Protist Predation Research Articles

Putative sponge biomarkers in unicellular Rhizaria question an early rise of animals.

The dawn of animals remains one of the most mysterious milestones in the evolution of life. The fossil lipids 24-isopropylcholestane and 26-methylstigmastane are considered diagnostic for demosponges-arguably the oldest group of living animals. The widespread occurrence and high relative abundance of these biomarkers in Ediacaran sediments from 635-541 million years (Myr) ago have been viewed as evidence for the rise of animals to ecological importance approximately 100 Myr before their rapid Cambrian radiation. Here we show that the biosynthesis of 24-isopropylcholestane and 26-methylstigmastane precursors is common among early-branching unicellular Rhizaria-heterotrophic protists that play an important role in trophic cycling and carbon export in the modern ocean. Negating these hydrocarbons as sponge biomarkers, our study places the oldest evidence for animals closer to the Cambrian Explosion. Cambrian silica hexactine spicules that are approximately 535 Myr old now represent the oldest diagnostic sponge remains, whereas approximately 558-Myr-old Dickinsonia and Kimberella (Ediacara biota) provide the most reliable evidence for the emergence of animals. The proliferation of predatory protists may have been responsible for much of the ecological changes during the late Neoproterozoic, including the rise of algae, the establishment of complex trophic relationships and the oxygenation of shallow-water habitats required for the subsequent ascent of macroscopic animals.

Hemimastigophora is a novel supra-kingdom-level lineage of eukaryotes.

Almost all eukaryote life forms have now been placed within one of five to eight supra-kingdom-level groups using molecular phylogenetics1-4. The 'phylum' Hemimastigophora is probably the most distinctive morphologically defined lineage that still awaits such a phylogenetic assignment. First observed in the nineteenth century, hemimastigotes are free-living predatory protists with two rows of flagella and a unique cell architecture5-7; to our knowledge, no molecular sequence data or cultures are currently available for this group. Here we report phylogenomic analyses based on high-coverage, cultivation-independent transcriptomics that place Hemimastigophora outside of all established eukaryote supergroups. They instead comprise an independent supra-kingdom-level lineage that most likely forms a sister clade to the 'Diaphoretickes' half of eukaryote diversity (that is, the 'stramenopiles, alveolates and Rhizaria' supergroup (Sar), Archaeplastida and Cryptista, as well as other major groups). The previous ranking of Hemimastigophora as a phylum understates the evolutionary distinctiveness of this group, which has considerable importance for investigations into the deep-level evolutionary history of eukaryotic life-ranging from understanding the origins of fundamental cell systems to placing the root of the tree. We have also established the first culture of a hemimastigote (Hemimastix kukwesjijk sp. nov.), which will facilitate future genomic and cell-biological investigations into eukaryote evolution and the last eukaryotic common ancestor.

Occasional long distance dispersal increases spatial synchrony of population cycles.

Spatially separated populations of the same species often exhibit correlated fluctuations in abundance, a phenomenon known as spatial synchrony. Dispersal can generate spatial synchrony. In nature, most individuals disperse short distances with a minority dispersing long distances. The effect of occasional long distance dispersal on synchrony is untested, and theoretical predictions are contradictory. Occasional long distance dispersal might either increase both overall synchrony and the spatial scale of synchrony, or reduce them. We conducted a protist microcosm experiment to test whether occasional long distance dispersal increases or decreases overall synchrony and the spatial scale of synchrony. We assembled replicate 15-patch ring metapopulations of the protist predator Euplotes patella and its protist prey Tetrahymena pyriformis. All metapopulations experienced the same dispersal rate, but differed in dispersal distance. Some metapopulations experienced strictly short distance (nearest neighbour) dispersal, others experienced a mixture of short- and long distance dispersal. Occasional long distance dispersal increased overall spatial synchrony and the spatial scale of synchrony for both prey and predators, though the effects were not statistically significant for predators. As predicted by theory, dispersal generated spatial synchrony by entraining the phases of the predator-prey cycles in different patches, a phenomenon known as phase locking. Our results are consistent with theoretical models predicting that occasional long distance dispersal increases spatial synchrony. However, our results also illustrate that the spatial scale of synchrony need not match the spatial scale of the processes generating synchrony. Even strictly short distance dispersal maintained high spatial synchrony for many generations at spatial scales much longer than the dispersal distance, thanks to phase locking.

