SOME EFFECTS OF DERIVATIVES OF VITAMIN K ON THE METABOLISM OF UNICELLULAR ALGAE

Vitamin K₁, 2-methyl-3-phytyl-1,4-naphthoquinone, is a substance found in all plant chloroplasts. It is, therefore, interesting to know whether it has any influence upon the metabolism of plants. Experiments made with the phytol-free derivatives like 2-methyl-1,4-naphthoquinone or the corresponding 3-oxy compound, phthiocol, gave the following results. These substances accelerate the respiration of Chlorella or Scenedesmus in a way similar to the action of the dinitrophenols. They inhibit photosynthesis and the compensation of respiration in the light strongly like hydroxylamine. In Scenedesmus they hinder the adaptation to the anaerobic utilization of hydrogen. If given after adaptation in amounts sufficient to stop photosynthesis they do not prevent photoreduction but rather stabilize this reaction against reversion. Their presence destroys the coupling between the reduction of carbon dioxide in the dark and the oxyhydrogen reaction in adapted algae. One can expect, therefore, that the natural vitamin K present in plants in concentrations of about 10^–3 M takes part in some metabolic reaction as a catalyst or regulator.     

Polarographische Messung der Nitritreduktion von Chlorella im monochromatischen Licht

Summary In green plant cells nitrite is reduced by two systems, one dependent on photosynthesis and the other upon respiration. Using a polarographic method for continuous measurement of nitrite uptake, the relationship between light driven and respiration linked nitrite reduction of Chlorella cells was studied.Photosynthetic nitrite reduction is characterized by a pronounced increase in the velocity of nitrite uptake upon illumination. After the light is turned off the velocity immediately returns to the preillumination value. Photosynthetic nitrite reduction of Chlorella is separated from respiration linked nitrite reduction by illumination with red light under anaerobic conditions; it is stimulated by CO₂ and is inhibited by DCMU, findings which confirm earlier observations.In white light a special blue light stimulation of nitrite uptake is overlapped by photosynthetic nitrite reduction. In contrast to photosynthetic nitrite reduction this type of light stimulation is characterized by a lag period of about I min from the onset of illumination; it continues about 10 min when the light is turned off. It is separated from photosynthetic nitrite reduction by irradiation of the algae with low intensities of short wavelength light (<500 nm). Blue light stimulation of nitrite uptake of Chlorella is strongly dependent on the developmental stage of the cells. It is observed with young cells (autospores) of synchronized algae only.There is no evidence for any connection between blue light stimulation of nitrite uptake and photosynthesis. From the sensitivity of this process towards anaerobic conditions and antimycin A it is concluded to be a stimulation of respiration linked nitrite reduction.Under conditions of low exogenous nitrite concentration a temporary inhibition of steady state dark nitrite reduction appears immediately after the light is turned off. From several observations it is concluded that the inhibition already exists during the preceding illumination and decreases the rate of total nitrite uptake in the light. This process is suppressed by inhibition of respiration as well as by the inhibitor of photosynthesis, DCMU.If nitrate is the source of nitrogen an excretion of nitrite is found following illumination. The kinetics of this process agree with those observed for the light induced inhibition of steady state dark nitrite reduction immediately after illumination.     

Hydrogen evolution by several algae

Summary Out of 33 strains of unicellular algae examined, H₂ evolution was observed only in species of Chlamydomonas, Chlorella and Scenedesmus. While the photoevolution of H₂ by these algae was generally stimulated both by an organic substrate and by the uncoupler CCCP¹, response to DCMU varied. On the basis of the response to DCMU, it was concluded that the mechanism of photoevolution of H₂ differed from one alga to another. The reaction in some algae appeared to be dependent on either the photooxidation of water or oxidative carbon metabolism for reductant; that in other algae was supported by reductant from both these sources.     

