliate species to starvation using three different optical methods. Two of these methods, i.e. ciliate mortality rates (δ) estimated from (i) numerical response experiments and (ii) the rate of decline (ROD) in cell numbers, investigated the response of the ciliate population using conventional light microscopy. The third method, imaging cytometry using a FlowCAM instrument, monitored single cells during the starvation experiment. Like light microscopy, the FlowCAM approach estimated δ based on ROD in the experimental containers. However, imaging cytometry also measured the relative cellular chlorophyll a content in the ciliates' food vacuoles as a proxy for the nutritional status of the cells. The linear decline of the cellular chl. a yielded an independent estimate of δ that was similar to δ calculated from ROD. Additionally, the FlowCAM measurements revealed a high degree of phenotypic plasticity of the ciliates when exposed to starvation.

Effects of Cryptocaryon irritans infection on the histopathology, oxidative stress, immune response, and intestinal microbiota in the orange-spotted grouper Epinephelus coioides.

Cryptocaryon irritans is a parasitic ciliate of marine fish, causing serious mortality and economic loss of grouper. In this study, the orange-spotted grouper (Epinephelus coioides) were separately exposed to C. irritans infection for 72 h at a dose of 5000 or 10000 active theronts per fish, and we evaluated the changes in histopathology, oxidative stress, immune response, and intestinal microbiota composition. The results showed that C. irritans infection caused pathological alteration on the skin, gills, and liver of E. coioides. Oxidative stress responses occurred in the liver and gills, reflected in the corresponding antioxidant enzyme and gene indexes. The mRNA expression levels of inflammation-related genes (IL-1ß, IL-6, and IL-8) and the mediators of apoptosis (casp3, casp9, and cytc) were increased in the liver and gills of the fish. C. irritans infection also affected the diversity and composition of intestinal microbiota. Specifically, the relative abundance of Firmicutes was increased, whereas that of Proteobacteria was decreased. Several potentially beneficial bacteria (Pandoraea, Clostridium sensu stricto 1, Christensenellaceae R-7 group, and Weissella) were decreased, whereas pathogenic bacteria (Streptococcus and Acinetobacter) were increased. In conclusion, this study reveals that C. irritans infection caused histopathology, immune disorders, and intestinal microbial community variation in E. coioides.

Host responses against the fish parasitizing ciliate Cryptocaryon irritans.

The parasitic ciliate Cryptocaryon irritans, which infects almost all marine fish species occurring in both tropical and subtropical regions throughout the world. The disease, cryptocaryonosis, accounts for significant economic losses to the aquaculture industry. This review attempts to provide a comprehensive overview of the biology of the parasite, host-parasite interactions and both specific and non-specific host defense mechanisms are responsible for the protection of fish against challenge infections with this ciliate. Also, this article reflects the current interest in this subject area and the quest to develop an available vaccine against the disease. Due to the high frequency of clinical fish cryptocaryonosis, the study of fish immune responses to C. irritans provides an optimal experimental model for understanding immunity against extracellular protozoa.

Cascading effects of prey identity on gene expression in a kleptoplastidic ciliate.

Kleptoplastidic, or chloroplast stealing, lineages transiently retain functional photosynthetic machinery from algal prey. This machinery, and its photosynthetic outputs, must be integrated into the host's metabolism, but the details of this integration are poorly understood. Here, we study this metabolic integration in the ciliate Mesodinium chamaeleon, a coastal marine species capable of retaining chloroplasts from at least six distinct genera of cryptophyte algae. To assess the effects of feeding history on ciliate physiology and gene expression, we acclimated M. chamaeleon to four different types of prey and contrasted well-fed and starved treatments. Consistent with previous physiological work on the ciliate, we found that starved ciliates had lower chlorophyll content, photosynthetic rates, and growth rates than their well-fed counterparts. However, ciliate gene expression mirrored prey phylogenetic relationships rather than physiological status, suggesting that, even as M. chamaeleon cells were starved of prey, their overarching regulatory systems remained tuned to the prey type to which they had been acclimated. Collectively, our results indicate a surprising degree of prey-specific host transcriptional adjustments, implying varied integration of prey metabolic potential into many aspects of ciliate physiology.

Effects of autophagy inhibition by 3-methyladenine on encystation, morphology, and metabolites of Cryptocaryon irritans.

