Economic and technical problems related to the reduction of petroleum resources require the valorisation of renewable raw material Recently, microalgae emerged as promising alternative feedstock that represents an enormous biodiversity with multiple benefits exceeding the potential of conventional agricultural feedstock Thus, this comprehensive review article spots the light on one of the most interesting microalga Chlorella vulgaris It assembles the history and a thorough description of its ultrastructure and composition according to growth conditions The harvesting techniques are presented in relation to the novel algo-refinery concept, with their technological advancements and potential applications in the market

/pdf/morphology-composition-production-processing-and-5458j8vfab.pdf

Morphology, composition, production, processing and applications of Chlorella vulgaris: A review

Viruses and viruslike particles of eukaryotic algae.

Some species of microalgae have high lipid yields; however, all species of microalgae, with the only known exception of Botryococcus braunii, have their lipids located inside the cells. The toughness of cell walls and cell membranes of microalgae makes the lipids not readily available for extraction and means that cell disruption an energy intensive process. The cell disruption energy required may become a critical consideration in the production of low valued commodities such as biofuels. This study provides an overview of microalgal cell disruption processes which are potentially suitable for large scale lipid extractions. The energy requirements of these processes were calculated and then compared with estimates of the theoretical minimum energy required for disruption. The results show that the mechanical disruption methods considered were highly energy inefficient when conducted under laboratory conditions and required a specific energy consumption of at least 33 MJ kg −1 of dry biomass. Thus the specific energy consumption is greater than the energy recoverable from the microalgae and is also a factor of 10 5 greater than that the estimated minimum theoretical energy consumption. This result clearly shows that further research and innovation is required for the sustainable cell disruption and lipid extraction from microalgae.

Disruption of microalgal cells for the extraction of lipids for biofuels: Processes and specific energy requirements

Animals acquire photosynthetically-fixed carbon by forming symbioses with algae and cyanobacteria. These associations are widespread in the phyla Porifera (sponges) and Cnidaria (corals, sea anemones etc.) but otherwise uncommon or absent from animal phyla. It is suggested that one factor contributing to the distribution of animal symbioses is the morphologically-simple body plan of the Porifera and Cnidaria with a large surface area:volume relationship wellsuited to light capture by symbiotic algae in their tissues. Photosynthetic products are released from living symbiont cells to the animal host at substantial rates. Research with algal cells freshly isolated from the symbioses suggests that low molecular weight compounds (e.g. maltose, glycerol) are the major release products but further research is required to assess the relevance of these results to the algae in the intact symbiosis. Photosynthesis also poses risks for the animal because environmental perturbations, especially elevated temperature or irradiance, can lead to the production of reactive oxygen species, damage to membranes and proteins, and ‘bleaching’, including breakdown of the symbiosis. The contribution of nonphotochemical quenching and membrane lipid composition of the algae to bleaching susceptibility is assessed. More generally, the development of genomic techniques to help understand the processes underlying the function and breakdown of function in photosynthetic symbioses is advocated.

/pdf/photosynthetic-symbioses-in-animals-3ib8aphi0n.pdf

Photosynthetic symbioses in animals.