Differential feeding by common heterotrophic protists on 12 different Alexandrium species

The genus Alexandrium often forms harmful algal blooms causing human illness and large-scale mortality of fish and shellfish. Thus, Alexandrium bloom dynamics are primary concerns for scientists, government officials, aquaculture farmers, and the public. To understand bloom dynamics, mortality due to predation needs to be assessed; however, interactions between many Alexandrium species and their potential predators have not previously been reported. Thus, feeding by five common heterotrophic dinoflagellates (Oxyrrhis marina, Gyrodinium dominans, Polykrikos kofoidii, Pfiesteria piscicida, and Oblea rotunda) and a naked ciliate (Strombidinopsis sp.) on 12 Alexandrium species was examined. Furthermore, the growth and ingestion rates of P. kofoidii on A. minutum CCMP 1888 (previously A. lusitanicum), A. minutum CCMP 113, and A. tamarense were measured as a function of prey concentration. The growth rates of P. kofoidii on the other Alexandrium species at single high prey concentrations were measured, at which the growth rates on A. minutum CCMP 1888 and A. tamarense were saturated. Feeding occurrence by these predators on 12 Alexandrium species could be categorized into 6 different prey groups. Each Alexandrium species was consumed by at least one predator; however, there was no Alexandrium species that was eaten by all six predators. Cells of A. minutum CCMP 1888, A. minutum CCMP 113, and A. tamarense were fed upon by four predators, but A. affine and A. pacificum by only one predator species, P. kofoidii or Strombidinopsis sp. Furthermore, A. minutum CCMP 1888 and A. tamarense supported high growth rates of P. kofoidii, but the other Alexandrium species did not support, but rather inhibited P. kofoidii growth. With increasing prey concentrations, the growth and ingestion rates of P. kofoidii on A. minutum CCMP 1888 and A. tamarense increased and became saturated, whereas those on A. minutum CCMP 113 continuously decreased. The maximum growth rates of P. kofoidii on A. tamarense and A. minutum CCMP 1888 were 1.010 and 0.765 d−1, respectively, and P. kofoidii maximum ingestion rates were 26.2 and 11.1 ng C predator−1d−1, respectively. In contrast, the growth rates of P. kofoidii on the other Alexandrium species at single high prey concentrations were almost zero (A. pacificum) or negative. Based on the feeding occurrence and growth and ingestion rates of predators on 12 Alexandrium species, it is suggested that common heterotrophic protistan predators respond differently to different Alexandrium species, and thus ecological niches of the Alexandrium species may be different from each other. These results may provide an insight into the roles of protistan predators in bloom dynamics of Alexandrium species.

High Grazing Rates on Cryptophyte Algae in Chesapeake Bay

Cryptophyte algae are globally distributed photosynthetic flagellates found in freshwater, estuarine, and neritic ecosystems. While cryptophytes can be highly abundant and are consumed by a wide variety of protistan predators, few studies have sought to quantify in situ grazing rates on their populations. Here we show that autumnal grazing rates on in situ communities of cryptophyte algae in Chesapeake Bay are high throughout the system, while growth rates, particularly in the lower bay, were low. Analysis of the genetic diversity of cryptophyte populations within dilution experiments suggests that microzooplankton may be selectively grazing the fastest-growing members of the population, which were generally Teleaulax spp. We also demonstrate that potential grazing rates of ciliates and dinoflagellates on fluorescently labeled (FL) Rhodomonas salina, Storeatula major, and Teleaulax amphioxeia can be high (up to 149 prey predator-1 d-1), and that a Gyrodinium sp. and Mesodinium rubrum could be selective grazers. Potential grazing was highest for heterotrophic dinoflagellates, but due to its abundance, M. rubrum also had a high overall impact. This study reveals that cryptophyte algae in Chesapeake Bay can experience extremely high grazing pressure from phagotrophic protists, and that this grazing likely shapes their community diversity.

