Coccoid Green Algae Research Articles

Changes of phytoplankton functional groups in a floodplain lake associated with hydrological perturbations

During the past decade, basic hydrological conditions of a floodplain lake in the middle Danube section have been altered with long-lasting extremely high flooding. The objective of the paper is to show the effectiveness of the functional approach to explain phytoplankton changes associated with hydrological events. Intensity and duration of flooding were qualified as the primary cause for the changes of functional groups. Flooding phase was characterised by diatoms (B, C, D, P, T B ) tolerant to water column mixing. Due to the dilution and washout effect their biomass was low during the long-lasting flooding despite their input from the river. Co-occurrence of coccoid green algae (X1, J, F) was associated with turbid and mixed waters. High-nutrient concentrations and water column stability during the long-term dry conditions led to the dominance and high biomass of cyanobacteria. Low-nitrogen H1 group was particularly sensitive to stress caused by flooding, while filamentous N2-fixing (S N ) and non N2-fixing species (S1) showed tolerance to short-term flooding. The development of euglenoids and dinoflagellates (W1, W2, L O ) was also associated with dry conditions and seasonal changes in autumn. The functional classification allows representing of the hydrological phases which characterise the phytoplankton succession in highly disturbed river-floodplain systems.

Hidden diversity of eukaryotic plankton in the soda lake Nakuru, Kenya, during a phase of low salinity revealed by a SSU rRNA gene clone library

A SSU rRNA gene clone library was constructed to establish the diversity of eukaryotic plankton in the African soda lake Nakuru during a phase of low salinity (9.7 ppt = hyposaline). Normally, the lake is mesosaline (up to 50 ppt) and its phytoplankton is dominated by few species of cyanobacteria, in particular Arthrospira fusiformis, which is the main food resource of Lesser Flamingos. Our study recovered a unique phytoplankton species composition characterized by a high diversity of monadoid and coccoid green algae. Out of 77 clones detected, 52 belonged to Chlorophyta. Many of the chlorophytes were transported from the catchment area into the lake through small seasonal rivers and an outflow of the Nakuru town sewage treatment plant. Other phylogenetic groups detected were Fungi, Cryptophyta, Jakobida, Alveolata, Stramenopiles, and Metazoa. Our findings reveal a hidden diversity, which would not have been detected by traditional observations.

Present state of the systematics of planktonic coccoid green algae of inland waters

This review discusses the main developments in the systematics of coccoid green algae over the last three decades. The relationships of key groups of planktonic coccoid green algae are shown in the phylogenetic trees of Chlorophyceae and Trebouxiophyceae. The trees clearly show that the morphology of these algae do not adequately reflect their phylogenetic position. Different phylogenetic species can be hidden under one and the same morphotype. As most of the genera have a polyphyletic origin, they are in need of a systematic re-evaluation. Species classification using the phylogenetic species concept resulted in the establishment of new genera with smaller numbers of species and the description of new species that are not distinguishable by light microscopy. An overview of genera is given in tables and the revised designations of species as contained in the harmonized taxon list of the European Water Framework Directive lists is provided. In this transitional phase from an artificial to a more natural systematics of algae, field biologists and ecologists as well as molecular biologists must strengthen their interdisciplinary cooperation. The alignment of eco-functional groups of algae with true species identities using the barcoding conception will provide a better understanding of the interaction between organisms and their environment.

Intersection of Chlorella, Muriella and Bracteacoccus: Resurrecting the genus Chromochloris Kol et Chodat (Chlorophyceae, Chlorophyta).

Coccoid green algae generally are difficult to classify because of their depauperate morphology, and for this reason many of them have a complicated taxonomic history. In the present study we use analyses of molecular sequence data to demonstrate the close relationship of two green coccoid taxa that have undergone multiple taxonomic transfers in the past, and argue for the recognition of a distinct lineage. We resurrect the genus Chromochloris to accommodate C. zofingiensis (= Muriella zofingiensis) and we provide data regarding the lineage's phylogenetic relationships to other chlorophyte genera, especially Bracteacoccus, Chlorella, Muriella, Mychonastes, and Pseudomuriella. In addition, we synonymize B. cinnabarinus and Bracteacoccus minutus with C. zofingiensis, and clarify the identity of three UTEX strains heretofore listed as Bracteacoccus.