Summary 449

I. INTRODUCTION 450

II. THE PARTNERS 451

1. Cyanobionts and their role 451

2. Hosts and their role 453

3. Location of cyanobionts in their hosts 455

III. INITIATION AND DEVELOPMENT OF SYMBIOSES 458

1. Initiation of symbioses 458

2. Geosiphon pyriforme 458

3. Cyanolichens 459

4. Liverworts and hornworts 460

5. Azolla 460

6. Cycads 461

7. Gunnera 461

IV. THE SYMBIOSES 462

1. Geographical distribution and ecological significance 462

2. Benefits to the partners 462

(a) Benefits to the cyanobionts 462

(b) Benefits to the hosts 463

3. Duration and stability 463

4. Mode of transmission and perpetuation 463

5. Recognition between the partners 464

6. Specificity and diversity 464

7. Symbiosis-related genes 465

8. Modifications of the cyanobiont 466

(a) Growth and morphology 466

(b) Photosynthesis and carbon metabolism 467

(c) Glutamine synthetase 467

(d) Heterocysts 469

(e) N2fixation 470

9. Nutrient exchange 471

(a) Carbon 471

(b) Nitrogen 472

V. EVOLUTIONARY ASPECTS 472

VI. ARTIFICIAL SYMBIOSES 474

VII. FUTURE OUTLOOK AND PERSPECTIVES 475

1. Cryptic symbioses 476

2. Developmental profile of symbiotic tissues 476

3. Sensing and signalling 476

4. Genetic aspects 476

5. Physiological and biochemical aspects of nutrient exchange 477

6. Microaerobiosis 477

7. Potential applications 477

Acknowledgements 477

References 477

Cyanobacteria are an ancient, morphologically diverse group of prokaryotes with an oxygenic photosynthesis. Many cyanobacteria also possess the ability to fix N2. Although well suited to an independent existence in nature, some cyanobacteria occur in symbiosis with a wide range of hosts (protists, animals and plants). Among plants, such symbioses have independently evolved in phylogenetically diverse genera belonging to the algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms. These are N2-fixing symbioses involving heterocystous cyanobacteria, particularly Nostoc, as cyanobionts (cyanobacterial partners). A given host species associates with only a particular cyanobiont genus but such specificity does not extend to the strain level. The cyanobiont is located under a microaerobic environment in a variety of host organs and tissues (bladder, thalli and cephalodia in fungi; cavities in gametophytes of hornworts and liverworts or fronds of the Azolla sporophyte; coralloid roots in cycads; stem glands in Gunnera). Except for fungi, the hosts form these structures ahead of the cyanobiont infection. The symbiosis lasts for one generation except in Azolla and diatoms, in which it is perpetuated from generation to generation. Within each generation, multiple fresh infections occur as new symbiotic tissues and organs develop. The symbioses are stable over a wide range of environmental conditions, and sensing–signalling between partners ensures their synchronized growth and development. The cyanobiont population is kept constant in relation to the host biomass through controlled initiation and infection, nutrient supply and cell division. In most cases, the partners have remained facultative, with the cyanobiont residing extracellularly in the host. However, in the water-fern Azolla and the freshwater diatom Rhopalodia the association is obligate. The cyanobionts occur intracellularly in diatoms, the fungus Geosiphon and the angiosperm Gunner a. Close cell–cell contact and the development of special structures ensure efficient nutrient exchange between the partners. The mobile nutrients are normal products of the donor cells, although their production is increased in symbiosis. Establishment of cyanobacterial–plant symbioses differs from chloroplast evolution. In these symbioses, the cyanobiont undergoes structural–functional changes suited to its role as provider of fixed N rather than fixed C, and the level of intimacy is far less than that of an organelle. This review provides an updated account of cyanobacterial–plant symbioses, particularly concerning developments during the past 10 yr. Various aspects of these symbioses such as initiation and development, symbiont diversity, recognition and signalling, structural–functional modifications, integration, and nutrient exchange are reviewed and discussed, as are evolutionary aspects and the potential uses of cyanobacterial–plant symbioses. Finally we outline areas that require special attention for future research. Not only will these provide information of academic interest but they will also help to improve the use of Azolla as green manure, to enable us to establish artificial N2-fixing associations with cereals such as rice, and to allow the manipulation of free-living cyanobacteria for photobiological ammonia or hydrogen production or for use as biofertilizers.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00720.x

Tansley Review No. 116: Cyanobacterium-plant symbioses.

Cell wall composition of chlorococcal algae

Isolated cell walls of mature Chlorella fusca consisted of about 80% carbohydrate, 7% protein, and 13% unidentified material. Mannose and glucose were present in a ratio of about 2.7:1 and accounted for most of the carbohydrate. Minor components were glucuronic acid, rhamnose, and traces of other sugars; galactose was absent. After treatment with 2 M trifluoroacetic acid or with 80% acetic acid/HNO3 (10/1, v/v), a residue with a mannose/glucose ratio of 0.3:1 was obtained, probably representing a structural polysaccharide. An X-ray diffraction diagram of the walls showed one diffuse reflection at 0.44 nm and no reflections characteristic of cellulose. Walls from young cells contained about 51% carbohydrate, 12% protein, and 37% unidentified material. Mannose and glucose were also the main sugars; their absolute amounts per wall increased 6–7 fold during cell growth. Walls isolated with omission of a dodecylsulphate/mercaptoethanol/urea extraction step had a higher protein content and, with young walls, a significantly higher glucose and fucose content. These data and other published cell wall analyses show a wide variability in cell wall composition of the members of the genus Chlorella.

