From termite symbionts with rotating heads to amoeboid cactus-dwellers, from cannibalistic ex-algae to ciliates thriving in saturated brine, 2023 brought us numerous new discoveries of weird and wonderful microbial eukaryotes. Let's explore some of the most interesting finds in this end of year recapitulation. This is quite a taxonomy-heavy read organized by eukaryotic supergroups and so you might want to consult the new eukaryotic tree of life with beautiful illustrations published by Patrick Keeling and Yana Eglit [1], or a slightly older paper about deep phylogeny of eukaryotes by Fabien Burki et al. [2]. Please note, that this list is by no means exhaustive and it necessarily reflects interests and biases of its author. If you don't find your favorite discovery below, you can share it with us on our social media. If you enjoy this article, you might also like similar summaries about new bacteria, fungi, and viruses by FEMS, or the monthly New Species posts on the Earthling Nature blog.
Stramenopiles
Ochrophytes are a highly diversified and successful group of photosynthetic stramenopiles comprising large multicellular phaeophyte seaweeds (eg. kelp or Sargassum) as well as diatoms living in glass shells, chrysophytes with silicaceous scales, and numerous other single-celled algae. From the many new taxa of microbial ochrophytes described in 2023, let's mention for example Lineaperpetua [3] - new genus of freshwater centric (round) diatoms, Amphora micrometra and Halamphora valdeminutissima [4] - two new species of small pennate (boat-shaped) diatoms from Bulgaria, and Synura rubra [5] - new freshwater chrysophyte species similar to S. sphagnicola.
In contrast to the well-studied ochrophytes, our understanding of heterotrophic stramenopiles remains rather patchy. Many deep-branching stramenopiles are still known only through molecular analyses and classified into provisional MAST (MArine STramenopiles) lineages. Anna Cho et al. [6] focused on MAST-6 and described four new species and two new genera of these sediment-dwelling flagellates. The same paper also describes a new member of the stramenopile class Placididea, a possibly halotolerant Haloplacidia sinai. Completely new deep-branching lineage of stramenopiles was recognized in a newly described algae-eating flagellate with a large ventral groove named Kaonashia insperata [7].
Alveolates
Ciliates are one of the best understood protistan groups and yet, every year brings discoveries of many new species of these highly diverse, successful, and charismatic microbes. Let's mention at least a few interesting examples from last year's harvest. Anaerobic ciliates are crucial for understanding evolutionary transitions to life without oxygen and they can be found in many independent lineages. New species of anaerobic ciliates were described in classes Muranotrichea [8] and Plagiopylea [9]. Kateřina Poláková et al. [10] studied the subclass Scuticociliatia, previously thought to be mostly aerobic, and found a broad diversity of anaerobes. These anaerobic scuticociliates form an evolutionarily distinct lineage that has been classified as the novel family Anaerocyclidiidae. Mahnaz Barmshuri et al. [11] described a new extremophilic ciliate Enchelyothrix muria living in saturated brines of a salt lake in Iran. Ciliates of the family Clevelandellidae are morphologically peculiar symbionts of wood-eating cockroaches. Six new species of the genus Clevelandella [12] were described from a broad spectrum of hosts using a combination of molecular and morphological methods.
Apart from ciliates, Alveolata comprises symbiotic apicomplexans, mostly photosynthetic dinoflagellates, and a number of smaller deep-branching lineages. Two new species and a new genus of gregarine apicomplexans [13] were described from darkling beetles living in the extremely dry environment of the Atacama desert. Coutea sabulosa [14] is a new genus and species of sand-dwelling dinoflagellates with unusual surface pattern. It is found in temperate and subtropical marine environments. Colponemids are deep-branching flagellates that are important for understanding overall evolutionary history of alveolates. Andrea Gigeroff et al. [15] described two new genera and species of colponemids, one alkaliphilic and other halophilic, both active predators of eukaryotes.
Rhizarians
Foraminifera are large rhizarian amoebae with shells that are often very elaborate and easy to identify in fossil record. This makes them very useful to geologists and paleontologists. Some Foraminifera shells are coiled with multiple chambers, resembling those of snails and ammonites, while other are simpler with a single chamber. Two new extant species [16] of the latter, so called monothalmous Foraminifera, were described from coastal Maharashtra, India. Amoebae of the genus Gromia are similar to monothalmous foraminifera but do not belong inside the group. Only a single species, G. oviformis, had been known from the Black sea until this year when Sylvain Kreuter et al. [17] identified three completely new Gromia species in samples from the Romanian continental shelf.