Morphological and 18S rRNA analysis of coccoid green algae isolated from lakes of Kızılırmak Delta

Molecular techniques are used today in many areas and have increased the confidence level of studies. The aim of the study was to compare the classical morphological characters of coccoid green algae under field and culture conditions by using modern molecular analysis. To achieve this aim, we first isolated the coccoid green algae from Liman and Cernek lakes. The morphological variations of green algae were observed under culture conditions and the phylogenetic relationships of these strains were defined according to the 18S rRNA gene sequences. According to sequence analysis, 2 of the isolated species had high similarity to Scenedesmus subspicatus (98%) and Desmodesmus sp. (100%).

Regulation of algal structure and function by nutrients and grazing in a boreal wetland

We evaluated the potential for grazers to regulate benthic algal biomass and taxonomic composition in an Alaskan marsh after enrichment with nutrients that are expected to increase in the region with ongoing climate change. We nested caged and uncaged substrates together inside mesocosm enclosures with natural abundances of snails or no snails and with or without nutrient enrichment (NO3 + PO4 + Si). Algal biomass was greater in all nutrient-enriched enclosures than in controls. Algal biomass was greater in enclosures where grazers were present but excluded by a cage than in enclosures where grazers were allowed to graze or where grazers were absent. In the presence of nutrients, grazed communities were dominated by small coccoid green algae and cyanobacteria, which were overgrown by filamentous green algae when grazers were excluded. In the absence of nutrients, grazers had little effect on algal biomass or taxonomic composition. However, grazers recycled a small but potentially important amount of nutrients in their waste, suggesting that consumer-driven nutrient recycling may have played a role in maintaining algal biomass when grazers were present. Our data show that grazers regulate algal responses to nutrients by suppressing algal accumulation but increasing productivity through nutrient recycling in a northern boreal wetland.

SSU rRNA GENE PHYLOGENY OF MORPHOSPECIES AFFILIATED TO THE BIOASSAY ALGA "SELENASTRUM CAPRICORNUTUM" RECOVERED THE POLYPHYLETIC ORIGIN OF CRESCENT-SHAPED CHLOROPHYTA(1).

The generic concept of coccoid green algae exhibiting a crescent-shaped morphotype is evaluated using SSU rRNA gene sequence analyses and light and electron microscopical observations. These common chlorophytes evolved polyphyletically in 10 different clades of the Chlorophyceae and three clades of the Trebouxiophyceae. Six clades are assigned to known genera of Selenastraceae: Kirchneriella, Nephrochlamys, Raphidocelis, Rhombocystis, Selenastrum, and Tetranephris. Four other clades, named following their present genus designation as Ankistrodesmus-like I and II and Monoraphidium-like I and II, require further investigation. One crescent-shaped morphotype, which evolved within the Trebouxiophyceae, is designated as Neocystis mucosa sp. nov. The other two lineages containing trebouxiophycean algae with this morphotype are the Elliptochloris and the Watanabea clades. The taxonomic placement of the widely used bioassay strain "Selenastrum capricornutum" NIVA-CHL 1 in the genus Raphidocelis (species name Raphidocelis subcapitata) is indicated by molecular data.

A Parachlorella kessleri (Trebouxiophyceae, Chlorophyta) strain from an extremely acidic geothermal pond in Argentina

Juárez Á.B., Vélez C.G., Iñiguez A.R., Martínez D.E., Rodríguez M.C., Vigna M.S. and Ríos de Molina M.C. 2011. A Parachlorella kessleri (Trebouxiophyceae, Chlorophyta) strain from an extremely acidic geothermal pond in Argentina. Phycologia 50: 413–421. DOI: 10.2216/10-79.1Acidic geothermal environments are rare and very few organisms can thrive in them. Chlorella or Chlorella-like species have been described in a variety of environments but their presence in extremely acidic waters have rarely been reported. A Chlorella-like alga (strain BAFC CA10) was isolated from Laguna Verde, a volcanic mesothermal acidic pond from northern Patagonia (Argentina). Applying a multimethod approach including microscopic, ultrastructural, chemical and phylogenetic analyses, the acidotolerant strain BAFC CA10 was identified as Parachlorella kessleri ( = Chlorella kessleri) on the basis of comparisons with the authentic strain SAG 211-11g. Our results confirmed the initial characterization of the Laguna Verde strain as C. kessleri, but since this species was transferred to the new genus Parachlorella, strain BAFC CA10 should be designated as P. kessleri. Histochemical staining and sugar composition of the cell-wall amorphous and fibrillar matrix, together with transmission electron microscopy of the cells, showed the distinctive characters of the species: (1) glucosamine was the exclusive component of the fibrillar cell wall; (2) rhamnose, galactose, glucose and xylose together with minor quantities of arabinose, mannose and fucose were present in the cell-wall hemicellulose matrix; (3) there was a single parietal chloroplast surrounding the entire cell with a small aperture (“mantel-shaped”); (4) there was one pyrenoid in the thickening of the chloroplast surrounded by two starch granules and bisected by two thylakoids; and (5) the cell wall was an electron-transparent homogeneous structure 60–80 nm thick. Phylogenetic analyses of concatenated small-subunit and internal transcribed sequence ribosomal DNA sequences of coccoid green algae within the Chlorella and the Parachlorella clades of the Chlorellaceae confirmed that strain BAFC CA10 is unequivocally related to the P. kessleri strains. Our results suggest that P. kessleri should be added to the list of species capable of adapting to live in extremely acidic environments.

