Trophic flexibility of marine diplonemids - switching from osmotrophy to bacterivory

Diplonemid protists possess exotic endomembrane machinery, impacting models of membrane trafficking in modern and ancient eukaryotes

Diplonemids are among the most abundant and species-rich protists in the oceans. Marine heterotrophic flagellates, including diplonemids, have been suggested to play important roles in global biogeochemical cycles. Diplonemids are also the sister taxon of kinetoplastids, home to trypanosomatid parasites of global health importance, and thus are informative about the evolution of kinetoplastid biology. However, the genomic and cellular complement that underpins diplonemids' highly successful lifestyle is underexplored. At the same time, our framework describing cellular processes may not be as broadly applicable as presumed, as it is largely derived from animal and fungal model organisms, a small subset of extant eukaryotic diversity. In addition to uniquely evolved machinery in animals and fungi, there exist components with sporadic (i.e., "patchy") distributions across other eukaryotes. A most intriguing subset are components ("jötnarlogs") stochastically present in a wide range of eukaryotes but lost in animal and/or fungal models. Such components are considered exotic curiosities but may be relevant to inferences about the complexity of the last eukaryotic common ancestor (LECA) and frameworks of modern cell biology. Here, we use comparative genomics and phylogenetics to comprehensively assess the membrane-trafficking system of diplonemids. They possess several proteins thought of as kinetoplastid specific, as well as an extensive set of patchy proteins, including jötnarlogs. Diplonemids apparently function with endomembrane machinery distinct from existing cell biological models but comparable with other free-living heterotrophic protists, highlighting the importance of including such exotic components when considering different models of ancient eukaryotic genomic complexity and the cell biology of non-opisthokont organisms.

Symbionts of predatory protists are widespread in the oceans and related to animal pathogens

Protists are major predators of ocean microbial life, with an ancient history of entanglements with prokaryotes, but their delicate cell structures and recalcitrance to culturing hinder exploration of marine symbioses. We report that tiny oceanic protistan predators, specifically choanoflagellates-the closest living unicellular relatives of animals-and uncultivated MAST-3 form symbioses with four bacterial lineages related to animal symbionts. By targeting living phagotrophs on ship expeditions, we recovered genomes from physically associated uncultivated Legionellales and Rickettsiales. The evolutionary trajectories of Marinicoxiellaceae, Cosmosymbacterales, Simplirickettsiaceae, and previously named Gamibacteraceae vary, including host-engagement mechanisms unknown in marine bacteria, horizontally transferred genes that mediate pathogen-microbiome interactions, and nutritional pathways. These symbionts and hosts occur throughout subtropical and tropical oceans. Related bacteria were detected in public data from freshwater, fish, and human samples. Symbiont associations with animal-related protists, alongside relationships to animal pathogens, suggest an unexpectedly long history of shifting associations and possibilities for host expansion as environments change.

Ecology of free-living freshwater heterotrophic euglenoids: A summarizing review

This review aims to compile sparse information on the ecology of freshwater heterotrophic euglenoids and synthesize the main phenomena and hypotheses from published results. Apparently, heterotrophic euglenoids play a very important role in the nutrient flow of water ecosystems and are irreplaceable heterotrophic contributors in benthic communities, as their total biomass is by far the largest among heterotrophic flagellates. Even though they are obviously a very crucial part of the diversity of freshwater heterotrophic protists, and likely the most represented (in terms of biovolume) group of heterotrophic flagellates, there have been only a few attempts to elucidate their ecological preferences, roles, niches, and importance. They exhibit three nutrition modes-bacterivory, eukaryovory, and osmotrophy-which are strategies closely related to their taxonomical groupings and phylogenetic positions. Unfortunately, the phylogeny of the majority of the species remains unknown, similar to their autecology. There are major problems with the quantitative research methodologies, which is a big challenge for future research to improve.

Gazing into the abyss: A glimpse into the diversity, distribution, and behaviour of heterotrophic protists from the deep-sea floor

The benthic biome of the deep-sea floor, one of the largest biomes on Earth, is dominated by diverse and highly productive heterotrophic protists, second only to prokaryotes in terms of biomass. Recent evidence suggests that these protists play a significant role in ocean biogeochemistry, representing an untapped source of knowledge. DNA metabarcoding and environmental sample sequencing have revealed that deep-sea abyssal protists exhibit high levels of specificity and diversity across local regions. This review aims to provide a comprehensive summary of the known heterotrophic protists from the deep-sea floor, their geographic distribution, and their interactions in terms of parasitism and predation. We offer an overview of the most abundant groups and discuss their potential ecological roles. We argue that the exploration of the biodiversity and species-specific features of these protists should be integrated into broader deep-sea research and assessments of how benthic biomes may respond to future environmental changes.

