Phylogenomic insights into the first multicellular streptophyte

Streptophytes are best known as the clade containing the teeming diversity of embryophytes (land plants).1,2,3,4 Next to embryophytes are however a range of freshwater and terrestrial algae that bear important information on the emergence of key traits of land plants. Among these, the Klebsormidiophyceae stand out. Thriving in diverse environments-from mundane (ubiquitous occurrence on tree barks and rocks) to extreme (from the Atacama Desert to the Antarctic)-Klebsormidiophyceae can exhibit filamentous body plans and display remarkable resilience as colonizers of terrestrial habitats.5,6 Currently, the lack of a robust phylogenetic framework for the Klebsormidiophyceae hampers our understanding of the evolutionary history of these key traits. Here, we conducted a phylogenomic analysis utilizing advanced models that can counteract systematic biases. We sequenced 24 new transcriptomes of Klebsormidiophyceae and combined them with 14 previously published genomic and transcriptomic datasets. Using an analysis built on 845 loci and sophisticated mixture models, we establish a phylogenomic framework, dividing the six distinct genera of Klebsormidiophyceae in a novel three-order system, with a deep divergence more than 830 million years ago. Our reconstructions of ancestral states suggest (1) an evolutionary history of multiple transitions between terrestrial-aquatic habitats, with stem Klebsormidiales having conquered land earlier than embryophytes, and (2) that the body plan of the last common ancestor of Klebsormidiophyceae was multicellular, with a high probability that it was filamentous whereas the sarcinoids and unicells in Klebsormidiophyceae are likely derived states. We provide evidence that the first multicellular streptophytes likely lived about a billion years ago.

Chloroplast genome evolution and phylogeny of the early-diverging charophycean green algae with a focus on the Klebsormidiophyceae and Streptofilum

The Klebsormidiophyceae are a class of green microalgae observed globally in both freshwater and terrestrial habitats. Morphology-based classification schemes of this class have been shown to be inadequate due to the simple morphology of these algae, the tendency of morphology to vary in culture versus field conditions, and rampant morphological homoplasy. Molecular studies revealing cryptic diversity have renewed interest in this group. We sequenced the complete chloroplast genomes of a broad series of taxa spanning the known taxonomic breadth of this class. We also sequenced the chloroplast genomes of three strains of Streptofilum, a recently discovered green algal lineage with close affinity to the Klebsormidiophyceae. Our results affirm the previously hypothesized polyphyly of the genus Klebsormidium as well as the polyphyly of the nominal species in this genus, K. flaccidum. Furthermore, plastome sequences strongly support the status of Streptofilum as a distinct, early-diverging lineage of charophytic algae sister to a clade comprising Klebsormidiophyceae plus Phragmoplastophyta. We also uncovered major structural alterations in the chloroplast genomes of species in Klebsormidium that have broad implications regarding the underlying mechanisms of chloroplast genome evolution.

Improved Methods for Acetocarmine and Haematoxylin Staining to Visualize Chromosomes in the Filamentous Green Alga Zygnema (Charophyta)

Genome sizes of Zygnema spp. vary greatly, being unknown whether polyploidization occurred. The exact number of chromosomes in this genus is unknown since counting methods established for higher plants cannot be applied to green algae. The massive presence of pectins and arabinogalactan proteins in the cell wall interferes with the uptake of staining solutions; moreover, cell divisions in green algae are not restricted to meristems as in higher plants, which is another limiting factor. Cell divisions occur randomly in the thallus, due to the intercalary growth of algal filaments. Therefore, we increased the number of cell divisions via synchronization by changing the light cycle (10:14 h light/dark). The number of observed mitotic stages peaked at the beginning of the dark cycle. This protocol describes two methods for the visualization of chromosomes in the filamentous green alga Zygnema. Existing protocols were modified, leading to improved acetocarmine and haematoxylin staining methods as investigated by light microscopy. A freeze-shattering approach with liquid nitrogen was applied to increase the accessibility of the haematoxylin dye. These modified protocols allowed reliable chromosome counting in the genus Zygnema. Key features Improved method for chromosome staining in filamentous green algae. Optimized for the Zygnema strains SAG 698-1a (Z. cylindricum), SAG 698-1b (Z. circumcarinatum), and SAG 2419 (Zygnema 'Saalach'). This protocol builds upon the methods of chromosomal staining in green algae developed by Wittmann (1965), Staker (1971), and Fujii and Guerra (1998). Cultivation and synchronization: 14 days; fixation and permeabilization: 24 h; staining: 1 h; image analysis and chromosome number quantification: up to 20 h.