Biomass and oil content of Chlorella sp., Haematococcus sp., Nannochloris sp. and Scenedesmus sp. under mixotrophic growth conditions in the presence of technical glycerol

The growth of algae strains Chlorella sp., Haematococcus sp., Nannochloris sp. and Scenedesmus sp. under mixotrophic conditions in the presence of different concentrations of technical glycerol was investigated with the aim of increasing biomass growth and algae oil content. The highest concentration of lipid obtained in media with 5 g L^?1 glycerol for Chlorella sp., Scenedesmus sp., Nannochloris sp. and Haematococcus sp. was 17.77, 22.34, 27.55 and 34.22 % larger than during the autotrophic growth of these species. Increases in triacylglycerols of up to ten times was observed for Scenedesmus sp. under mixotrophic conditions (using 10 g L^?1 glycerol), whereas an increase of 2.28 times was found for Haematococcus sp. The content of saturated fatty acids of Scenedesmus, Chlorella, Haematococcus and Nannochloris was 67.11, 34.63, 23.39 and 24.23 %, and the amount of unsaturated fatty acids was 32.9, 65.06, 79.61 and 75.78 % of total fatty acids, respectively. Growth on technical glycerol of these strains with light produced higher biomass concentrations and lipid content compared with autotrophic growth. The fatty acid content of oils from these species suggests their potential use as biodiesel feedstock.     

Factors that Control the Species Composition of Freshwater Phytoplankton,with Special Attention to Nutrient Concentrations

Variations of species composition and population size of planktonic algae were studied in relation to the different nutrient levels in a eutrophic pond and an oligotrophic lake. The results obtained were discussed in comparison with the changes in photosynthesis and chlorophyll of several algal populations cultured under different nutrient conditions. As based on unialgal culture experiments two types of algae (the Chlorella-type and the Melosira-type) could be distinguished with regard to variations of photosynthesis and chlorophyll in response to different nutrient levels. Distribution of the Chlorella-type algae may be confined to eutrophic waters, while the Melosira-type algae can be distributed widely in waters from oligotrophic to eutrophic.     

Photoadaptation of two members of the Chlorophyta (Scenedesmus and Chlorella) in laboratory and outdoor cultures: changes in chlorophyll fluorescence quenching and the xanthophyll cycle

The role of the xanthophyll cycle in the adaptation of two chlorococcal algae Scenedesmus quadricauda and Chlorella sorokiniana to high irradiance was studied under laboratory and outdoor conditions. We wished to elucidate whether the xanthophyll cycle plays a key role in dissipating the excesses of absorbed light, as in higher plants, and to characterise the relationship between chlorophyll fluorescence parameters and the content of xanthophyll-cycle pigments. The xanthophyll cycle was found to be operative in both species; however, its contribution to overall non-photochemical quenching (NPQ) could only be distinguished in Scenedesmus (15–20% of total NPQ). The Scenedesmus cultures showed a larger pool of xanthophyll-cycle pigments than Chlorella, and lower sensitivity to photoinhibition as judged from the reduction of maximum quantum yield of photosystem II. In general, both algae had a larger xanthophyll-cycle pool when grown outdoors than in laboratory cultures. Comparing the two species, Scenedesmus exhibited a higher capacity to adapt to high irradiance, due to an effective quenching mechanism and high photosynthetic capacity; in contrast, Chlorella represents a species with a larger antennae system, less-efficient quenching and lower photosynthetic performance. Non-photochemical quenching (NPQ) induced through the xanthophyll cycle can, to a limited extent, represent a regulatory factor in diluted algal cultures grown in outdoor solar photobioreactors, as well as in natural algal phytoplankton populations exposed transiently to high irradiance. However, it does not play an appreciable role in dense, well-mixed microalgal suspensions. Received: 6 August 1998 / Accepted: 12 February 1999     

The capability of the algaeChlorella vulgaris andScenedesmus obliquus of utilizing amino acids as the sole nitrogen source

The capability of utilizing 20 amino acids and 2 amides as the sole nitrogen source for growth was studied in two green algae (Chlorophyceae). A comparison was made of the growth rate of algae in a mineral nutrient solution containing nitrate as the nitrogen source, with that in the same solution in which nitrogen in the form of nitrate was substituted by an equivalent nitrogen amount in the form of various amino acids. In addition to this, another series of experiments was carried out in whioh both culture media were supplied with glucose. The results show that both algae utilize a series of amino acids in dependence of their structure (mostly 3-carbon amino acids). The growth rate ofChlorella in the presence of these sources is the same as in nitrate, that ofScenedesmus even much higher. In the cultures containing glucose both algal species exhibit a higher growth rate in the media with the nitrate nitrogen source than in those with amino acids (with the exception of glycine inScenedesmus).     