Gene expression dynamics of natural assemblages of heterotrophic flagellates during bacterivory

BACKGROUND

Marine heterotrophic flagellates (HF) are dominant bacterivores in the ocean, where they represent the trophic link between bacteria and higher trophic levels and participate in the recycling of inorganic nutrients for regenerated primary production. Studying their activity and function in the ecosystem is challenging since most of the HFs in the ocean are still uncultured. In the present work, we investigated gene expression of natural HF communities during bacterivory in four unamended seawater incubations.

RESULTS

The most abundant species growing in our incubations belonged to the taxonomic groups MAST-4, MAST-7, Chrysophyceae, and Telonemia. Gene expression dynamics were similar between incubations and could be divided into three states based on microbial counts, each state displaying distinct expression patterns. The analysis of samples where HF growth was highest revealed some highly expressed genes that could be related to bacterivory. Using available genomic and transcriptomic references, we identified 25 species growing in our incubations and used those to compare the expression levels of these specific genes. Video Abstract CONCLUSIONS: Our results indicate that several peptidases, together with some glycoside hydrolases and glycosyltransferases, are more expressed in phagotrophic than in phototrophic species, and thus could be used to infer the process of bacterivory in natural assemblages.

Functional differentiation of Sec13 paralogues in the euglenozoan protists

The β-propeller protein Sec13 plays roles in at least three distinct processes by virtue of being a component of the COPII endoplasmic reticulum export vesicle coat, the nuclear pore complex (NPC) and the Seh1-associated (SEA)/GATOR nutrient-sensing complex. This suggests that regulatory mechanisms coordinating these cellular activities may operate via Sec13. The NPC, COPII and SEA/GATOR are all ancient features of eukaryotic cells, and in the vast majority of eukaryotes, a single Sec13 gene is present. Here we report that the Euglenozoa, a lineage encompassing the diplonemid, kinetoplastid and euglenid protists, possess two Sec13 paralogues. Furthermore, based on protein interactions and localization studies we show that in diplonemids Sec13 functions are divided between the Sec13a and Sec13b paralogues. Specifically, Sec13a interacts with COPII and the NPC, while Sec13b interacts with Sec16 and components of the SEA/GATOR complex. We infer that euglenozoan Sec13a is responsible for NPC functions and canonical anterograde transport activities while Sec13b acts within nutrient and autophagy-related pathways, indicating a fundamentally distinct organization of coatomer complexes in euglenozoan flagellates.

Recent expansion of metabolic versatility in Diplonema papillatum, the model species of a highly speciose group of marine eukaryotes

BACKGROUND

Diplonemid flagellates are among the most abundant and species-rich of known marine microeukaryotes, colonizing all habitats, depths, and geographic regions of the world ocean. However, little is known about their genomes, biology, and ecological role.

RESULTS

We present the first nuclear genome sequence from a diplonemid, the type species Diplonema papillatum. The ~ 280-Mb genome assembly contains about 32,000 protein-coding genes, likely co-transcribed in groups of up to 100. Gene clusters are separated by long repetitive regions that include numerous transposable elements, which also reside within introns. Analysis of gene-family evolution reveals that the last common diplonemid ancestor underwent considerable metabolic expansion. D. papillatum-specific gains of carbohydrate-degradation capability were apparently acquired via horizontal gene transfer. The predicted breakdown of polysaccharides including pectin and xylan is at odds with reports of peptides being the predominant carbon source of this organism. Secretome analysis together with feeding experiments suggest that D. papillatum is predatory, able to degrade cell walls of live microeukaryotes, macroalgae, and water plants, not only for protoplast feeding but also for metabolizing cell-wall carbohydrates as an energy source. The analysis of environmental barcode samples shows that D. papillatum is confined to temperate coastal waters, presumably acting in bioremediation of eutrophication.

CONCLUSIONS

Nuclear genome information will allow systematic functional and cell-biology studies in D. papillatum. It will also serve as a reference for the highly diverse diplonemids and provide a point of comparison for studying gene complement evolution in the sister group of Kinetoplastida, including human-pathogenic taxa.