Xylochloris irregularis gen. et sp. nov. (Trebouxiophyceae, Chlorophyta), a novel subaerial coccoid green alga

Neustupa J., Eliáš M., Škaloud P., Němcová Y. and Šejnohová L. 2011. Xylochloris irregularis gen. et sp. nov. (Trebouxiophyceae, Chlorophyta), a novel subaerial coccoid green alga. Phycologia 50: 57–66. DOI: 10.2216/08-64.1The phylogenetic diversity of subaerial coccoid green algae remains still poorly explored. We characterised in detail two unicellular green algae found on tropical trees in Singapore. Light microscopy revealed morphological identity of these two strains. Depending on the age of cultures, the cells were spherical to cylindrical, and ranged in size from 13.5 to 20.5 µm. Each cell contained a pyrenoid-bearing parietal chloroplast that was typically somewhat detached from the plasma membrane on its parietal side. The cells reproduced by 4–16 globular autospores. The 18S rRNA gene sequences of the two strains differed by only a single nucleotide, indicating probable conspecificity. Because the strains were morphologically most comparable to species of the genus Parietochloris, we determined the 18S rRNA gene sequences from authentic strains of three Parietochloris species (P. alveolaris, P. cohaerens and P. ovoidea) for comparison. Molecular phylogenetic analyses placed all five examined strains into the class Trebouxiophyceae. The two novel tropical strains were found to be an independent lineage without an obvious sister group. The type species of the genus Parietochloris, P. alveolaris formed a monophyletic lineage with Parietochloris pseudalveolaris. Finally, P. cohaerens and P. ovoidea fell into another independent clade that also contained Lobosphaera tirolensis, L. incisa and Myrmecia bisecta, indicating that the genus Parietochloris as previously defined is polyphyletic. Based on our morphological and molecular phylogenetic data, we describe the two novel tropical strains as representatives of a new trebouxiophycean genus and species, Xylochloris irregularis gen. et sp. nov.

Optimization of fluorescence in situ hybridization (FISH) for the identification of two polar coccoid green algae species

Standard FISH protocols using fluorochrome labeled oligonucleotide probes have been successfully applied for in situ detection. However, optimized protocols of FISH for specific eukaryotes in marine environments are often not developed. This study optimized the conditions of fluorescence in situ hybridization (FISH) by using two polar isolated microalgae. The modified conditions were as follows: (1) 10 mg?mL -1 lysozyme solution pretreatment at 37°C for 30 min; (2) the hybridization buffer including 20% formamide; (3) the hybridization condition was 47°C for 6 h. The cells enumerated by FISH were compared with those enumerated by flow cytometry (FCM) and DAPI to confirm the cell loss and hybridization efficiency. The optimized protocol was also successfully applied to Arctic Ocean samples, which were found to be dominated by Micromonas sp. The modified protocol showed a high relative efficiency and could be successfully applied for the detection of specific microbial eukaryotes in environmental samples.

