Explore the vast range of titles available.

\"This edited volume analyzes the evolution of international disaster law as a field that encompasses new ideas about human rights, sovereignty, and technology\"-- Provided by publisher.

Rapidly improving DNA sequencing technology has revolutionized our ability to efficiently survey the biodiversity of microbial life. We are now equipped to investigate protistan richness and community dynamics on scales that would not have been imaginable with traditional observational methods. However, for most taxa the relationship between DNA sequences and morphologically-defined species is poorly understood, and morphology has remained the cornerstone of taxonomy for centuries. To better utilize the wealth of sequence data being collected, we must understand how it relates to entities such as individuals, populations, and species. Here we use a combined microscopy and sequencing approach to unveil the striking intragenomic and intraspecies genetic variation in one group of ecologically-important marine protists, the polycystine Radiolaria. Long-read 18S rRNA gene amplicon data from 173 isolated and morphologically-identified radiolarians showed that the vast majority (90%) yielded multiple sequence variants per specimen. Furthermore, every morphospecies analyzed displayed a range of different genetic signatures. Intraspecies genetic variability was expressed as specimens having different assemblages of ASVs, different dominant ASVs, or having no ASVs in common with other specimens of the same morphospecies. By integrating morphological and molecular information, we begin to parse the genetic richness of Radiolaria in ocean environments, as well as illuminate relationships between taxa, and their poorly-known life stages. Our findings emphasize the need to account for protists’ taxon-specific sequence variability, particularly their intragenomic and intraspecies genetic variation, in interpreting metabarcoding diversity survey data.

Sinking particles are a critical conduit for the export of organic material from surface waters to the deep ocean. Despite their importance in oceanic carbon cycling and export, little is known about the biotic composition, origins, and variability of sinking particles reaching abyssal depths. Here, we analyzed particle-associated nucleic acids captured and preserved in sediment traps at 4,000-m depth in the North Pacific Subtropical Gyre. Over the 9-month time-series, Bacteria dominated both the rRNA-gene and rRNA pools, followed by eukaryotes (protists and animals) and trace amounts of Archaea. Deep-sea piezophile-like Gammaproteobacteria, along with Epsilonproteobacteria, comprised >80% of the bacterial inventory. Protists (mostly Rhizaria, Syndinales, and ciliates) and metazoa (predominantly pelagic mollusks and cnidarians) were the most common sinking particle-associated eukaryotes. Some near-surface water-derived eukaryotes, especially Foraminifera, Radiolaria, and pteropods, varied greatly in their abundance patterns, presumably due to sporadic export events. The dominance of piezophile-like Gammaproteobacteria and Epsilonproteobacteria, along with the prevalence of their nitrogen cycling-associated gene transcripts, suggested a central role for these bacteria in the mineralization and biogeochemical transformation of sinking particulate organic matter in the deep ocean. Our data also reflected several different modes of particle export dynamics, including summer export, more stochastic inputs from the upper water column by protists and pteropods, and contributions from sinking mid- and deep-water organisms. In total, our observations revealed the variable and heterogeneous biological origins and microbial activities of sinking particles that connect their downward transport, transformation, and degradation to deep-sea biogeochemical processes.

Key Points Single-celled eukaryotes (protists) constitute a tremendously diverse group of microorganisms. These species exhibit a wide range of nutritional modes (many species possess multiple nutritional modes simultaneously) and are essential components at several trophic levels within food webs. Genetic analyses of protists have lagged behind those of other microbial taxa because protists have much larger genomes and more complicated gene expression patterns. Consequently, we have very limited knowledge about gene number, identity and function within many protistan lineages. Widespread application of targeted gene sequencing (most notably, of small-subunit rRNA genes) has greatly improved our knowledge of eukaryote phylogeny and provided a framework for an emerging taxonomy incorporating morphological and molecular information. A recently developed alternative approach to provide genetic information for ecologically important protistan taxa is transcriptome sequencing of cultured species. Transcriptomes are providing vital databases of genes for species that lack sequenced genomes. Transcriptomic studies of cultures and natural assemblages of phototrophic protists (phytoplankton) are revealing complex metabolic responses to environmental conditions (such as nutrient limitation and light regime), pathways that are involved in toxin production by some harmful algal species and changes in gene expression that are related to shifts in nutritional mode for mixotrophic species. The application of transcriptomic approaches to the study of protistan symbioses, predator–prey interactions and protist–bacterium interactions are beginning to reveal the molecular signalling that is involved in the recognition and response between microorganisms, providing insights into the origin of eukaryotic organelles and the structure of aquatic food webs. We now have an improved understanding of the physiological responses of ecologically relevant protistan species and trophic groups to environmental changes. This understanding, which has been garnered through omics studies, is being harnessed to improve the predictive capabilities of global biogeochemical models. Protists are an important part of the marine food web. In this Review, Caron et al . summarize recent insights from transcriptomic studies of cultured and free-living protists and discuss how these findings highlight the functions and interactions of these single-celled eukaryotes in the global oceans. Protists, which are single-celled eukaryotes, critically influence the ecology and chemistry of marine ecosystems, but genome-based studies of these organisms have lagged behind those of other microorganisms. However, recent transcriptomic studies of cultured species, complemented by meta-omics analyses of natural communities, have increased the amount of genetic information available for poorly represented branches on the tree of eukaryotic life. This information is providing insights into the adaptations and interactions between protists and other microorganisms and macroorganisms, but many of the genes sequenced show no similarity to sequences currently available in public databases. A better understanding of these newly discovered genes will lead to a deeper appreciation of the functional diversity and metabolic processes in the ocean. In this Review, we summarize recent developments in our understanding of the ecology, physiology and evolution of protists, derived from transcriptomic studies of cultured strains and natural communities, and discuss how these novel large-scale genetic datasets will be used in the future.