Unlike Foraminifera and Gromia, vampyrellids are naked amoebae without shells. Their name derives from a remarkable way of feeding that some of them exhibit: they puncture cell wall of algae and suck out their cytoplasm. New vampyrellid genus [18] with two species, Pseudovampyrella closterii and P. minor, was described in German moorlands. Both species feed on protoplasts of green algae of the genus Closterium. Cercozoa is a large and highly diversified group of rhizarians which is difficult to define morphologically. Eduardo Acosta et al. [19] described four new species of bacterivorous cercozoan amoebae and flagellates living on cacti in the Atacama desert.
Video of Pseudovampyrella closterii eating a Closterium. From Suthaus & Hess, 2023:
Haptists
Haptophytes are unicellular algae characterized by two almost identical flagella and a third appendage called haptonema. Coccolithophores are those haptophytes which cover their cells in hard calcareous plates. They are among the most abundant phytoplankton in the ocean and play important roles in marine ecology. Three new species of the coccolithophore genus Ericiolus [20] were described based on samples from the low photic zone of various ocean localities. Unlike in most other coccolithophores, calcareous plates of Ericiolus look like little stars and have no circular components. Benoît Véron et al. [21] described two new species belonging to Pavlovophyceae, a group of naked haptophytes with no hard plates.
With their thin radial microtubule-supported pseudopods and predatory lifestyle, centrohelid heliozoans have little in common with haptophytes, and yet molecular phylogenetics clearly places them together in one group called Haptista. Kenneth Nicholls [22] published results of nearly five decades of sampling for centrohelids in Canadian coastal waters and freshwater habitats. Among other results, his survey yielded 22 species new to science.
Archaeplastids
Archaeplastida is best known as the home of land plants and multicellular red algae. However, this large group of eukaryotes also includes numerous lineages of single-celled algae and their heterotrophic relatives. Zygnematophyceae are one of the closest lineages to land plants. They live as single cells or simple filaments, have no flagella, and sexually reproduce through a unique type of conjugation. Among them can be found the especially beautiful Desmidiaceae with cells resembling flat discs divided into multiple symmetrically organized branches so they look like green microscopic snowflakes. New Desmidiaceae species, Micrasterias levergerensis [23], was found in the Pantanal of Brazil. One of the most diverse groups of green algae is the class Chlorophyceae comprising unicellular, colonial, and even multicellular algae. Karolina Fučíková et al. [24] identified a new genus and species of Chlorophyceae in samples of desert soil from Joshua Tree National Park in California. The new alga, named Johansenicoccus eremophilus, has remarkably derived ribosomal genes.
Rhodelphidia are an evolutionary oddity. These are heterotrophic flagellates actively hunting bacteria or small protists and yet they are closely related to red algae and clearly derived from photosynthetic ancestors. This makes them unique and very interesting subjects for evolutionary protistologists. When they were first described in 2019 [25], only two species were known. Now, Kristina Prokina et al. [26] found a third species, Rhodelphis mylnikovi, which is larger than the other two rhodelphids, and shows a tendency to cannibalism.
Amoebozoans
When a protists doesn't have fixed shape and uses pseudopods for moving around or eating, we call it amoeboid. Not all amoeboid protists are members of Amoebozoa but all amoebozoans are at least somewhat amoeboid. Arcellinida compensate for their lack of cellular rigidity by living in vase-shaped shells which they either secret themselves or construct from found materials. Giulia Ribeiro et al. [27] described four new species of arcellinid amoebae while working on a database of molecular markers specific for this group. Other four species were discovered [28] during exploration of arcellinid diversity in environments with varying salinity. Arcellinida are most often found in soils and fresh water, but some of the newly described species show a notable tolerance to high salt contents in their environment which they apparently developed independently from each other.
Unlike Arcellinida, most amoebozoans live true to their shapeshifting nature. New species of these naked amoebae was found at the bottom of the Pacific ocean, right next to the Mariana trench. Mayorella marianaensis [29] is the first member of its genus known to live in the deep sea. Certain amoebozoans managed to enter the macroscopic world by creating large fruiting bodies resembling mushrooms. These structures are either multicellular (cellular slime molds of the class Dictyosteliida) or formed by a single huge cell with numerous nuclei (plasmodial slime molds of the class Myxogastria). New genus and species of beautiful yellow plasmodial slime molds [30] was found in Tasmania. You can watch a talk about the discovery of Tasmaniomyxa umbilicata by one of the co-authors, Dmytro Leontyev, on YouTube.
