Glycolate metabolism in green algae

Using ¹⁴C-labelled substrates, the succession of the single steps in the glycolate metabolism was investigated in Mougeotia scalaris and Eremosphaera viridis , which, within the group of green algae, are representatives of the evolutionary lines of Charophyta and Chlorophyta , respectively. In both algae the same metabolites are formed as in higher plants, although in Eremosphaera , which in contrast to Mougeotia does not possess leaf peroxisomes, all reactions are exclusively mitochondrial. Concomitant with the oxidation of glycolate, the synthesis of ATP was demonstrated in Eremosphaera . Formation of tartronic semi-aldehyde or other products different from those in land plants could not be demonstrated in either of these algae. Excretion of glycolate by Mougeotia and Eremosphaera is enhanced by decreasing the CO₂ concentration as well as by increasing the light intensity, but is completely stopped about 14 h later. Whereas increasing enzyme activities of the glycolate pathway apparently reduces glycolate excretion in Mougeotia , activation of CO₂ pumps seems to be the dominant reaction to prevent glycolate excretion in Eremosphaera . Mesostigma viride is one of the phylogenetically oldest algae in the group of Charophyceae . As this alga has already been demonstrated to contain microbodies with enzymes of leaf peroxisomes, the peroxisomal glycolate pathway must have originated at a very early stage. Surprisingly, the organelles from Mesostigma contain also the glyoxysomal marker enzyme isocitrate lyase suggesting these microbodies to be prototypes from which both glyoxysomes and leaf peroxisomes evolved.     

Biosynthetic mechanism of glycolate in Chromatium I. Glycolate pathway

The pattern of radioactivity distribution in several amino acidsof Chromatium cells exposed to ¹⁴CO₂ was determined. By transferringthe bacterial cells from an atmosphere of nitrogen to oxygenthere occurred a transient decrease of ¹⁴CO₂ incorporation intoaspartate and glutamate, whereas that into glycine showed aprominent increase. The labeling of both serine and alaninedid not show a marked change under such conditions. The, activitiesof glycolate oxidase and glycolate dehydrogenase in crude extractsof the bacterial cells were very low. The formation of glycolic acid only occurred during the oxidativemetabolism of Chromatium cells grown on bicarbonate as a C source,being negligibly small in bacteria under nitrogen or after growthon malate or acetate. The activities of both ribulose- 1,5-bisphosphateoxygenase and phosphoglycolate phosphatase in the extract preparedfrom the bicarbonate-grown bacterial cells were very low andapparently could not account for the glycolic acid formationthrough these enzymic reactions. Metabolic patterns of glycolicacid in Chromatium are discussed in relation to the photorespiratoryphenomenon. (Received February 24, 1975; )     

Serpins in plants and green algae

Control of proteolysis is important for plant growth, development, responses to stress, and defence against insects and pathogens. Members of the serpin protein family are likely to play a critical role in this control through irreversible inhibition of endogenous and exogenous target proteinases. Serpins have been found in diverse species of the plant kingdom and represent a distinct clade among serpins in multicellular organisms. Serpins are also found in green algae, but the evolutionary relationship between these serpins and those of plants remains unknown. Plant serpins are potent inhibitors of mammalian serine proteinases of the chymotrypsin family in vitro but, intriguingly, plants and green algae lack endogenous members of this proteinase family, the most common targets for animal serpins. An Arabidopsis serpin with a conserved reactive centre is now known to be capable of inhibiting an endogenous cysteine proteinase. Here, knowledge of plant serpins in terms of sequence diversity, inhibitory specificity, gene expression and function is reviewed. This was advanced through a phylogenetic analysis of amino acid sequences of expressed plant serpins, delineation of plant serpin gene structures and prediction of inhibitory specificities based on identification of reactive centres. The review is intended to encourage elucidation of plant serpin functions.     

Hydrocarbons in green and blue-green algae

Liquid column chromatography and thin-layer chromatography were used to determine the total content of hydrocarbons and gas chromatography was used to evaluate composition of hydrocarbons in green algae (Chlorella kessleri, C. vulgaris, Chlorella sp.,Scenedesmus acutus, S. acuminatus, S. obliquus) and the blue-green alga (Spirulina platensis) cultivated under autotrophic or heterotrophic conditions. InC. kessleri cultivated under heterotrophic conditions the content of hydrocarbons was found to be about 10^-2 % (per dry mass), whereas under autotrophic conditions it was about 10^-3 % (per dry mass). The highest content of hydrocarbons was detected in species of the genusScenedesmus cultivated autotrophically (10^-1 %). Heptadecane and hexacosane were found as major alkanes, 1-heptadecene was detected among alkenes.     

Rhodopsin-mediated photosensing in green flagellated algae

Green flagellated algae possess a primitive visual system that regulates the activity of their motor apparatus. Photoexcitation of a rhodopsin-type photoreceptor protein gives rise to the photoreceptor current, which, above a certain threshold of stimulus intensity, induces the flagellar current. It is probable that the photoinduced alteration in flagellar beating is governed by changes in intracellular Ca2+ concentration. This rhodopsin-mediated sensory system serves to align the swimming path with the direction of the light stimulus, whereas processes of energy metabolism determine whether the oriented movement is directed towards or away from the light source.     

