Sei Suzuki‐Tellier

Accumulating evidence suggests that the Last Eukaryotic Common Ancestor (LECA) resembled contemporary excavates, a non‐monophyletic group of deep branching flagellates. Here we explore the functional ecology of distantly related deep‐branching “excavate‐like” flagellates (in Alveolata and Provora) that share with many excavates a vane‐bearing posterior flagellum beating in association with a ventral groove. This arrangement is key to generating a feeding current in the bacterivorous typical excavates. However, the four species examined here are motile “hunters” that randomly encounter eukaryotic prey. Initial prey attachment may be facilitated by the force generated by the vaned flagellum but eventually by firing extrusomes. Propulsion is driven by the two flagella that are directed backwards. Despite minor differences, the propulsion and hunting strategies are similar between these two deep‐branching groups and distinctly different from those found in typical excavates. While the vane‐and‐groove morphology may help prey attachment, simulations show that it seems energetically disadvantageous to propulsion. Thus, its overall role, origin, and phylogenetic implications remain unclear. This study adds to our knowledge of the functional ecological range of living vane‐ and groove‐bearing flagellates. Given their significance to understanding early eukaryote evolutionary history, this may in turn shed light on the deep history of eukaryote life.

Flagellates, unicellular organisms equipped with one or a few flagella, are phylogenetically and functionally hugely diverse. Yet, most studies have focused on a few model organisms and on the role of the flagellum in propulsion, ignoring the fundamental role of the flagellum in foraging. The number and position of flagella vary between species; the flagella may be naked or equipped with vanes or hairs; the kinematics and wave patterns vary and may be planar or three-dimensional; and the flagellum may extend from the surface of the cell or lie within a groove on the cell surface. All these features impact the fluid dynamics and functioning of the flagellum. Here we explore some of this functional diversity with a focus on the fluid dynamics of phagotrophic foraging. Finally, we identify gaps in our knowledge of flagellar functioning in this diverse and ecologically significant group of organisms. This article is part of the theme issue ‘Biological fluid dynamics: emerging directions’.

Most excavates, a paraphyletic assemblage of flagellates, typically present an active vaned flagellum that drives a feeding current through a ventral groove for predation. However, some have “atypical” morphologies. Here, we describe the foraging mechanisms in heteroloboseid flagellates (Discoba) that have a groove but lack the seemingly crucial vane. The percolomonads barbeliid AE‐1 and Percolomonas doradorae form a functional vane with four adjacent flagella undulating with lateral asymmetry, creating an erratic flow that rapidly “sucks” water into the groove and expels it on the other side. This flow attenuates rapidly away from the cell, consistent with the flagellar pump acting as an instantaneous point sink. Conversely, Pharyngomonas kirbyi generates a continuous flow through the groove with two asynchronously moving posterior flagella. Despite the unexplained fluid dynamics, this flow has a further reach, consistent with describing the flagellar pump as a point force (stokeslet). While the volumetric flow rate through the groove—a measure of the maximum potential clearance rate—of the two percolomonads is similar to clearance rates estimated for other phagotrophic flagellates, it is an order of magnitude lower for Ph. kirbyi, which may afford lower rates due to high prey concentration in its hypersaline environment.

The phagotrophic flagellates described as “typical excavates” have been hypothesized to be morphologically similar to the Last Eukaryotic Common Ancestor and understanding the functional ecology of excavates may therefore help shed light on the ecology of these early eukaryotes. Typical excavates are characterized by a posterior flagellum equipped with a vane that beats in a ventral groove. Here, we combined flow visualization and observations of prey capture in representatives of the three clades of excavates with computational fluid dynamic modeling, to understand the functional significance of this cell architecture. We record substantial differences amongst species in the orientation of the vane and the beat plane of the posterior flagellum. Clearance rate magnitudes estimated from flow visualization and modeling are both like that of other similarly sized flagellates. The interaction between a vaned flagellum beating in a confinement is modeled to produce a very efficient feeding current at low energy costs, irrespective of the beat plane and vane orientation and of all other morphological variations. Given this predicted uniformity of function, we suggest that the foraging systems of typical excavates studied here may be good proxies to understand those potentially used by our distant ancestors more than 1 billion years ago.