First description of a laccase-like enzyme in soil algae

Laccases (EC 1.10.3.2) are versatile multi-copper oxidases so far found in higher plants, fungi, insects, prokaryotes and lichens. In the present study, the production of an extracellular laccase-like enzyme by the coccoid green soil alga Tetracystis aeria was investigated and the enzyme was partly characterized, thereby providing the first description of a laccase-like enzyme in soil algae. Enzyme production in algae cultures was considerably increased by addition of the fungal laccase inducer copper sulphate. Maximal enzyme production was observed during the stationary growth phase. Peroxidase or tyrosinase activity was not detected. The native enzyme exhibits an apparent molecular mass of about 212 kDa as observed with size exclusion chromatography and about 210-260 kDa as estimated by zymograms. The enzyme efficiently oxidizes 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), 2,6-dimethoxyphenol (2,6-DMP), syringaldazine (SGZ) and the anthraquinone dye Acid Blue 62, while guaiacol and Remazol Brilliant Blue R are only poorly oxidized. The apparent kinetic parameters obtained for ABTS, 2,6-DMP and SGZ oxidation are within the range reported for fungal laccases. Oxidation of the phenolic substrate 2,6-DMP displays a remarkably high pH optimum (pH 8.0-8.5), which is interesting with respect to potential biotechnological applications.

Phylogenetic position of Ooplanctella planoconvexa gen. et comb. nova and Echinocoleum elegans (Oocystaceae, Trebouxiophyceae, Chlorophyta).

The coccoid green algae Coenochloris planoconvexa and Echinocoleum elegans are assigned to different families (Radiococcaceae and Oocystaceae, respectively), but display similar ray-like mucilaginous envelopes. Molecular analysis of the 18S rRNA gene of C. planoconvexa CAUP H 5502 and E. elegans SAG 37.93 revealed that they both belong to Oocystaceae. However, the two strains were not a part of a monophyletic cluster. Furthermore, their morphology differed, namely in the shape of their mucilaginous envelopes, and in their ability to form four-celled units in a broadened sporangial wall. Because the name Coenochloris cannot be used for members of Oocystaceae, a new genus Ooplanctella, with the type species O. planoconvexa comb. nova, is proposed.

Hylodesmus singaporensis gen. et sp. nov., a new autosporic subaerial green alga (Scenedesmaceae, Chlorophyta) from Singapore

The algal flora of subaerial habitats in the tropics remains largely unexplored, despite the fact that it potentially encompasses a wealth of new evolutionary diversity. Here we present a detailed morphological and molecular characterization of an autosporic coccoid green alga isolated from decaying wood in a natural forest in Singapore. Depending on culture conditions, this alga formed globular to irregularly oval solitary cells. Autosporulation was the only mode of reproduction observed. The cell periphery was filled with numerous vacuoles, and a single parietal chloroplast contained a conspicuous pyrenoid surrounded by a bipartite starch envelope. The cell wall was composed of a thick inner layer and a thin trilaminar outer layer, and the cell surface was ornamented with a few delicate ribs. Phylogenetic analyses of 18S rRNA gene sequences placed our strain in the family Scenedesmaceae (Sphaeropleales, Chlorophyceae) as a strongly supported sister branch of the genus Desmodesmus. Analyses of an alternative phylogenetic marker widely used for the Scenedesmaceae, the ITS2 region, confirmed that the strain is distinct from any scenedesmacean alga sequenced to date, but is related to the genus Desmodesmus, despite lacking the defining phenotypic features of Desmodesmus (cell wall with four sporopolleninic layers ornamented with peculiar submicroscopic structures). Collectively, our results establish that we identified a novel, previously undocumented, evolutionary lineage of scenedesmacean algae necessitating its description as a new species in a new genus. We propose it be named Hylodesmus singaporensis gen. et sp. nov. A cryopreserved holotype specimen has been deposited into the Culture Collection of Algae of Charles University in Prague, Czech Republic (CAUP) as CAUP C-H8001.

Toxic cyanobacteria at Nakuru sewage oxidation ponds – A potential threat to wildlife

Phytoplankton composition and biomass, and microcystin content were determined on diverse dates between November 2001 and June 2006 in the final oxidation pond of the Nakuru town sewage treatment plant. The oxidation ponds as well as the rivulet that drains the same are important water sources for some wildlife in the park. The phytoplankton composition of the pond studied mostly comprised coccoid green alga species. However, occasionally cyanobacteria or euglenoids were dominant. Among the cyanobacteria, Microcystis sp. made periodic appearance in the phytoplankton, and was the dominant species on some occasions. Total phytoplankton biomass varied widely from 48 to 135 mg L −1 (wet weight) while cyanobacteria biomass ranged from undetectable levels to 130 mg L −1. Most phytoplankton biomass was due to one or a few species. Detectable cyanotoxin concentrations (sum of microcystins) of up to 551.08 μg mg −1 dry weight (DW) of cyanobacteria biomass or 0.28 μg mg −1 DW of total phytoplankton biomass were recorded in samples collected on different dates. Microcystin content did not appear to correspond to the biomass of cyanobacteria suggesting that toxin production is possibly triggered by environmental changes or changes in the proportion of toxic strains. An occasional presence of microcystins in the pond water suggests that the wildlife species, which regularly use the ponds as drinking water sources are potentially exposed to intoxication. A close monitoring of pond water phytoplankton composition is necessary to accurately quantify the potential impact of cyanotoxins on these wildlife species.