Ribonucleotide reductase activity in green algae

A ribonucleoside diphosphate reductase is demonstrated in the algae, $\text{Scenedesmus}\text{obliquus}$ and $\text{Chlorella}\text{pyrenoidosa}$. In synchronized cultures an activity maximum at the 12th hour of the cell cycle coincides with maximum DNA production. Induction of reductase activity is prevented by cycloheximide. The enzyme requires dithiols for reduction of CDP $\text{in}\text{vitro}$; it is not significantly stimulated by iron or magnesium ions nor dependent upon deoxyadenosylcobalamin. ATP stimulates the reaction but dATP or dTTP act as inhibitors. The ribonucleotide reductase of green algae differs from the B₁₂-requiring enzyme characterized in $\text{Euglena}\text{gracilis}$.     

Peroxisomal targeting signals in green algae

Peroxisomal enzymatic proteins contain targeting signals (PTS) to enable their import into peroxisomes. These targeting signals have been identified as PTS1 and PTS2 in mammalian, yeast, and higher plant cells; however, no PTS2-like amino acid sequences have been observed in enzymes from the genome database of Cyanidiochyzon merolae (Bangiophyceae), a primitive red algae. In studies on the evolution of PTS, it is important to know when their sequences came to be the peroxisomal targeting signals for all living organisms. To this end, we identified a number of genes in the genome database of the green algae Chlamydomonas reinhardtii, which contains amino acid sequences similar to those found in plant PTS. In order to determine whether these sequences function as PTS in green algae, we expressed modified green fluorescent proteins (GFP) fused to these putative PTS peptides under the cauliflower mosaic virus 35S promoter. To confirm whether granular structures containing GFP–PTS fusion proteins accumulated in the peroxisomes of Closterium ehrenbergii, we observed these cells after the peroxisomes were stained with 3, 3′-diaminobenzidine. Our results confirm that the GFP–PTS fusion proteins indeed accumulated in the peroxisomes of these green algae. These findings suggest that the peroxisomal transport system for PTS1 and PTS2 is conserved in green algal cells and that our fusion proteins can be used to visualize peroxisomes in live cells.     

Fatty acids filamentous green algae

Fatty acid analyses of several filamentous green algae were conducted using gas-liquid chromatography. Two bryophytes were also examined. Qualitatively, the genera of algae studied were divided into two groups: (A) algae that have significant amounts of polyunsaturated C₂₀ fatty acids and (B) algae that lack or only have very small amounts of the C₂₀ acids. On the basis of fatty acid content, the algae of Group A more closely resemble the bryophytes than do the algae of Group B. Culture age was shown to cause quantitative but not qualitative variations in fatty acid content. It is evident from this study that extrapolation to land plants, from studies on the fatty acid content of the green algae, should include the filamentous forms.     

Ultrastructural localization of polysaccharides and N-actetyl-D-galactosamine in the secretory pathway of green algae (Desmidiaceae)

In order to characterize the secretory pathway leading to multipolar tip growth in green algae of the family Desmidiaceae, different general polysaccharide stainings, such as the periodic acid-silver hexamine method and the periodic acid-silver proteinate method as well as different lectins specific to defined sugar residues have been employed. General polysaccharide stainings label different kinds of secretory vesicles starting from the onset of vesicle production up to their delivery into the primary cell wall, however, the discrimination of Golgi products is possible using lectins. Both gold-labelled lectins from Helix pomatia and from Glycine max with affinity to N-acetyl-D-galactosamine only produce labelling of primary wall material containing 'dark vesicles' on the ultrathin sections of high-pressure frozen and freeze-substituted Micrasterias cells, whereas other vesicle types remain unstained. 'Dark vesicles' are labelled when still attached to trans-Golgi compartments, when distributed throughout the cytoplasm or when fusing with the plasma membrane with the same staining intensity which indicates that the sugars detected by the methods used are present from the onset of visible vesicle production. Gold-labelling of N-acetyl-D-galactosamine is also observed in the primary cell wall. In control experiments the staining vanishes when ultrathin sections are pre-incubated with N-acetyl-D-galactosamine. Various other lectins with affinity to different sugar residues than N-acetyl-D-galactosamine do not produce staining of the cell wall nor of any kind of secretory vesicles. As N-acetyl-D-galactosamine is usually not present in N-linked polysaccharides the results point towards the presence of O-linked-glycoproteins in the primary cell wall of desmids.Key words: Golgi, lectins, Micrasterias, N-acetyl-D-galactosamine, secretion, vesicles.      

Hydrogen production by photosynthetic green algae

Oxygenic photosynthetic organisms such as cyanobacteria, green algae and diatoms are capable of absorbing light and storing up to 10-13% of its energy into the H-H bond of hydrogen gas. This process, which takes advantage of the photosynthetic apparatus of these organisms to convert sunlight into chemical energy, could conceivably be harnessed for production of significant amounts of energy from a renewable resource, water. The harnessed energy could then be coupled to a fuel cell for electricity generation and recycling of water molecules. In this review, current biochemical understanding of this reaction in green algae, and some of the major challenges facing the development of future commercial algal photobiological systems for H2 production have been discussed.     

Iron supply and hydrogenase activity in green algae

Summary An addition of EDTA to the culture medium considerably increases H₂ metabolism of green algae. However, even under these conditions, several hours of anaerobic incubation (adaptation) are necessary for the optimum of hydrogenase activity to be reached. In addition, the stability of H₂ metabolism during longer periods of anaerobiosis is much better.