Bioprospecting and indexing the microalgal diversity of different ecological habitats of India

Our study reports the collection, biodiversity analyses, isolation and identification of microalgae from different habitats of India. Cyanophyceae and Chlorophyceae were the most dominant algal groups recorded, with the highest number being recorded for non-heterocystous cyanobacteria (48), followed by 44 unicellular forms. Sagar Island, Sunderbans recorded the greatest number of algae, and unicellular/colonial green algae were present in all the samples. Shannon’s Diversity Index was highest in Koikhali, Sunderbans, followed by Rushikulya River, Odisha. Selective enrichment, purification through serial dilution followed by plating and regular observations led to the isolation of sixteen strains. Identification was done by using microscopic observations, supported with standard monographs and classified as belonging to seven genera (Chlorella, Chlorococcum, Kirchneria, Scenedesmus, Chlamydomonas, Tetracystis and Ulothrix). 18S rDNA sequencing was undertaken for four strains. The set of sixteen strains were screened under standard cultural conditions for their growth kinetics and Chlorella sorokiniana MIC-G5, followed by Chlorella sp. MIC-G4 exhibited the highest growth rates. The strain Chlorococcum sp. MIC-G2 recorded highest chlorophyll, while MIC-G3 ranked highest for carbohydrates. The study aided in identifying the dominant microalgae in the diverse habitats and characterizing their growth rate and carbohydrate content, providing a valuable germplasm for further utilization in agriculture and industry.     

The systematics of Zoochlorella revisited employing an integrative approach

Symbiosis of green algae with protozoa and invertebrates has been studied for more than 100 years. Endosymbiotic green algae are widely distributed in ciliates (e.g. Paramecium, Stentor, Climacostomum, Coleps, Euplotes), heliozoa (e.g. Acanthocystis) and invertebrates (e.g. Hydra, Spongilla), and have traditionally been identified as named or unnamed species of Chlorella Beij. or Zoochlorella K. Brandt or referred to as Chlorella‐like algae or zoochlorellae. We studied 17 strains of endosymbionts isolated from various hosts and geographical localities using an integrative approach (nuclear encoded small subunit and internal transcribed spacer regions of rRNA gene sequences including their secondary structures, morphology, physiology and virus sensitivity). Phylogenetic analyses have revealed them to be polyphyletic. The strains examined belong to five independent clades within the Trebouxiophyceae (Choricystis‐, Elliptochloris‐, Auxenochlorella‐ and Chlorella‐clades) and Chlorophyceae (Scenedesmus‐clade). The most studied host organism, Paramecium bursaria, harbours endosymbionts representing at least five different species. On the basis of our results, we propose a taxonomic revision of endosymbiotic ‘Chlorella’‐like green algae. Zoochlorella conductrix K. Brandt is transferred to Micractinium Fresen. and Zoochlorella parasitica K. Brandt to Choricystis (Skuja) Fott. It was shown that Choricystis minor (Skuja) Fott, the generitype, is a later heterotypic synonym of Choricystis parasitica (K. Brandt) comb. nov. A new species, Chlorella heliozoae, is proposed to accommodate the endosymbiont of Acanthocystis turfacea.     