Implications of selective predation on the macroevolution of eukaryotes: evidence from Arctic Canada.

Existing paleontological data indicate marked eukaryote diversification in the Neoproterozoic, ca. 800 Ma, driven by predation pressure and various other biotic and abiotic factors. Although the eukaryotic record remains less diverse before that time, molecular clock estimates and earliest crown-group affiliated microfossils suggest that the diversification may have originated during the Mesoproterozoic. Within new assemblages of organic-walled microfossils from the ca. 1150 to 900 Ma lower Shaler Supergroup of Arctic Canada, numerous specimens from various taxa display circular and ovoid perforations on their walls, interpreted as probable traces of selective protist predation, 150-400 million years before their first reported incidence in the Neoproterozoic. Selective predation is a more complex behavior than phagotrophy, because it requires sensing and selection of prey followed by controlled lysis of the prey wall. The ca. 800 Ma eukaryotic diversification may have been more gradual than previously thought, beginning in the late Mesoproterozoic, as indicated by recently described microfossil assemblages, in parallel with the evolution of selective eukaryovory and the spreading of eukaryotic photosynthesis in marine environments.

Evolution of STEC virulence: Insights from the antipredator activities of Shiga toxin producing E. coli

Shiga toxin-producing Escherichia coli (STEC) are a diverse group of strains that are implicated in over 270,000 cases of human illness annually in the United States alone. Shiga toxin (Stx), encoded by a resident temperate lambdoid bacteriophage, is the main STEC virulence factor. Although the population structure of E. coli O157:H7, the most common disease-causing STEC strain, is highly homogenous, the range of clinical illness caused by this strain varies by dramatically outbreak, suggesting that human virulence is evolving. However, the factors governing this variation in disease severity are poorly understood. STEC evolved from an O55:H7-like progenitor into a human pathogen. In addition to causing human disease, Stx released from STEC kill bacterivorous protist predators and enhance bacterial survival in the face of protist predation. Cattle are the primary reservoir for STEC and protists and bacteria occur together within the ruminant intestinal tract. Cattle associated STEC are not highly pathogenic to humans. These observations suggest that disease causing STEC strains evolved from cattle-associated “antipredator” STEC strains. To test this idea and to gain insight into the features that govern the evolution of STEC from a commensal strain of ruminants strain to virulent human pathogen, we compared the predation resistance of STEC strains isolated from asymptomatic infected cows and human patients. We find that STEC O157:H7 progenitor lineages and clades are more effective than human associated ones at killing the types of protist predators. In addition, our results indicate that the presence of Stx2c-containing bacteriophage is associated with more efficient amoeba killing. Also, these phage apparently also encode Q21-like version of the Q antitermination protein, the protein that controls expression of Stx.

Tetrahymena thermophila Predation Enhances Environmental Adaptation of the Carp Pathogenic Strain Aeromonas hydrophila NJ-35.

Persistence of Aeromonas hydrophila in aquatic environments is the principle cause of fish hemorrhagic septicemia. Protistan predation has been considered to be a strong driving force for the evolution of bacterial defense strategies. In this study, we investigated the adaptive traits of A. hydrophila NJ-35, a carp pathogenic strain, in response to Tetrahymena thermophila predation. After subculturing with Tetrahymena, over 70% of A. hydrophila colonies were small colony variants (SCVs). The SCVs displayed enhanced biofilm formation, adhesion, fitness, and resistance to bacteriophage infection and oxidative stress as compared to the non-Tetrahymena-exposed strains. In contrast, the SCVs exhibited decreased intracellular bacterial number in RAW264.7 macrophages and were highly attenuated for virulence in zebrafish. Considering the outer membrane proteins (OMPs) are directly involved in bacterial interaction with the external surroundings, we investigated the roles of OMPs in the antipredator fitness behaviors of A. hydrophila. A total of 38 differentially expressed proteins were identified in the SCVs by quantitative proteomics. Among them, three lipoproteins including SurA, Slp, and LpoB, and a serine/threonine protein kinase (Stpk) were evidenced to be associated with environmental adaptation of the SCVs. Also, the three lipoproteins were involved in attenuated virulence of SCVs through the proinflammatory immune response mediated by TLR2. This study provides an important contribution to the understanding of the defensive traits of A. hydrophila against protistan predators.