Massive Accumulation of Strontium and Barium in Diplonemid Protists

Barium and strontium are often used as proxies of marine productivity in palaeoceanographic reconstructions of global climate. However, long-searched biological drivers for such correlations remain unknown. Here, we report that taxa within one of the most abundant groups of marine planktonic protists, diplonemids (Euglenozoa), are potent accumulators of intracellular barite (BaSO₄), celestite (SrSO₄), and strontiobarite (Ba,Sr)SO₄. In culture, Namystinia karyoxenos accumulates Ba²⁺ and Sr²⁺ 42,000 and 10,000 times higher than the surrounding medium, forming barite and celestite representing 90% of the dry weight, the greatest concentration in biomass known to date. As heterotrophs, diplonemids are not restricted to the photic zone, and they are widespread in the oceans in astonishing abundance and diversity, as their distribution correlates with environmental particulate barite and celestite, prevailing in the mesopelagic zone. We found diplonemid predators, the filter-feeding zooplankton that produces fecal pellets containing the undigested celestite from diplonemids, facilitating its deposition on the seafloor. To the best of our knowledge, evidence for diplonemid biomineralization presents the strongest explanation for the occurrence of particulate barite and celestite in the marine environment. Both structures of the crystals and their variable chemical compositions found in diplonemids fit the properties of environmentally sampled particulate barite and celestite. Finally, we propose that diplonemids, which emerged during the Neoproterozoic era, qualify as impactful players in Ba²⁺/Sr²⁺ cycling in the ocean that has possibly contributed to sedimentary rock formation over long geological periods. IMPORTANCE We have identified that diplonemids, an abundant group of marine planktonic protists, accumulate conspicuous amounts of Sr²⁺ and Ba²⁺ in the form of intracellular barite and celestite crystals, in concentrations that greatly exceed those of the most efficient Ba/Sr-accumulating organisms known to date. We propose that diplonemids are potential players in Ba²⁺/Sr²⁺ cycling in the ocean and have possibly contributed to sedimentary rock formation over long geological periods. These organisms emerged during the Neoproterozoic era (590 to 900 million years ago), prior to known coccolithophore carbonate biomineralization (~200 million years ago). Based on reported data, the distribution of diplonemids in the oceans is correlated with the occurrence of particulate barite and celestite. Finally, diplonemids may provide new insights into the long-questioned biogenic origin of particulate barite and celestite and bring more understanding of the observed spatial-temporal correlation of the minerals with marine productivity used in reconstructions of past global climate.

Water masses shape pico-nano eukaryotic communities of the Weddell Sea

Polar oceans belong to the most productive and rapidly changing environments, yet our understanding of this fragile ecosystem remains limited. Here we present an analysis of a unique set of DNA metabarcoding samples from the western Weddell Sea sampled throughout the whole water column and across five water masses with different characteristics and different origin. We focus on factors affecting the distribution of planktonic pico-nano eukaryotes and observe an ecological succession of eukaryotic communities as the water masses move away from the surface and as oxygen becomes depleted with time. At the beginning of this succession, in the photic zone, algae, bacteriovores, and predators of small eukaryotes dominate the community, while another community develops as the water sinks deeper, mostly composed of parasitoids (syndinians), mesoplankton predators (radiolarians), and diplonemids. The strongly correlated distribution of syndinians and diplonemids along the depth and oxygen gradients suggests their close ecological link and moves us closer to understanding the biological role of the latter group in the ocean ecosystem.

Gene Transfer Agents in Bacterial Endosymbionts of Microbial Eukaryotes

Gene transfer agents (GTAs) are virus-like structures that package and transfer prokaryotic DNA from donor to recipient prokaryotic cells. Here, we describe widespread GTA gene clusters in the highly reduced genomes of bacterial endosymbionts from microbial eukaryotes (protists). Homologs of the GTA capsid and portal complexes were initially found to be present in several highly reduced alphaproteobacterial endosymbionts of diplonemid protists (Rickettsiales and Rhodospirillales). Evidence of GTA expression was found in polyA-enriched metatranscriptomes of the diplonemid hosts and their endosymbionts, but due to biases in the polyA-enrichment methods, levels of GTA expression could not be determined. Examining the genomes of closely related bacteria revealed that the pattern of retained GTA head/capsid complexes with missing tail components was common across Rickettsiales and Holosporaceae (Rhodospirillales), all obligate symbionts with a wide variety of eukaryotic hosts. A dN/dS analysis of Rickettsiales and Holosporaceae symbionts revealed that purifying selection is likely the main driver of GTA evolution in symbionts, suggesting they remain functional, but the ecological function of GTAs in bacterial symbionts is unknown. In particular, it is unclear how increasing horizontal gene transfer in small, largely clonal endosymbiont populations can explain GTA retention, and, therefore, the structures may have been repurposed in endosymbionts for host interactions. Either way, their widespread retention and conservation in endosymbionts of diverse eukaryotes suggests an important role in symbiosis.