A phylogenomically informed five-order system for the closest relatives of land plants

The evolution of streptophytes had a profound impact on life on Earth. They brought forth those photosynthetic eukaryotes that today dominate the macroscopic flora: the land plants (Embryophyta).¹ There is convincing evidence that the unicellular/filamentous Zygnematophyceae-and not the morphologically more elaborate Coleochaetophyceae or Charophyceae-are the closest algal relatives of land plants.^2-6 Despite the species richness (>4,000), wide distribution, and key evolutionary position of the zygnematophytes, their internal phylogeny remains largely unresolved.^7,8 There are also putative zygnematophytes with interesting body plan modifications (e.g., filamentous growth) whose phylogenetic affiliations remain unknown. Here, we studied a filamentous green alga (strain MZCH580) from an Austrian peat bog with central or parietal chloroplasts that lack discernible pyrenoids. It represents Mougeotiopsis calospora PALLA, an enigmatic alga that was described more than 120 years ago⁹ but never subjected to molecular analyses. We generated transcriptomic data of M. calospora strain MZCH580 and conducted comprehensive phylogenomic analyses (326 nuclear loci) for 46 taxonomically diverse zygnematophytes. Strain MZCH580 falls in a deep-branching zygnematophycean clade together with some unicellular species and thus represents a formerly unknown zygnematophycean lineage with filamentous growth. Our well-supported phylogenomic tree lets us propose a new five-order system for the Zygnematophyceae and provides evidence for at least five independent origins of true filamentous growth in the closest algal relatives of land plants. This phylogeny provides a robust and comprehensive framework for performing comparative analyses and inferring the evolution of cellular traits and body plans in the closest relatives of land plants.

Transcriptomic and Metabolomic Response to High Light in the Charophyte Alga Klebsormidium nitens

The characterization of the molecular mechanisms, such as high light irradiance resistance, that allowed plant terrestralization is a cornerstone in evolutionary studies since the conquest of land by plants played a pivotal role in life evolution on Earth. Viridiplantae or the green lineage is divided into two clades, Chlorophyta and Streptophyta, that in turn splits into Embryophyta or land plants and Charophyta. Charophyta are used in evolutionary studies on plant terrestralization since they are generally accepted as the extant algal species most closely related to current land plants. In this study, we have chosen the facultative terrestrial early charophyte alga Klebsormidium nitens to perform an integrative transcriptomic and metabolomic analysis under high light in order to unveil key mechanisms involved in the early steps of plants terrestralization. We found a fast chloroplast retrograde signaling possibly mediated by reactive oxygen species and the inositol polyphosphate 1-phosphatase (SAL1) and 3'-phosphoadenosine-5'-phosphate (PAP) pathways inducing gene expression and accumulation of specific metabolites. Systems used by both Chlorophyta and Embryophyta were activated such as the xanthophyll cycle with an accumulation of zeaxanthin and protein folding and repair mechanisms constituted by NADPH-dependent thioredoxin reductases, thioredoxin-disulfide reductases, and peroxiredoxins. Similarly, cyclic electron flow, specifically the pathway dependent on proton gradient regulation 5, was strongly activated under high light. We detected a simultaneous co-activation of the non-photochemical quenching mechanisms based on LHC-like stress related (LHCSR) protein and the photosystem II subunit S that are specific to Chlorophyta and Embryophyta, respectively. Exclusive Embryophyta systems for the synthesis, sensing, and response to the phytohormone auxin were also activated under high light in K. nitens leading to an increase in auxin content with the concomitant accumulation of amino acids such as tryptophan, histidine, and phenylalanine.

Cryogenian Glacial Habitats as a Plant Terrestrialisation Cradle - The Origin of the Anydrophytes and Zygnematophyceae Split