Interactions between the Newly Described Small- and Fast-Swimming Mixotrophic Dinoflagellate Yihiella yeosuensis and Common Heterotrophic Protists.

The mixotroph Yihiella yeosuensis is a small- and fast-swimming dinoflagellate. To investigate its protistan predators, interactions between Y.yeosuensis and 11 heterotrophic protists were explored. No potential predators were able to feed on actively swimming Y. yeosuensis cells, which escaped via rapid jumps, whereas Aduncodinium glandula, Oxyrrhis marina, and Strombidinopsis sp. (approximately 150μm in cell length) were able to feed on weakly swimming cells that could not jump. Furthermore, Gyrodinium dominans, Luciella masanensis, and Pfiesteria piscicida were able to feed on heat-killed Yihiella cells, whereas Gyrodinium moestrupii, Noctiluca scintillans, Oblea rotunda, Polykrikos kofoidii, and Strombidium sp. (20μm) did not feed on them. Thus, the jumping behavior of Y.yeosuensis might be primarily responsible for the observed lack of predation. With increasing Yihiella concentration, the growth rate of O.marina decreased, whereas that of Strombidinopsis did not change. However, with increasing Yihiella concentration (up to 530ng C/ml), the ingestion rate of Strombidinopsis on Yihiella increased linearly. The highest ingestion rate was 24.1ng C per predator per d. The low daily carbon acquisition from Yihiella relative to the body carbon content of Strombidinopsis might be responsible for its negligible growth. Thus, Y.yeosuensis might have an advantage over its competitors due to its low mortality rate.

Interactions between predation and disturbances shape prey communities

Ecological disturbances are important drivers of biodiversity patterns. Many biodiversity studies rely on endpoint measurements instead of following the dynamics that lead to those outcomes and testing ecological drivers individually, often considering only a single trophic level. Manipulating multiple factors (biotic and abiotic) in controlled settings and measuring multiple descriptors of multi-trophic communities could enlighten our understanding of the context dependency of ecological disturbances. Using model microbial communities, we experimentally tested the effects of imposed disturbances (i.e. increased dilution simulating density-independent mortality as press or pulse disturbances coupled with resource deprivation) on bacterial abundance, diversity and community structure in the absence or presence of a protist predator. We monitored the communities immediately before and after imposing the disturbance and four days after resuming the pre-disturbance dilution regime to infer resistance and recovery properties. The results highlight that bacterial abundance, diversity and community composition were more affected by predation than by disturbance type, resource loss or the interaction of these factors. Predator abundance was strongly affected by the type of disturbance imposed, causing temporary relief of predation pressure. Importantly, prey community composition differed significantly at different phases, emphasizing that endpoint measurements are insufficient for understanding the recovery of communities.

With a pinch of extra salt-Did predatory protists steal genes from their food?

The cellular adjustment of Bacteria and Archaea to high-salinity habitats is well studied and has generally been classified into one of two strategies. These are to accumulate high levels either of ions (the “salt-in” strategy) or of physiologically compliant organic osmolytes, the compatible solutes (the “salt-out” strategy). Halophilic protists are ecophysiological important inhabitants of salt-stressed ecosystems because they are not only very abundant but also represent the majority of eukaryotic lineages in nature. However, their cellular osmostress responses have been largely neglected. Recent reports have now shed new light on this issue using the geographically widely distributed halophilic heterotrophic protists Halocafeteria seosinensis, Pharyngomonas kirbyi, and Schmidingerothrix salinarum as model systems. Different approaches led to the joint conclusion that these unicellular Eukarya use the salt-out strategy to cope successfully with the persistent high salinity in their habitat. They accumulate various compatible solutes, e.g., glycine betaine, myo-inositol, and ectoines. The finding of intron-containing biosynthetic genes for ectoine and hydroxyectoine, their salt stress–responsive transcription in H. seosinensis, and the production of ectoine and its import by S. salinarum come as a considerable surprise because ectoines have thus far been considered exclusive prokaryotic compatible solutes. Phylogenetic considerations of the ectoine/hydroxyectoine biosynthetic genes of H. seosinensis suggest that they have been acquired via lateral gene transfer by these bacterivorous Eukarya from ectoine/hydroxyectoine-producing food bacteria that populate the same habitat.