Encystment is crucial for defense and reproduction in Cryptocaryon irritans. Therefore, understanding the encystment-related events in the protomont stage can help prevent and control C. irritans. Autophagy promotes protozoan parasite encystation. However, 3MA can inhibit autophagy. In this study, the effects of autophagy inhibition on encystation, survival rate, ultrastructural features, and metabolomic profiles of C. irritans, were evaluated using protomonts treated with 3MA (20 mM). The treatment with 3MA for about 4 h significantly lowered survival and encystation rates of protomonts to about 86.44% and 76.08%, respectively. Microstructural observations showed that the 3MA-treated protomonts showed deformed cell membranes and the cytoplasmic content spill. Furthermore, observation of the ultrastructure of 3MA-treated protomonts showed the destruction of organelles (Golgi bodies and mucocyst) and a lack of autophagosomes. However, no abnormality was observed in the control experiments. Furthermore, the metabolic analysis revealed suppression of metabolites, such as lipids, amino acids, and carbohydrates. These results demonstrate that 3MA can inhibit autophagy in C. irritans, thus hindering encystation, suppressing the metabolism of metabolites, and altering morphological ultrastructure in these parasites.

Treatments of orange-spotted grouper (Epinephelus coioides) against Cryptocaryon irritans with •OH, ClO2 or HCHO: Survival, physiological and histological response.

Cryptocaryon irritans causes one of the most serious diseases in various wild and cultured marine fish, leading to mass mortality and economic loss. In this study, hydroxyl radical (•OH) solution produced by strong ionization discharge combined with water jet cavitation effect was injected into orange-spotted grouper (Epinephelus coioides) aquaculture tanks for C. irritans control. The results showed that all C. irritans theronts were inactivated by •OH solution at concentrations of 0.5 mg/L within 2 min. •OH could induce alteration of shape, the absence of motility and macronucleus dispersion in theronts. A possible explanation was that the macronucleus of C. irritans might be damaged by •OH; as a result, its metabolism and life activities were disturbed. The •OH treatment increased the survival rate of E. coioides challenged with C. irritans from 64.7 ± 8.0% (mean ± SD) to 100% and reduced their infection intensity significantly. Stress response biomarkers such as malonaldehyde, glutathione, glutathione peroxidase, superoxide dismutase (SOD) and catalase levels in the gills of E. coioides at different time points were analysed. The SOD activity in the •OH group first decreased and then recovered to the initial level at the end of the experiment. The other stress response biomarkers had no significant difference from that in the uninfected control group after •OH treatment. Additionally, the gill of E. coioides in the •OH group exhibited slight and reversible transformation compared with the uninfected control group. Compared with •OH treatment, chlorine dioxide and formalin treatment reduced the survival rate, induced oxidative damage and changed the histological gill structure in E. coioides. In conclusion, •OH could be applied effectively to control C. irritans infection without affecting the normal physiological condition of E. coioides.

Single-cell analysis of habituation in Stentor coeruleus.

Although learning is often viewed as a unique feature of organisms with complex nervous systems, single-celled organisms also demonstrate basic forms of learning. The giant ciliate Stentor coeruleus responds to mechanical stimuli by contracting into a compact shape, presumably as a defense mechanism. When a Stentor cell is repeatedly stimulated at a constant level of force, it will learn to ignore that stimulus but will still respond to stronger stimuli. Prior studies of habituation in Stentor reported a graded response, suggesting that cells transition through a continuous range of response probabilities. By analyzing single cells using an automated apparatus to deliver calibrated stimuli, we find that habituation occurs via a single step-like switch in contraction probability within each cell, with the graded response in a population arising from the random distribution of switching times in individual cells. This step-like response allows Stentor behavior to be represented by a simple two-state model whose parameters can be estimated from experimental measurements. We find that transition rates depend on stimulus force and also on the time between stimuli. The ability to measure the behavior of the same cell to the same stimulus allowed us to quantify the functional heterogeneity among single cells. Together, our results suggest that the behavior of Stentor is governed by a two-state stochastic machine whose transition rates are sensitive to the time series properties of the input stimuli.

Planktonic ciliate community driven by environmental variables and cyanobacterial blooms: A 9-year study in two subtropical reservoirs.

It is well-established that environmental variability and cyanobacterial blooms have major effects on the assembly and functioning of bacterial communities in both marine and freshwater habitats. It remains unclear, however, how the ciliate community responds to such changes over the long-term, particularly in subtropical lake and reservoir ecosystems. We analysed 9-year planktonic ciliate data series from the surface water of two subtropical reservoirs to elucidate the role of cyanobacterial bloom and environmental variabilities on the ciliate temporal dynamics. We identified five distinct periods of cyanobacterial succession in both reservoirs. Using multiple time-scale analyses, we found that the interannual variability of ciliate communities was more strongly related to cyanobacterial blooms than to other environmental variables or to seasonality. Moreover, the percentage of species turnover across cyanobacterial bloom and non-bloom periods increased significantly with time over the 9-year period. Phylogenetic analyses further indicated that 84 %-86 % of ciliate community turnover was governed by stochastic dispersal limitation or undominated processes, suggesting that the ciliate communities in subtropical reservoirs were mainly controlled by neutral processes. However, short-term blooms increased the selection pressure and drove 30 %-53 % of the ciliate community turnover. We found that the ciliate community composition was influenced by environmental conditions with nutrients, cyanobacterial biomass and microzooplankton having direct and/or indirect significant effects on the ciliate taxonomic or functional community dynamics. Our results provide new insights into the long-term temporal dynamics of planktonic ciliate communities under cyanobacterial bloom disturbance.