For tens of millions of years (Ma), the terrestrial habitats of Snowball Earth during the Cryogenian period (between 720 and 635 Ma before present-Neoproterozoic Era) were possibly dominated by global snow and ice cover up to the equatorial sublimative desert. The most recent time-calibrated phylogenies calibrated not only on plants but on a comprehensive set of eukaryotes indicate that within the Streptophyta, multicellular charophytes (Phragmoplastophyta) evolved in the Mesoproterozoic to the early Neoproterozoic. At the same time, Cryogenian is the time of the likely origin of the common ancestor of Zygnematophyceae and Embryophyta and later, also of the Zygnematophyceae-Embryophyta split. This common ancestor is proposed to be called Anydrophyta; here, we use anydrophytes. Based on the combination of published phylogenomic studies and estimated diversification time comparisons, we deem it highly likely that anydrophytes evolved in response to Cryogenian cooling. Also, later in the Cryogenian, secondary simplification of multicellular anydrophytes and loss of flagella resulted in Zygnematophyceae diversification as an adaptation to the extended cold glacial environment. We propose that the Marinoan geochemically documented expansion of first terrestrial flora has been represented not only by Chlorophyta but also by Streptophyta, including the anydrophytes, and later by Zygnematophyceae, thriving on glacial surfaces until today. It is possible that multicellular early Embryophyta survived in less abundant (possibly relatively warmer) refugia, relying more on mineral substrates, allowing the retention of flagella-based sexuality. The loss of flagella and sexual reproduction by conjugation evolved in Zygnematophyceae and zygomycetous fungi during the Cryogenian in a remarkably convergent way. Thus, we support the concept that the important basal cellular adaptations to terrestrial environments were exapted in streptophyte algae for terrestrialization and propose that this was stimulated by the adaptation to glacial habitats dominating the Cryogenian Snowball Earth. Including the glacial lifestyle when considering the rise of land plants increases the parsimony of connecting different ecological, phylogenetic, and physiological puzzles of the journey from aquatic algae to terrestrial floras.

Desiccation tolerance in streptophyte algae and the algae to land plant transition: evolution of LEA and MIP protein families within the Viridiplantae

The present review summarizes the effects of desiccation in streptophyte green algae, as numerous experimental studies have been performed over the past decade particularly in the early branching streptophyte Klebsormidium sp. and the late branching Zygnema circumcarinatum. The latter genus gives its name to the Zygenmatophyceae, the sister group to land plants. For both organisms, transcriptomic investigations of desiccation stress are available, and illustrate a high variability in the stress response depending on the conditions and the strains used. However, overall, the responses of both organisms to desiccation stress are very similar to that of land plants. We highlight the evolution of two highly regulated protein families, the late embryogenesis abundant (LEA) proteins and the major intrinsic protein (MIP) family. Chlorophytes and streptophytes encode LEA4 and LEA5, while LEA2 have so far only been found in streptophyte algae, indicating an evolutionary origin in this group. Within the MIP family, a high transcriptomic regulation of a tonoplast intrinsic protein (TIP) has been found for the first time outside the embryophytes in Z. circumcarinatum. The MIP family became more complex on the way to terrestrialization but simplified afterwards. These observations suggest a key role for water transport proteins in desiccation tolerance of streptophytes.

Structural and functional similarities and differences in nucleolar Pumilio RNA-binding proteins between Arabidopsis and the charophyte Chara corallina

BACKGROUND

Pumilio RNA-binding proteins are evolutionarily conserved throughout eukaryotes and are involved in RNA decay, transport, and translation repression in the cytoplasm. Although a majority of Pumilio proteins function in the cytoplasm, two nucleolar forms have been reported to have a function in rRNA processing in Arabidopsis. The species of the genus Chara have been known to be most closely related to land plants, as they share several characteristics with modern Embryophyta.

RESULTS

In this study, we identified two putative nucleolar Pumilio protein genes, namely, ChPUM2 and ChPUM3, from the transcriptome of Chara corallina. Of the two ChPUM proteins, ChPUM2 was most similar in amino acid sequence (27% identity and 45% homology) and predicted protein structure to Arabidopsis APUM23, while ChPUM3 was similar to APUM24 (35% identity and 54% homology). The transient expression of 35S:ChPUM2-RFP and 35S:ChPUM3-RFP showed nucleolar localization of fusion proteins in tobacco leaf cells, similar to the expression of 35S:APUM23-GFP and 35S:APUM24-GFP. Moreover, 35S:ChPUM2 complemented the morphological defects of the apum23 phenotypes but not those of apum24, while 35S:ChPUM3 could not complement the apum23 and apum24 mutants. Similarly, the 35S:ChPUM2/apum23 plants rescued the pre-rRNA processing defect of apum23, but 35S:ChPUM3/apum24^+/- plants did not rescue that of apum24. Consistent with these complementation results, a known target RNA-binding sequence at the end of the 18S rRNA (5'-GGAAUUGACGG) for APUM23 was conserved in Arabidopsis and C. corallina, whereas a target region of ITS2 pre-rRNA for APUM24 was 156 nt longer in C. corallina than in A. thaliana. Moreover, ChPUM2 and APUM23 were predicted to have nearly identical structures, but ChPUM3 and APUM24 have different structures in the 5th C-terminal Puf RNA-binding domain, which had a longer random coil in ChPUM3 than in APUM24.