The microbiome associated with two Synechococcus ribotypes at different levels of ecological interaction.

Planktonic cyanobacteria belonging to the genus Synechococcus are ubiquitously distributed in marine and fresh waters, substantially contributing to total carbon fixation on a global scale. While their ecological relevance is acknowledged, increasing resolution in molecular techniques allows disentangling cyanobacteria's role at the micro-scale, where complex microbial interactions may drive the overall community assembly. The interplay between phylogenetically different Synechococcus clades and their associated bacterial communities can affect their ecological fate and susceptibility to protistan predation. In this study, we experimentally promoted different levels of ecological interaction by mixing two Synechococcus ribotypes (MW101C3 and LL) and their associated bacteria, with and without a nanoflagellate grazer (Poterioochromonas sp.) in laboratory cultures. The beta-diversity of the Synechococcus-associated microbiome in laboratory cultures indicated that the presence of the LL ribotype was the main factor determining community composition changes (41% of total variance), and prevailed over the effect of protistan predation (18% of total variance). Our outcomes also showed that species coexistence and predation may promote microbial diversity, thus highlighting the underrated ecological relevance of such micro-scale factors.

A Generalist Protist Predator Enables Coexistence in Multitrophic Predator-Prey Systems Containing a Phage and the Bacterial Predator Bdellovibrio

Complex ecosystems harbor multiple predator and prey species whose direct and indirect interactions are understudied. In particular, the combined effects of predator diversity and resource preference on prey removal are not known. To better understand the effect of interspecies interactions combinations of micro-predators – i.e. protists (generalists), predatory bacteria (semi-specialists), and phages (specialists) – and bacterial prey were tracked over a 72-hour period in miniature membrane bioreactors. While specialist predators alone drove their preferred prey to extinction, the inclusion of a generalist resulted in uniform losses among prey species. Most importantly, presence of a generalist predator enabled coexistence of all predators and prey. As the generalist predator also negatively affected the other predators we suggest that resource partitioning between predators and the constant availability of resources for bacterial growth due to protist predation stabilizes the system and keeps its diversity high. The appearance of resistant prey strains and subsequent evolution of specialist predators unable to infect the ancestral prey implies that multitrophic communities are able to persist and stabilize themselves. Interestingly, the appearance of BALOs and phages unable to infect their prey was only observed for the BALO or phage in the absence of additional predators or prey species indicating that competition between predators might influence coevolutionary dynamics.

Grazing of leaf-associated Cercomonads (Protists: Rhizaria: Cercozoa) structures bacterial community composition and function.

Preferential food selection in protists is well documented, but we still lack basic understanding on how protist predation modifies the taxonomic and functional composition of bacterial communities. We conducted feeding trials using leaf-associated cercomonad Cercozoa by incubating them on a standardized, diverse bacterial community washed from plant leaves. We used a shotgun metagenomics approach to investigate the taxonomic and functional changes of the bacterial community after five days protist predation on bacteria. Predation-induced shifts in bacterial community composition could be linked to phenotypic protist traits. Protist reproduction rate, morphological plasticity and cell speed were most important in determining bacterial community composition. Analyses of co-occurrence patterns showed less complex correlations between bacterial taxa in the protist-grazed treatments with a higher proportion of positive correlations than in non-grazed controls, suggesting that predation reduced the influence of strong competitors. Protist predation influenced 14 metabolic core functions including membrane transport from which type VI secretion systems were in particular upregulated. In view of the functional importance of bacterial communities in the phyllosphere and rhizosphere of plants, a more detailed understanding of predator-prey interactions, changes in microbial composition and function, and subsequent repercussions on plant performance are clearly required.