Effect of algal and animal diets on life history of the freshwater copepod <Emphasis Type="Italic">Eucyclops serrulatus</Emphasis> (Fischer, 1851)

We studied the survivorship and reproduction of the cyclopoid Eucyclops serrulatus using four diet types: algae Chlorella vulgaris and Scenedesmus acutus separately and each together with the rotifer Brachionus havanaensis. We used equal biomass (3.9 mg C l⁻¹) of C. vulgaris and S. acutus while B. havanaensis was offered at a density of 0.5 ind. ml⁻¹. Regardless of the food type, the average lifespan of female E.␣serrulatus varied from 30 to 50 days, and those of males from 22 to 27 days. When Chlorella was offered alone as food, the female lifespan was 40% lower than when fed together with B. havanaensis. The number of nauplii per female (gross reproductive rate, GRR) was higher on Scenedesmus alone than on Chlorella alone or Chlorella and Scenedesmus offered with Brachionus. Thus, GRR was not significantly influenced by the presence of rotifer in mixed diet. Net reproductive rate (R _o) and the rate of population increase (r) were similar at all the four food types. This implies that the diet-related differences of GRR did not ultimately translate to the differences in R _o or r, due to the survivorship patterns of the adult females.     

Effect of two microalgae concentrations on body size and egg size of the rotifer <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis>

The rotifer, Brachionus calyciflorus, was grown with two algae species (Chlorella sp. and Scenedesmus obliquus) at different concentrations (0.1, 1 and 10 × 10⁶ cells ml⁻¹). The body size (lorica biovolume) of individual rotifer and their egg size were measured when the populations were roughly in the exponential phase of population growth. The body size of the rotifers differed significantly (P < 0.05) among the two algae species used, however this effect was not observed for egg size. The body size of rotifers fed on higher densities of Chlorella sp. (10 × 10⁶ cells ml⁻¹) was significantly larger than for those fed on lower and medium densities (0.1 and 1 × 10⁶ cells ml⁻¹). Body size and egg size of rotifers fed with different amounts of Scenedesmus did not differ significantly. The egg size was significantly larger at higher food level of Chlorella. A significantly positive correlation was observed between the adult rotifer body size and their egg size.     

Growth of Corbicula fluminea (bivalvia) fed artificial and algal diets

Laboratory experiments were conducted to assess the growth of Corbicula fluminea Müller (1774) fed mixed diets of the green alga Ankistrodesmus and five artificial foods (nine-grain cereal, rice flour, rye bran, denatured brewer's yeast and Ralston Purina trout chow). Clams lost weight after 30 days on each artificial diet; however, weight losses were significantly less than those of control clams which were not fed (p < 0.05). In contrast, clam growth was positive in each treatment which included Ankistrodesmus. There was no significant difference in the tissue growth of clams fed on an Ankistrodesmus monoculture and Ankistrodesmus supplemented with each artificial diet (p > 0.2). Therefore, we conclude that all of the artificial diets evaluated were of nutritional value. However, the alga Ankistrodesmus appears to be a superior food item. In subsequent experiments, clams were fed on diets consisting of various combinations of six genera of green algae (Pedinomonas, Ankistrodesmus, Chlamydomonas, Chlorella, Scenedesmus, and Selenastrum). All trialgal combinations of Chlamydomonas, Chlorella, Scenedesmus, and Ankistrodesmus and a dialgal culture of Chlamydomonas and Chlorella resulted in strong positive tissue growth. No other dialgal or monoalgal culture resulted in similar growth. Cultures of Selenastrum were found to be toxic, and their filtrate was determined to inhibit Corbicula filtration.     

Glycolate biosynthesis by Scenedesmus and Chlorella in the presence or absence of NaHCO3

Summary Both Scenedesmus and Chlorella excreted comparable quantities of glycolate. Glycolate formation was dependent upon light and oxygen, but occured in the absence of added CO₂ or NaHCO₃ for net photosynthesis. In an environment of 3000 ft. c. light and an atmosphere of oxygen, about 35 g glycolate were excreted per hour per milliliter 1% (v/v) algae without NaHCO₃ or CO₂. Upon addition of NaHCO₃ the rate increased to about 55 g. Glycolate formation in the light in the absence of CO₂ may result from photometabolism of algal polysaccharides.Glycolate excretion by Scenedesmus occurred at all pH values between 6.5 and 9.5 and was not related to utilization of bicarbonate. Scenedesmus obliquus excreted glycolate when existing in plates of four or eight cells, but not when present as small individual cells.At pH 9 ¹⁴C fixation by Scenedesmus was faster than fixation by Chlorella. There was no significant difference in products of ¹⁴C fixation formed by Scenedesmus at pH values between 6.5 and 9.5.For unknown reasons -hydroxy-2-pyridinemethanesulfonate stimulated CO₂ fixation by Scenedesmus by at least 100%. This sulfonate had no effect on glycolate excretion nor upon the distribution of ¹⁴C among the products of ¹⁴CO₂ fixation by Scenedesmus.Supported in part by NSF Grant GB-4154 and published with the approval of the Director of the Michigan Agricultural Experiment Station as journal article No. 3946. The research was initiated during the period when N. E. Tolbert was supported in part by a National Institutes of Health Senior Fellowship at the Biochemisches Institut, Universität, Freiburg/Br., Germany.     