CONCLUSIONS

ChPUM2 of C. corallina was functional in Arabidopsis, similar to APUM23, but ChPUM3 did not substitute for APUM24 in Arabidopsis. Protein homology modeling showed high coverage between APUM23 and ChPUM2, but displayed structural differences between APUM24 and ChPUM3. Together with the protein structure of ChPUM3 itself, a short ITS2 of Arabidopsis pre-rRNA may interrupt the binding of ChPUM3 to 3'-extended 5.8S pre-rRNA.

Comprehensive Genome-Wide Classification Reveals That Many Plant-Specific Transcription Factors Evolved in Streptophyte Algae

Plant genomes encode many lineage-specific, unique transcription factors. Expansion of such gene families has been previously found to coincide with the evolution of morphological complexity, although comparative analyses have been hampered by severe sampling bias. Here, we make use of the recently increased availability of plant genomes. We have updated and expanded previous rule sets for domain-based classification of transcription associated proteins (TAPs), comprising transcription factors and transcriptional regulators. The genome-wide annotation of these protein families has been analyzed and made available via the novel TAPscan web interface. We find that many TAP families previously thought to be specific for land plants actually evolved in streptophyte (charophyte) algae; 26 out of 36 TAP family gains are inferred to have occurred in the common ancestor of the Streptophyta (uniting the land plants—Embryophyta—with their closest algal relatives). In contrast, expansions of TAP families were found to occur throughout streptophyte evolution. 17 out of 76 expansion events were found to be common to all land plants and thus probably evolved concomitant with the water-to-land-transition.

Xyloglucan evolution and the terrestrialization of green plants

Xyloglucan (XyG) is the major noncellulosic nonpectic matrix polysaccharide in cell walls of most land plants. Initially thought to be restricted to land plants, the last decade has seen the detection of XyG and the discovery of synthesis and modification/degradation genes in charophycean green algae (CGA). Recently, a totally new function of XyG was discovered as a potent soil aggregator released by roots and rhizoids of all major groups of land plants. In this Viewpoint, I show the presence of a complex XyG genetic machinery in most CGA groups. I discuss the context of XyG evolution in light of the terrestrialization of early CGA that gave rise to embryophytes and its possible role in early soil formation.

Why we need more non-seed plant models

Contents 355 I. 355 II. 356 III. 356 IV. 357 V. 358 VI. 359 359 References 359 SUMMARY: Out of a hundred sequenced and published land plant genomes, four are not of flowering plants. This severely skewed taxonomic sampling hinders our comprehension of land plant evolution at large. Moreover, most genetically accessible model species are flowering plants as well. If we are to gain a deeper understanding of how plants evolved and still evolve, and which of their developmental patterns are ancestral or derived, we need to study a more diverse set of plants. Here, I thus argue that we need to sequence genomes of so far neglected lineages, and that we need to develop more non-seed plant model species.

First discovery of the charophycean green alga Lychnothamnus barbatus (Charophyceae) extant in the New World

PREMISE OF THE STUDY

Although some species of Characeae, known as stoneworts, can be found on every continent except Antarctica, many species and some genera have limited geographic distributions. The genus Lychnothamnus, represented by a single extant species L. barbatus, was known only from scattered localities in Europe and Australasia until it was recently discovered in North America.

METHODS

Morphological identifications were made from specimens collected in Minnesota and Wisconsin, USA. DNA sequences were obtained for three plastid-encoded genes (atpB, psbC, rbcL) from seven putative Lychnothamnus samples from two states in the USA Distribution and abundance were estimated in each lake using point intercept surveys where surveyors sampled aquatic vegetation.

KEY RESULTS

Fourteen lakes in Wisconsin and two lakes in Minnesota, USA, were found to harbor Lychnothamnus barbatus. These represent the first report of this rare charophycean extant in the New World. The North American specimens matched the morphological description for L. barbatus and were compared directly with the neotype. Phylogenetic results using three plastid-encoded genes confirmed the identification placing New World samples with those from Europe and Australasia. Our phylogenetic analyses also confirmed the sister relationship between L. barbatus and Nitellopsis obtusa.