Competition and niche separation of pelagic bacteria in freshwater habitats.

Freshwater bacterioplankton assemblages are composed of sympatric populations that can be delineated, for example, by ribosomal RNA gene relatedness and that differ in key ecophysiological properties. They may be free-living or attached, specialized for particular concentrations or subsets of substrates, or invest a variable amount of their resources in defence traits against protistan predators and viruses. Some may be motile and tactic whereas others are not, with far-reaching implications for their respective life styles and niche partitioning. The co-occurrence of competitors with overlapping growth requirements has profound consequences for thestability of community functions; it can to some extent be explained by habitat factors such as the microscale complexity and spatiotemporal variability of the lacustrine environments. On the other hand, the composition and diversity of freshwater microbial assemblages also reflects non-equilibrium states, dispersal and the stochasticity of community assembly processes. This review synoptically discusses the competition and niche separation of heterotrophic bacterial populations (defined at various levels of phylogenetic resolution) in the pelagic zone of inland surface waters from a variety of angles, focusing on habitat heterogeneity and the resulting biogeographic distribution patterns, the ecophysiological adaptations to the substrate field and the interactions of prokaryotes with predators and viruses.

The Role of Microbes in the Nutrition of Detritivorous Invertebrates: A Stoichiometric Analysis.

Detritus represents an important pool in the global carbon cycle, providing a food source for detritivorous invertebrates that are conspicuous components of almost all ecosystems. Our knowledge of how these organisms meet their nutritional demands on a diet that is typically comprised of refractory, carbon-rich compounds nevertheless remains incomplete. “Trophic upgrading” of detritus by the attached microbial community (enhancement of zooplankton diet by the inclusion of heterotrophic protozoans) represents a potential source of nutrition for detritivores as both bacteria and their flagellated protistan predators are capable of biosynthesizing essential micronutrients such as polyunsaturated fatty acids (PUFAs). There is however a trade-off because although microbes enhance the substrate in terms of its micronutrient content, the quantity of organic carbon is diminished though metabolic losses as energy passes through the microbial food web. Here, we develop a simple stoichiometric model to examine this trade-off in the nutrition of detritivorous copepods inhabiting the mesopelagic zone of the ocean, focusing on their requirements for carbon and an essential PUFA, docosahexaenoic acid (DHA). Results indicate that feeding on microbes may be a highly favorable strategy for these invertebrates, although the potential for carbon to become limiting when consuming a microbial diet exists because of the inefficiencies of trophic transfer within the microbial food web. Our study highlights the need for improved knowledge at the detritus-microbe-metazoan interface, including interactions between the physiology and ecology of the associated organisms.

The prey's scent - Volatile organic compound mediated interactions between soil bacteria and their protist predators.

Protists are major predators of bacteria in soils. However, it remains unknown how protists sense their prey in this highly complex environment. Here, we investigated whether volatile organic compounds (VOCs) of six phylogenetic distinct soil bacteria affect the performance of three different soil protists and how that relates to direct feeding interactions. We observed that most bacteria affected protist activity by VOCs. However, the response of protists to the VOCs was strongly dependent on both the bacterial and protist interacting partner. Stimulation of protist activity by volatiles and in direct trophic interaction assays often coincided, suggesting that VOCs serve as signals for protists to sense suitable prey. Furthermore, bacterial terpene synthase mutants lost the ability to affect protists, indicating that terpenes represent key components of VOC-mediated communication. Overall, we demonstrate that volatiles are directly involved in protist-bacterial predator-prey interactions.