Mucosal immunoglobulin response in Epinephelus coioides after Cryptocaryon irritans infection.

The teleost mucosal immune system consists mainly of the skin, gills and gut, which play crucial roles in local immune responses against invading organisms. Immunoglobulins are essential molecules in adaptive immunity that perform crucial biological functions. In our study, a mucosal immunity model was constructed in Epinephelus coioides groupers after Cryptocaryon irritans infection, according to previous experience. Total IgM and IgT in the groupers increased in the serum and mucus in the immune group, whereas only pathogen-specific IgM were detected existence. More critically, pathogen-specific IgM was detected in the head kidney, gill and skin supernatants, thus suggesting that the systematic immune and mucosal immune system secreted immunoglobulins. Furthermore, an early response in the skin was observed, on the basis of the detection of pathogen-specific IgM in the skin supernatant. In conclusion, this research characterized the grouper IgM and IgT in mucosal immune responses to pathogens in the gills and skin, thus providing a theoretical basis for future studies on vaccines against C. irritans.

Trade-offs of lipid remodeling in a marine predator-prey interaction in response to phosphorus limitation.

Phosphorus (P) is a key nutrient limiting bacterial growth and primary production in the oceans. Unsurprisingly, marine microbes have evolved sophisticated strategies to adapt to P limitation, one of which involves the remodeling of membrane lipids by replacing phospholipids with non-P-containing surrogate lipids. This strategy is adopted by both cosmopolitan marine phytoplankton and heterotrophic bacteria and serves to reduce the cellular P quota. However, little, if anything, is known of the biological consequences of lipid remodeling. Here, using the marine bacterium Phaeobacter sp. MED193 and the ciliate Uronema marinum as a model, we sought to assess the effect of remodeling on bacteria-protist interactions. We discovered an important trade-off between either escape from ingestion or resistance to digestion. Thus, Phaeobacter grown under P-replete conditions was readily ingested by Uronema, but not easily digested, supporting only limited predator growth. In contrast, following membrane lipid remodeling in response to P depletion, Phaeobacter was less likely to be captured by Uronema, thanks to the reduced expression of mannosylated glycoconjugates. However, once ingested, membrane-remodeled cells were unable to prevent phagosome acidification, became more susceptible to digestion, and, as such, allowed rapid growth of the ciliate predator. This trade-off between adapting to a P-limited environment and susceptibility to protist grazing suggests the more efficient removal of low-P prey that potentially has important implications for the functioning of the marine microbial food web in terms of trophic energy transfer and nutrient export efficiency.

Impacts of inorganic nutrients on the physiology of a mixoplanktonic ciliate and its cryptophyte prey.

Many marine planktonic ciliates retain functional chloroplasts from their photosynthetic prey and use them to incorporate inorganic carbon via photosynthesis. While this strategy provides the ciliates with carbon, little is known about their ability to incorporate major dissolved inorganic nutrients, such as nitrogen and phosphorus. Here, we studied how ciliates respond to different concentrations of dissolved inorganic nitrogen and phosphorus. Specifically, we tested the direct and indirect effects of nutrient availability on the ciliate Strombidium cf. basimorphum fed the cryptophyte prey Teleaulax amphioxeia. We assessed responses in the rates of growth, ingestion, photosynthesis, inorganic nutrient uptake, and excretion. Our results show that the prey changed its carbon content depending on the nutrient concentrations. Low inorganic nutrient concentrations increased S. cf. basimorphum growth and prey ingestion. The higher carbon content of the prey under these low nutrient conditions likely supported the growth of the ciliate, while the higher carbon:nutrient stoichiometry of the prey led to the higher ingestion rates. The low carbon content of the prey at high nutrient concentrations resulted in reduced growth of S. cf. basimorphum, which indicates that carbon acquired via photosynthesis in the ciliate cannot compensate for the ingestion of prey with low carbon content. In conclusion, our findings show S. cf. basimorphum is not able to utilize dissolved inorganic nitrogen and phosphorus for growth, and this species seems to be well adapted to exploit its prey when grown at low nutrient conditions.