CONCLUSIONS

Because this taxon is not known for aggressive invasiveness in its native range, it may have existed in heretofore-undiscovered native populations, although the possibility that it is a recent introduction cannot be eliminated. The potential for discovery of novel lineages of green algae in even well-studied regions is apparently far from exhausted.

F-actin reorganization upon de- and rehydration in the aeroterrestrial green alga Klebsormidium crenulatum

Filamentous actin (F-actin) is a dynamic network involved in many cellular processes like cell division and cytoplasmic streaming. While many studies have addressed the involvement of F-actin in different cellular processes in cultured cells, little is known on the reactions to environmental stress scenarios, where this system might have essential regulatory functions. We investigated here the de- and rehydration kinetics of breakdown and reassembly of F-actin in the streptophyte green alga Klebsormidium crenulatum. Measurements of the chlorophyll fluorescence (effective quantum yield of photosystem II [ΔF/Fm']) via pulse amplitude modulation were performed as a measure for dehydration induced shut down of physiological activity, which ceased after 141±15min at ∼84% RH. We hypothesized that there is a link between this physiological parameter and the status of the F-actin system. Indeed, 20min of dehydration (ΔF/Fm'=0) leads to a breakdown of the fine cortical F-actin network as visualized by Atto 488 phalloidin staining, and dot-like structures remained. Already 10min after rehydration a beginning reassembly of F-actin is observed, after 25min the F-actin network appeared similar to untreated controls, indicating a full recovery. These results demonstrate the fast kinetics of F-actin dis- and reassembly likely contributing to cellular reorganization upon rehydration.

Cloning, molecular characterization, and phylogeny of two evolutionary distinct glutamine synthetase isoforms in the green microalga Haematococcus pluvialis (Chlorophyceae)

Haematococcus pluvialis (Chlorophyta) is a widely used microalga of great economic potential, yet its molecular genetics and evolution are largely unknown. We present new detailed molecular and phylogenetic analysis of two glutamine synthetase (GS) enzymes and genes (gln) under the Astaxanthin-inducing conditions of light- and nitrogen-stress. Structure analysis identified key residues and confirmed two decameric GS₂ holoenzymes, a cytoplasmic enzyme, termed GS_2c , and a plastidic form, termed GS_2p , due to chloroplast-transit peptides at its N-terminus. Gene expression analysis showed dissociation of mRNA, protein, and enzyme activity levels for both GS₂ under different growth conditions, indicating the strong post-transcriptional regulation. Data-mining identified novel and specified published gln genes from Prasinophyceae, Chlorophyta, Trebouxiophyceae, Charophyceae, Bryophyta, Lycopodiophyta, Spermatophyta, and Rhodophyta. Phylogenetic analysis found homologues to the cytosolic GS_2c of H. pluvialis in all other photo- and non-photosynthetic Eukaryota. The chloroplastic GS_2p was restricted to Chlorophyta, Bryophyta, some Proteobacteria and Fungii; no homologues were identified in Spermatophyta or other Eukaryota. This indicates two independent prokaryotic donors for these two gln genes in H. pluvialis. Combined phylogenetic analysis of GS, chl-b synthase, elongation factor, and light harvesting complex homologues project a newly refined model of Viridiplantae evolution. Herein, a GS₁ evolved into the cytosolic GS_2c and was passed on to all Eukaryota. Later, the chloroplastic GS_2p entered the Archaeplastida lineage via a horizontal gene transfer at the divergence of Chlorophyta and Rhodophyta lineages. GS_2p persisted in Chlorophyta and Bryophyta, but was lost during Spermatophyta evolution. These data suggest the revision of GS classification and nomenclature, and extend our understanding of the photosynthetic Eukaryota evolution.

Why Plants Were Terrestrial from the Beginning

The current hypothesis is that land plants originated from a charophycean green alga and that a prominent feature for adaptation to land was their development of alternating life cycles. Our work on cell wall evolution and morphological and physiological observations in the charophycean green algae challenged us to reassess how land plants became terrestrial. Our hypothesis is simple in that the charophycean green algae ancestors were already living on land and had been doing so for some time before the emergence of land plants. The evolution of alternate life cycles merely made the ancestral land plants evolutionary successful and had nothing to do with terrestrialization per se.