The Synechococcus cell surface protein SwmA increases vulnerability to predation by flagellates and ciliates

AbstractThe genus Synechococcus, a major contributor to ocean productivity, exhibits wide genetic variability and a distinct biogeography of genetic subgroups. Synechococcus sp. strain WH8102 is a motile Sargasso Sea isolate belonging to Clade III in subcluster 5.1. Non‐flagellar motility in WH8102 depends upon two large cell surface proteins, SwmA and SwmB. In the marine plankton, Synechococcus can experience high rates of mortality from protist predators, suggesting strong selective pressure for the maintenance of anti‐predation defenses. We used knockout mutants deficient in large cell surface proteins (SwmA‐ and SwmB‐) to test the hypothesis that these proteins defend WH8102 against predation by ciliates and flagellates, important consumers of Synechococcus in aquatic ecosystems. Contrary to our hypothesis, we found that wild‐type WH8102 were preyed upon at higher rates than SwmA‐ by ciliates and by two of three tested flagellate species. Thus the SwmA protein, constitutively produced by WH8102, made this strain more rather than less vulnerable to predation. SwmA proteins link to form the S layer, a cell coating known to promote adhesion in other microbial systems, suggesting the predation vulnerability arises from increased attachment between consumers and prey. Given the SwmA‐related enhancement of predation rates, our findings suggest two possibilities for WH8102 and perhaps other Clade III Synechococcus: (1) factors other than protist predation select for production of the S layer; (2) ingestion actually increases fitness, by exposing digestion‐resistant cells to beneficial conditions in the food vacuole.

Protistan predation interferes with bacterial long-term adaptation to substrate restriction by selecting for defence morphotypes.

Bacteria that are introduced into aquatic habitats face a low substrate environment interspersed with rare productive 'hotspots', as well as high protistan grazing. Whereas the former condition should select for growth performance, the latter should favour traits that reduce predation mortality, such as the formation of large cell aggregates. However, protected morphotypes often convey a growth disadvantage, and bacteria thus face a trade-off between investing in growth or defence traits. We set up an evolutionary experiment with the freshwater isolate Sphingobium sp. strain Z007 that conditionally increases aggregate formation in supernatants from a predator-prey coculture. We hypothesized that low substrate levels would favour growth performance and reduce the aggregated subpopulation, but that the concomitant presence of a flagellate predator might conserve the defence trait. After 26 (1-week) growth cycles either with (P+) or without (P-) predators, bacteria had evolved into strikingly different phenotypes. Strains from P- had low numbers of aggregates and increased growth yield, both at the original rich growth conditions and on various single carbon sources. By contrast, isolates from the P+ treatment formed elevated proportions of defence morphotypes, but exhibited lower growth yield and metabolic versatility. Moreover, the evolved strains from both treatments had lost phenotypic plasticity of aggregate formation. In summary, the (transient) residence of bacteria at oligotrophic conditions may promote a facultative oligotrophic life style, which is advantageous for survival in aquatic habitats. However, the investment in defence against predation mortality may constrain microbial adaptation to the abiotic environment.

Competition and protist predation are important regulators of riverine bacterial community composition and size distribution

ABSTRACTAmong the bacterivorous protists, heterotrophic nanoflagellates (HNFs) are considered to be the main grazers of bacteria in freshwaters due to their size-selective grazing. In this work, we assessed the change of a riverine bacterial community in controlled incubations, where HNFs’ predation pressure was initially released through filtration. Filtration did not prevent the passage of cysts, which grew in the enrichments afterwards. Data on the composition of the bacterial community were gathered by Catalyzed Reporter Deposition Fluorescent In situ Hybridization (CARD-FISH) using 16S probes targeting phylogenetic groups. Bacterial cell size was also examined using image analysis. Overall, the initial filtration directly (through release of predation pressure) or indirectly (through competition among bacterial groups) affected the bacterial community composition. When nanoflagellate abundance rose, a reduction of bacterial abundance and changes in cell size distribution were observed. Gamma-Proteobacteria and Actinobacteria were the groups showing the greatest reduction in abundance. Beta-Proteobacteria showed a reduction of cell size and were found in aggregates. Alpha-Proteobacteria and Actinobacteria developed two distinct filamentous morphotypes: short, segmented rods and long chains of rods. Our results showed that the release of the predation pressure and the successive rise of the nanoflagellates changed the bacterial community in terms of composition at large phylogenetic scale. HNF grazing is highly group-specific and seems to reconstruct the community based on cell size, and thus, not only drastically changing the bacterial community composition, but also increasing its functional diversity.