Diversity and geographic distribution of desmids and other coccoid green algae

Taxonomic diversity of desmids and other coccoid green algae is discussed in relation to different species concepts. For want of unambiguous criteria about species delimitation, no reliable estimations of global species richness can be given. Application of the biological species concept is seriously hampered by lack of sexual reproduction in many species. Molecular analyses demonstrated cases of close affiliation between morphologically highly different taxa and, contrary, examples of little relationship between morphologically similar taxa. Despite the fact that desmids and chlorococcal algae, because of their microbial nature, can be readily distributed, cosmopolitan species are relatively scarce. The geographic distribution of some well-recognizable morphospecies is discussed in detail. Of some species a recent extension of their area could be established, e.g., in the desmids Micrasterias americana and Euastrum germanicum, and in the chlorococcaleans Desmodesmus perforatus and Pediastrum simplex.

HEMIFLAGELLOCHLORIS KAZAKHSTANICA GEN. ET SP. NOV.: A NEW COCCOID GREEN ALGA WITH FLAGELLA OF CONSIDERABLY UNEQUAL LENGTHS FROM A SALINE IRRIGATION LAND IN KAZAKHSTAN (CHLOROPHYCEAE, CHLOROPHYTA)1

A coccoid green alga, Hemiflagellochloris kazakhstanica S. Watanabe, S. Tsujimura, T. Misono, S. Nakamura et H. Inoue, gen. et sp. nov., was described from soil samples of a saline irrigation land in Ili River basin, Kazakhstan. This alga had a parietal chloroplast with a pyrenoid, which was covered with starch segments and penetrated with thylakoid membranes. Reproduction occurred by the formation of aplanospores and zoospores. The aplanospores frequently formed tetrad aggregations in a mother cell. The zoospores were covered by a single‐layered cell wall and lacked stigmata. The zoospores had two flagella of considerably unequal lengths; the longer flagellum was 17–19 lm in length and the shorter one was 9–10 lm. The flagellar apparatus architecture was of the clockwise orientation group type in the Chlorophyceae. Molecular phylogenetic analysis using 18S and 28S rDNA sequence data resolved this organism in a separate clade from the green algae that had flagella of slightly unequal lengths. It was suggested that features such as inequality in flagellar lengths, parallel exsertion of basal bodies, and subapical position of the flagellar apparatus were sporadically evolved.

Diversity and ecology of small coccoid green algae from Lake Itasca, Minnesota, USA, including Meyerella planktonica, gen. et sp. nov.

M.W. Fawley, K.P. Fawley and H.A. Owen. 2005. Diversity and ecology of small coccoid green algae from Lake Itasca, Minnesota, USA, including Meyerella planktonica, gen. et sp. nov. Phycologia 44: 35–48.We characterized 81 isolates of picoplankton and other small planktonic coccoid green algae from Lake Itasca, Itasca State Park, MN, USA. Sequence analysis and examination by light microscopy revealed three common algae: Pseudodictyosphaerium/Korshpalmella spp. (Chlorophyceae), Choricystis spp. (Trebouxiophyceae) and a previously undescribed autosporic coccoid. The Pseudodictyosphaerium and Korshpalmella species that were evaluated all possessed identical 18S ribosomal DNA (rDNA) and rbcL sequences, but this 18S rDNA sequence differed from the published 18S rDNA sequence for P. jurisii isolated from Europe. These chlorophycean algae were isolated from samples collected from Lake Itasca during all seasons. The 18S rDNA sequence of some of the Itasca isolates of Choricystis was nearly identical to the published sequence for C. minor; however, six different rbcL sequences were detected among the Choricystis isolates and none of these rbcL sequences matched published sequences for isolates from several lakes in Europe. The different rbcL sequence types of Choricystis had distinct seasonal distributions in Lake Itasca. These results extend our knowledge of the distribution of Pseudodictyosphaerium and Choricystis and suggest that Choricystis is even more diverse than previously thought. In addition, the distinct sequences of marker genes for Lake Itasca isolates indicate that these organisms have diverged from the European isolates. The previously undescribed alga isolated from Lake Itasca is a sister taxon to the trebouxiophyte lineage that includes Chlorella vulgaris, but differs from true Chlorella species in that it lacks a pyrenoid. Based upon this difference and molecular analyses, these isolates are described as Meyerella planktonica, gen. et sp. nov. (Trebouxiophyceae, Chlorophyta). All M. planktonica isolates examined possessed identical 18S rDNA, rbcL and ribosomal internal transcribed spacer sequences. Meyerella planktonica was isolated almost exclusively from winter and spring samples.