Rates of photosynthesis of two lichens and of their isolated algal components

Abstract

The rate of photosynthesis of two lichen species (Peltigera leucophlebia and Ramalina farinacea) was found to be 30 to 40% that of spinach leaf dises and 20% that of the free-living alga Chlorella when the results were expressed on a per mg chlorophyll basis. When the algae were isolated from the thalli, the rate of photosynthesis per mg chlorophyll increased for Ramalina farinacea and decreased for Peltigera leucophlebia. Product analysis indicated that the products of photosynthesis depended on the association of the alga with the fungus: algae isolated from the thalli showed a «shift» in products from sugars and sugar alcohols. to compounds such as organic acids. The results suggest that a symbiotic relationship with a fungus alters both the rate and products of algal photosynthesis.     

CRYPTIC SPECIES OF SCENEDESMUS (CHLOROPHYTA) FROM DESERT SOIL COMMUNITIES OF WESTERN NORTH AMERICA1

Nine isolates of unicellular green algae were obtained from six geographically separate desert microbiotic crust communities in western North America. Microscopically, eight isolates strongly resembled unicellular forms of Scenedesmus obliquus (Turpin) Kützing. They are oval or crescent shaped, often flattened on one side, with knobby cell apices. SEM indicated a lack of wall ornamentation. Fine filaments connecting cells pole to pole were observed in some isolates, as previously documented in Scenedesmus (Dactylococcus) dissociatus and S. obliquus. The ninth isolate was spherical, without knobby apices or connections between cells, and was similar to unicellular forms that were originally classified as species of Chlorella (Scenedesmus vacuolatus and S. rubescens). None of the isolates formed coenobia in liquid culture. Phylogenetic analysis of the 18S rRNA gene placed all desert isolates in the genus Scenedesmus, separating them into two or three weakly resolved groups along with published sequences of other Scenedesmus isolates. Phylogenetic analyses of the internal transcribed spacer region revealed well‐supported lineages of desert algae that were unsupported with 18S data alone. The eight S. obliquus‐like desert strains formed two distinct clades that excluded the S. obliquus strains from geographically widespread nondesert habitats. The ninth strain was outside of the S. obliquus group, associated with S. raciborskii and S. pectinatus. These results demonstrate three lineages of Scenedesmus from desert soils and provide robust support for the presence of cryptic species in S. obliquus, a morphospecies that is said to have a cosmopolitan distribution. Three new species of Scenedesmus are described.     

Thermal Acclimation of Respiration and Photosynthesis in the Marine Macroalga Gracilaria lemaneiformis (Gracilariales,Rhodophyta)

The responses of respiration and photosynthesis to temperature fluctuations in marine macroalgae have the potential to significantly affect coastal carbon fluxes and sequestration. In this study, the marine red macroalga Gracilaria lemaneiformis was cultured at three different temperatures (12, 19, and 26°C) and at high‐ and low‐nitrogen (N) availability, to investigate the acclimation potential of respiration and photosynthesis to temperature change. Measurements of respiratory and photosynthetic rates were made at five temperatures (7°C–33°C). An instantaneous change in temperature resulted in a change in the rates of respiration and photosynthesis, and the temperature sensitivities (i.e., the Q₁₀ value) for both the metabolic processes were lower in 26°C‐grown algae than 12°C‐ or 19°C‐grown algae. Both respiration and photosynthesis acclimated to long‐term changes in temperature, irrespective of the N availability under which the algae were grown; respiration displayed strong acclimation, whereas photosynthesis only exhibited a partial acclimation response to changing growth temperatures. The ratio of respiration to gross photosynthesis was higher in 12°C‐grown algae, but displayed little difference between the algae grown at 19°C and 26°C. We propose that it is unlikely that respiration in G. lemaneiformis would increase significantly with global warming, although photosynthesis would increase at moderately elevated temperatures.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