Oospore dimensions and morphology in North American Tolypella (Charophyceae, Charophyta)

Characteristics of the oospores have been used to delimit sections and, in some cases, species in the genus Tolypella A. Braun. To test the utility of oospore characters for identifying North American species of Tolypella, we investigated oospores from field-collected and herbarium specimens. Oospore dimensions (length, width, and length to width ratio) and morphology (color, ridge number and shape, wall ornamentation, and basal impression number) were measured. Oospore dimensions were statistically analyzed and oospore morphology was studied with light and scanning electron microscopy. Statistical analyses showed significant differences in length, width, and length to width ratios among most Tolypella species and populations but there was considerable overlap, which suggested that species identification based on oospore measurements alone is not wholly reliable. In addition, oospore morphology was not unique for every species.

Phylogeny of North American Tolypella (Charophyceae, Charophyta) based on plastid DNA sequences with a description of Tolypella ramosissima sp. nov

Characeae (Charophyceae, Charophyta) contains two tribes with six genera: tribe Chareae with four genera and tribe Nitelleae, which includes Tolypella and Nitella. This paper uses molecular and morphological data to elucidate the phylogeny of Tolypella species in North America. In the most comprehensive taxonomic treatment of Characeae, 16 Tolypella species worldwide were subsumed into two species, T. intricata and T. nidifica, in two sections, Rothia and Tolypella respectively. It was further suggested that Tolypella might be a derived group within Nitella. In this investigation into species diversity and relationships in North American Tolypella, sequence data from the plastid genes atpB, psbC, and rbcL were assembled for a broad range of charophycean and land plant taxa. Molecular data were used in conjunction with morphology to test monophyly of the genus and species within it. Phylogenetic analyses of the sequence data showed that Characeae is monophyletic but that Nitelleae is paraphyletic with Tolypella sister to a monophyletic Nitella + Chareae. The results also supported the monophyly of Tolypella and the sections Rothia and Tolypella. Morphologically defined species were supported as clades with little or no DNA sequence differences. In addition, molecular data revealed several lineages and a new species (T. ramosissima sp. nov.), which suggests greater species diversity in Tolypella than previously recognized.

Oospore Wall Ornamentation in the Genus Tolypella (Charales, Charophyceae)

The ultrastructural features of oospore wall ornamentation in the genus Tolypella were examined using scanning electron microscopy (SEM). The taxonomic relationships among several species were discussed on the basis of oospore ultrastructure and measurements. In the case of T. glomerata and T. nidifica, our results support the status of separate species. Close relationships and transitional forms may exist between T. nidifica and T. normaniana, and not only in oospore wall ornamentation. Oospores of T. hispanica exhibited the same distinct type of reticulate oospore wall as previously reported, but our results do not support the recognition of T. hispanica as a separate species. Ultrastructure of the oospore walls of T. prolifera and T. intricata was almost identical, suggesting that these species are closely related. We therefore reject previous suggestions that morphological characteristics of oospores as observed in SEM are sufficient for identification of individual species. Although significant differences were found among oospores in individual species of Tolypella, large variation among populations, and among individuals belonging to the same population, caused substantial overlap among species.

A CHARACEAN THALLUS WITH ATTACHED GYROGONITES AND ASSOCIATED FOSSIL CHAROPHYTES FROM THE MAASTRICHTIAN OF THE EASTERN PYRENEES (CATALONIA, SPAIN)(1)

The new species Clavatoraxis microcharophorus is described from the Lower Maastrichtian of the Eastern Pyrenees (Catalonia, Spain). Microchara sp. gyrogonites were found in anatomical connection with this thallus, attached to bract-cell rosettes and coated by a structural tunica, formed by an expanded bract cell. This is a feature unknown in extant characeans, which only display lime incrustations similar to tunicae in extremely alkaline and well-illuminated environments. This is the first time that a complete fossil characean is described. The attribution of characean vegetative remains to the genus Clavatoraxis shows that this genus is not exclusive of clavatoraceans as previously thought. The taphonomic study of C. microcharophorus sp. nov. and associated fossil charophytes, along with sedimentological and microfacies analyses, has enabled us to characterize the habitat of this species in the Maastrichtian lake of Vallcebre. They grew forming meadows, and their remains were deposited in the poorly oxygenated lake bottom, where they were well preserved. A number of other characeans and porocharaceans were living in shallower belts. This was the case for Peckichara sp. and Munieria grambasti in the freshwater lacustrine meadows. Another species, Feistiella malladae, was found parautochthonous in brackish lakes.