Ochromonas spp. strains CCMP1393 and BG-1 are phagotrophic phytoflagellates with different nutritional strategies. Strain CCMP1393 is an obligate phototroph while strain BG-1 readily grows in continuous darkness in the presence of bacterial prey. Growth and gene expression of strain CCMP1393 were investigated under conditions allowing phagotrophic, mixotrophic, or phototrophic nutrition. The availability of light and bacterial prey led to the differential expression of 42% or 45-59% of all genes, respectively. Data from strain CCMP1393 were compared to those from a study conducted previously on strain BG-1, and revealed notable differences in carbon and nitrogen metabolism between the 2 congeners under similar environmental conditions. Strain BG-1 utilized bacterial carbon and amino acids through glycolysis and the tricarboxylic acid cycle, while downregulating light harvesting and carbon fixation in the Calvin cycle when both light and bacteria were available. In contrast, the upregulation of genes related to photosynthesis, light harvesting, chlorophyll synthesis, and carbon fixation in the presence of light and prey for strain CCMP1393 implied that this species is more phototrophic than strain BG-1, and that phagotrophy may have enhanced phototrophy. Cellular chlorophyll a content was also significantly higher in strain CCMP1393 supplied with bacteria compared to those without prey. Our results thus point to very different physiological strategies for mixotrophic nutrition in these closely related chrysophyte species.

Characterizing ecological relationships between viruses, bacteria and protists in the ocean are critical to understanding ecosystem function, yet these relationships are infrequently investigated together. We evaluated these relationships through microbial association network analysis of samples collected approximately monthly from March 2008 to January 2011 in the surface ocean (0–5 m) at the San Pedro Ocean Time series station. Bacterial, T4-like myoviral and protistan communities were described by Automated Ribosomal Intergenic Spacer Analysis and terminal restriction fragment length polymorphism of the gene encoding the major capsid protein (g23) and 18S ribosomal DNA, respectively. Concurrent shifts in community structure suggested similar timing of responses to environmental and biological parameters. We linked T4-like myoviral, bacterial and protistan operational taxonomic units by local similarity correlations, which were then visualized as association networks. Network links (correlations) potentially represent synergistic and antagonistic relationships such as viral lysis, grazing, competition or other interactions. We found that virus–bacteria relationships were more cross-linked than protist–bacteria relationships, suggestive of increased taxonomic specificity in virus–bacteria relationships. We also found that 80% of bacterial–protist and 74% of bacterial–viral correlations were positive, with the latter suggesting that at monthly and seasonal timescales, viruses may be following their hosts more often than controlling host abundance.

Literature review and synthesis of growth rates of aquatic protists focused on the role of temperature in the formation of massive annual algal blooms in high-latitude ecosystems. Maximal growth rates of herbivorous protists equaled or exceeded maximal growth rates of phototrophic protists at temperatures above 15°C. Maximal growth rates of herbivorous protists declined more rapidly with decreasing temperature than did those of phototrophic protists, and at the very low temperatures common to high-latitude ecosystems, the maximal growth rates of herbivorous protists were less than half the maximal growth rates of phototrophic protists. Growth rates of herbivorous protists were consistently lower than those of bacterivorous protists and were unrelated to differences in cell volume between the two groups. Linear equations describing the relationship of the natural log of maximal growth rates of bacterivorous and herbivorous protists to temperature were generated and compared to published information for maximal growth rates of phototrophic protists and copepods. The three heterotrophic groups had similar slopes (0.12 for bacterivorous protists, 0.10 for herbivorous protists, and 0.13 for copepods) that were approximately double that of phototrophic protists (0.06). The massive annual algal blooms observed in high latitudes are due in part to a fundamental difference in the relationship between growth and temperature for phototrophic protists and their grazers.

Large amoeba-like organisms known as Rhizaria have often been overlooked in studies of ocean biology and biogeochemistry. Underwater imaging and ecological network analyses are revealing their roles. See Article p.465 & Letter p.504 Rhizaria are major players in ocean ecology Ocean ecosystems are inhabited by planktonic organisms spanning a wide size range, with large zooplankton feeding on smaller species and thereby contributing to the marine food web and carbon cycling. However, our understanding of the role and contribution of fragile and large zooplankton to the marine ecosystem is limited. Using data collected by an in situ imaging system during the Tara Oceans global survey, Tristan Biard et al . quantified the respective contributions of Rhizaria (a broad phylogenetic group of marine protists) and other zooplankton larger than 600 μm, finding that they represent a much larger biomass than previously appreciated, contributing up to 5.2% of the total oceanic biota carbon reservoir. These findings highlight the important contribution of Rhizaria to plankton biomass, primary productivity and other biogeochemical processes in the oceans.