Effects of oxytetracycline on populations and community metabolism of an aquatic microcosm

To investigate the ecological impacts of a widely-used antibiotic agent oxytetracycline (OTC), we prepared an aquatic microcosm consisting of two green algae (Chlorella sp. and Scenedesmus sp.), a blue-green alga (Tolypothrix sp.), a protozoan (Cyclidium sp.), two rotifers (Philodina sp. and Lecane sp.), an aquatic oligochaete (Aeolosoma hemprichi), and bacteria. We investigated the effects of OTC on population abundance of composite species, gross primary production (GPP), and community respiration (CR). We linked population abundance, GPP and CR to understand the relationship between these measures. First, ‘experimental respiration rate of heterotrophs’ (HR_exp) was obtained by CR minus algal respiration that is calculated by 0.35 × GPP. Next, the population respiration of each heterotroph was calculated from the direct measure of population abundance and the assumed constant of per-capita respiration. We defined the sum of the population respirations across all heterotrophs as ‘theoretical respiration rate of heterotrophs’ (HR_theo). Finally, relative community metabolism (RCM) of the heterotrophic community was obtained by calculating the ratio of HR_exp to HR_theo that indicates changes in specific respiration rates as a whole in the community. The influence of OTC on RCM was larger than on CR, thus the effect of OCT on the metabolism of heterotrophs was far more severe than expected from CR. RCM can magnify the rate of change in the original data, and facilitate the detection of influences in ecotoxicological studies.     

Formation of N-Carboxymethyl Amino Acid from the Reaction of α-Amino Acid with Glyoxal

Enrichment cultures in a medium containing 0.1% methanol and 0.1% bicarbonate at pH 7.0 under anaerobic conditions in the light became mainly green in color. Forty-four enrichment cultures, which showed abundant growth, were obtained from 46 different sources and found to contain cells of methanol-utilizing bacteria and green algae as predominant members. From these enrichment cultures, two strains of bacteria and two strains of algae were isolated. The microorganisms isolated were designated as bacterium No. 7, bacterium No. 8, Chlorella sp. A-1 and Chlorella sp. B-1, respectively. Stable mixed cultures were easily formed by mixing the isolated cultures of bacteria and algae. Both methanol and bicarbonate were necessary for the growth of the mixed cultures under anaerobic-light conditions. Growth behavior of the mixed cultures was examined on a medium containing 0.1% methanol and 0.1 % bicarbonate at 30°C in the light (about 6000 lx). The maximum specific growth rate for the cultures, µ_max, was 0.092 hr^?1 (doubling time, 7.5 hr). The maximum cell yield was 0.87 g dry-cell weight per g of methanol used. The protein content of the biomass was 65%.     

Phylogenetic position of someChlorella species within the chlorococcales based upon complete small-subunit ribosomal RNA sequences

Summary Complete small-subunit rRNA (16S-like rRNA) coding region sequences were determined for eight species of the Chlorococcales (Chlorophyceae). The genera investigated includePrototheca, Ankistrodesmus, Scenedesmus, and fiveChlorella species. Distance matrix methods were used to infer a phylogenetic tree that describes evolutionary relationships between several plant and green algal groups. The tree exhibits a bifurcation within the Chlorococcales consistent with the division into Oocystaceae and Scenedesmaceae, but three of the fiveChlorella species are more similar to other algae than toChlorella vulgaris. All of the sequences contain primary and secondary structural features that are characteristic of 16S-like rRNAs of chlorophytes and higher plants.Anikstrodesmus stipitatus, however, contains a 394-bp group I intervening sequence in its 16S-like rRNA coding region.