Nomenclatural and taxonomic changes in the classification of "green" algae

The current classification of "green" algae of the Ukraine is presented. The system confirms the morphological-cytological and molecular-biological results of previous studies. A new family Bracteacoccaceae (Sphaeropleales, Chlorophyceae) is described. An independent family Scotiellocystoidaceae (Chlorococcales, Chlorophyceae) is further described and certain nomenclatural changes at the species and infraspecific levels among coccoid green algae are given.

Phylogenetic relationship of Chlorella and Parachlorella gen. nov. (Chlorophyta, Trebouxiophyceae)

Chlorella is one of the archetypical forms of coccoid green algae and one of the best-studied phototrophic eukaryotes. However, its systematics remains enigmatic due to conflicts between morphological and molecular phylogenetic approaches. The sequences of the 18S ribosomal RNA gene of nine strains of Chlorella and related taxa, and the ITS2 region of 17 strains of Chlorellaceae were determined and included in phylogenetic analyses. The secondary structure of the ITS2 region of C. vulgaris was compared to that of Parachlorella beijerinckii. All phylogenetic analyses showed that the Chlorellaceae form a clade within the Trebouxiophyceae. The Chlorellaceae studied here are divided into two sister groups: (1) the Parachlorella-clade with Parachlorella beijerinckii gen. et sp. nov. and P. kessleri comb. nov. as well as Diclosler acuatus and Closteriopsis acicularis; and (2) the Chlorella-clade including the ‘true’ spherical Chlorella species C. vulgaris, C. lobophora and C.sorokiniana. The latter are intermixed with taxa that differ in morphology and were formerly classified in other families of coccoid green algae: Actinastrum hantzschii (formerly Coelastraceae), Diacanthos belenophorus (formerly Oocystaceae), Dictyosphaerium pulchellum (formerly Botryococcaceae), Didymogenes anomala and D. palatina (formerly Scenedesmaceae) and Micractinium pusilium (formerly Micractiniaceae). It was shown that morphological criteria traditionally used for classification (e.g. spines, mucilaginous envelopes or the formation of colonies or coenobia) are burdened with a high degree of phenotypic plasticity. These plastic traits represent adaptations to environmental factors in order to optimize floating in the water column and to resist grazing pressure. A genus and species concept in Chlorellaceae remains provisional, because, in most cases, there is still a lack of classical and molecular approaches to identify clear phylogenetic lineages.

Systematics of Coccoid Green Algae: Morphology vs. 18S rRNA Gene Phylogeny

AbstractClassical approaches using morphological characters for the circumscription of taxa in coccoid green algae do not adequately reflect the phylogenetic relationships. Phylogenetic trees based on 18S rRNA gene sequence analysis provide new insights into the systematics of these algae. We give a review and discuss the content of orders of coccoid green algae and evaluate the usefulness of some traditionally used diacritic features. Whereas the content of the orders Chlorococcales and Chlorellales needs to be reduced, the order Sphaeropleales comprises more families than in the traditional circumscription. According to molecular biological investigations the Neochloridaceae, Selenastraceae and Scenedesmaceae formerly classified in the Chlorococcales and later into the Chlorellales belong to the Sphaeropleales. The Hydrodictyaceae, formerly a member of the Protosiphonales, are also to be grouped in the Sphaeropleales. Some members of the Ulotrichales, which propagate by binary fission (e.g. Nannochloris and Catena) cluster together with autosporine coccoid taxa within the Trebouxiophyceae. With the example of members of the Selenastraceae, Oocystaceae and Chlorellacea it is shown that the morphology of cells as well as the formation of coenobia, spines, mucilage and incrustations cannot be used to separate genera.