Ancyromonads
The bean-shaped ancyromonads just can't find their home on the eukaryotic tree of life. These tiny and ubiquitous flagellates may possibly form a very deep lineage on their own, and that’s why evolutionary protistologists are so interested in their diversity. Naoji Yubuki et al. [31] published a large scale study of ancyromonads in which they combined molecular environmental surveys, observations of live cell behavior, and morphological descriptions. Their results confirm a global distribution of ancyromonads in a broad spectrum of aquatic environments from the deep sea to freshwater lakes and suggest a marine origin of the entire group with multiple transitions to fresh water. They also described three new genera and seven new species.
CRuMs
Mantamonads are marine flagellates whose flattened cell shape with lateral wings resembles manta rays. They are found in sediments where they glide along surfaces on one of their flagella and eat bacteria. Similarly to ancyromonads, they were considered an orphan lineage, but recent molecular phylogenetic studies placed them in relationship with two other groups of enigmatic flagellates: collodictyonids and rigifilids. These three lineages are now classified in a supergroup called "CRuMs" which appears to be related to Amoebozoa and Opisthokonta (containing animals and fungi). Jazmin Blaz et al. [32] described two new species of mantamonads, Mantamonas vickermani and M. sphyraenae, and managed to gather a wealth of genetic data including a high-quality genome for one of them.
Metamonads
The anaerobic metamonads are known for their highly reduced [33] or even completely missing mitochondria [34]. Their position on the eukaryotic tree of life [35] as well as their role in early evolution of eukaryotes [36] remain controversial. Parabasalia are almost exclusively symbiotic metamonads exhibiting two very different cell types. Small parabasalids have several flagella and often cause health issues to their vertebrate hosts. Large parabasalids known as hypermastigids have many clusters of flagella and are found exclusively in digestive tracts of insects. Leila Tuzlak et al. [37] found two new species of small parabasalids in laboratory mice. The discovery of Tritrichomonas musculus and T. casperi is remarkable in light of how well studied these ubiquitous model rodents are.
Elisabeth Hehenberger et al. [38] described a new genus and four new species of hypermastigids from termites. All new species have large cells with many flagella and nuclei, and they presumably help their termite hosts with digestion of wood as is expected for hypermastigids. The new genus Daimonympha, however, is delightfully bizarre. The entire front third of the cell including the numerous flagella and nuclei smoothly and continuously rotates. Why and how do they do this? How do they deal with their cell membrane being constantly stretched and their cytoplasm stirred? We simply don't know.
Video of Daimonympha friedkini with its rotating "head". From Hehenberger et al., 2023:
Anaeramoebae is a recently described [39] group of amoeboid metamonads [40]. They live in oxygen depleted marine sediments and benefit from symbiosis with sulfate reducing bacteria [41]. Two new species of anaeramoebae [42] were described from coastal marine sediments. One of them, Anaeramoeba pumila, shows a very different type of symbiosis with bacteria, that still needs to be characterized in detail.
Discobids
Euglenophytes are mostly freshwater algae known for their bright green color and red eyespots. However, certain euglenophytes apparently abandoned their pretty colors and peaceful photosynthetic lifestyle to become parasites of invertebrates. New species of parasitic euglenophytes [47] was found in crustaceans and flatworms living in rice fields in Japan. This deadly microbe belonging to an otherwise free-living photosynthetic genus Euglenaformis was named E. parasitica. You can read more about this discovery in a blogpost by Tommy Leung. Ploeotids are a paraphyletic assemblage of primarily heterotrophic euglenids which means they form a large part of the backbone of euglenid tree of life and unlike their relatives, the euglenophytes, they never acquired a plastid. Gordon Lax et al. [43] described two new genera and three new species of ploeotids from marine and freshwater sediments of British Columbia.
Jakobids are tiny flagellates that eat bacteria. Their mitochondria have one of the largest and most conserved genomes among all eukaryotes. One new genus and two new species [44] described from marine and freshwater sediments were placed in the jakobid suborder Ophirinina and their phylogenetic analysis helped to establish Ophirinina as sister to all other jakobids. Surprisingly, mitochondrial genome of one of the new species includes a machinery for removing group II introns which was not previously identified in jakobids although it is present in other protistan lineages. The often amoeboid heteroloboseans are related to euglenids and jakobids but they don't share any morphological characteristics. Most heteroloboseans feed on bacteria but there are also species which prefer eukaryotic algae. New algivorous heterolobosean amoeba Euplaesiobystra perlucida [45] was found to wreak havoc in cultures of the diatom Phaeodactylum tricornutum intended for industrial production of dietary supplements. Another new amoeboid heterolobosean of the genus Allovahlkampfia was found in desert cactus soil from Atacama in the same study as the abovementioned cactus-dwelling cercozoans [19].
Lukas V. F. Novak, December 2023
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