Phagotrophic flagellates are the main consumers of bacteria and picophytoplankton. Despite their ecological significance in the ‘microbial loop’, many of their predation mechanisms remain unclear. ‘Typical excavates’ bear a ventral groove, where prey is captured for ingestion. The consequences of feeding through a ‘semi‐rigid’ furrow on the prey size range have not been explored. An unidentified moving element called ‘the wave’ that sweeps along the bottom of the groove toward the site of phagocytosis has been observed in a few species; its function is unclear. We investigated the presence, behavior, and function of the wave in four species from the three excavate clades (Discoba, Metamonada, and Malawimonadida) and found it present in all studied cases, suggesting the potential homology of this feature across all three groups. The wave displayed a species‐specific behavior and was crucial for phagocytosis. The morphology of the feeding groove had an upper‐prey size limit for successful prey captures, but smaller particles were not constrained. Additionally, the ingestion efficiencies were species dependent. By jointly studying these feeding traits, we speculate on adaptations to differences in food availability to better understand their ecological functions.

The phagotrophic flagellates described as ‘typical excavates’ have been hypothesized to be morphologically similar to the Last Eukaryotic Common Ancestor and understanding the functional ecology of excavates may therefore help shed light on the ecology of these early eukaryotes. Typical excavates are characterized by a posterior flagellum equipped with a vane that beats in a ventral groove. Here, we combined flow visualization and observations of prey capture in representatives of the three clades of excavates with computational fluid dynamic modelling, to understand the functional significance of this cell architecture. We record substantial differences amongst species in the orientation of the vane and the beat plane of the posterior flagellum. Clearance rate magnitudes estimated from flow visualization and modelling are like that of other similarly sized phagotrophic flagellates. The interaction between a vaned flagellum beating in a confinement is modelled to produce a very efficient feeding current at low energy costs, irrespective of the beat plane and vane orientation and of all other morphological variations. Given this predicted uniformity of function, we suggest that the foraging systems of typical excavates studied here may be good proxies to understand those potentially used by our distant ancestors more than 1 billion years ago. Significance Human sperm reminds us of our ancestry: flagellates, unicellular organisms equipped with a flagellum. The last common eukaryotic ancestor (LECA) was a flagellate. Phylogenetic analyses suggest that Excavates, an assemblage of flagellates, are the living organisms most similar to LECA. They have distinct characteristics in common: a ventral groove within which a vaned flagellum is beating. We show how the shared morphology and foraging behavior among 3 excavate clades is fluid dynamically efficient. A similar flagellar arrangement, potentially homologous to that found in the excavates, is found among flagellates from other deep branches of the eukaryotic tree, suggesting that the typical excavate foraging system studied here may have been used by our distant ancestors more than 1 billion years ago.

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their microscale world, viscosity impedes predator–prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 10⁶ times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 10⁴ bacterial preys per day. We used high‐speed video microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point force models allowed us to estimate the forces required to generate the feeding flows, between 4 and 13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked (smooth) flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their micro-scale world, viscosity impedes predator-prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 10 ⁶ times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 10 ⁴ bacterial prey per day. We used high-speed video-microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey-handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point-force models allowed us to estimate the forces required to generate the feeding flows, between 4-13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.

A fundamental concept in evolutionary theory is the last universal common ancestor (LUCA) from which all living organisms originated. While hypotheses regarding the phenotype of LUCA do exist, most are at best based on gene presence or absence. However, despite recent attempts to link genes and phenotypic traits in prokaryotes, it is still inherently difficult to predict phenotype based on the presence or absence of genes alone. Here, we apply a novel approach based on phylogenetically informed ancestral character reconstruction with 22 fundamental descriptors of prokaryotic cells to reconstruct LUCA’s phenotype. We infer that the last universal common ancestor of all living organisms was likely an ovoid cell with a large genome, which probably had a cell wall and was actively motile. Potentially obtaining energy from the oxidation of inorganic compounds, tolerant of saltwater and thriving at temperatures above 70°C, LUCA was probably found living freely in anaerobic conditions in water of neutral pH. Our results depict LUCA as likely to be a far more complex cell than has previously been proposed, challenging the evolutionary model of increased complexity through time in prokaryotes. Given current estimates for the emergence of LUCA we suggest that early life very rapidly evolved considerable cellular complexity.