/pdf/composition-of-the-cell-wall-of-chlorella-fusca-3q2kbe7kr6.pdf

Composition of the cell wall of Chlorella fusca

Chlorella sorokiniana strain 211-40c, a symbiotic Chlorella isolated from a freshwater sponge, excreted between 3% and 5% of assimilated 14CO2 as glucose in the light, with a pH optimum around 5. This percentage increased when the illuminance was lowered (to 15% at 20 lx). Release of [14C]glucose continued in the dark and could be inhibited by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). Net efflux of glucose occurred even at a concentration ratio of extracellular/intracellular glucose of 4. This, together with the sensitivity to FCCP, is taken as evidence for active transport. Exogenous [14C]glucose was taken up by the cells under conditions of net glucose efflux, showing uptake and excretion to take place simultaneously.

Glucose excretion by the symbiotic Chlorella of Spongilla fluviatilis.

A cell-wall-degrading activity was solubilized from young cells and from mother cell walls of Chlorella fusca by treatment with LiCl. The cytoplasmic enzyme hexokinase was not detectable in these extracts. The LiCl-solubilized activity increased in the cell cycle parallel to the release of autospores. The enzyme was purified on a chromatofocusing column followed by gel filtration. Sodium dodecyl sulfate/polyacryl amide gel electrophoresis of the purified enzyme revealed a molecular weight of 44 kDa, whereas gel filtration indicated a molecular weight of 25 kDa. Cell-wall-lytic activity and β-1,4-mannanase activity coeluted in gel filtration and were separated from β-d-fucosidase activity. The enzyme degraded isolated cell walls and ivory nut mannan primarily to oligosaccharides with an estimated degree of polymerization ≧6. The soluble degradation products of the cell wall consisted of 92–96% mannose and 4–8% glucose. It is concluded that the cell-wall-lytic activity is caused by an endo-mannanase. In vivo, this enzyme probably degrades the mother cell wall and, after autospore release, remains bound to it as well as to the surface of the daughter cells by ionic forces. The identity of this bound enzyme with a soluble wall-degrading enzyme previously obtained from mother cells is discussed.

/pdf/cell-wall-bound-lytic-activity-in-chlorella-fusca-function-2vowxreq7n.pdf

Cell-wall-bound lytic activity in Chlorella fusca: function and characterization of an endo-mannanase.

The soluble fraction of homogenates of synchronous Chlorella fusca was tested for carbohydrate-lyzing activities. With isolated cell walls and β-1,4-mannan or carboxymethyl cellulose as substrates, a sharp increase in activity occurred shortly before release of the daughter cells followed by a decline during release. The lytic activities were partially purified by ammonium sulphate precipitation and analyzed by gel filtration on a calibrated column. Apparent molecular weights were 27,000 for cell wall autolysin(s) and β-1,4-mannanase, 36,000 for carboxymethyl cellulase and ≧70,000 for another β-1,4-mannanase. Incubation of isolated cell walls with an enzyme preparation purified by ammonium sulphate precipitation resulted in release of up to 70% of the cell wall carbohydrate as monosaccharide, predominantly mannose and glucose. The carbohydrate released in vivo into the culture medium shortly before and during liberation of the daughter cells consisted largely of polymeric material with rhamnose, fucose and mannose as main constitutents. Upon poisoning the cells with NaN3 or carbonyl cyanide p-trifluoromethoxy-phenylhydrazone, however, a monosaccharide fraction consisting of mannose and glucose was predominant in the medium. It is suggested that the major products of cell wall lysis in vivo are monosaccharides which are rapidly taken up and metabolized by the developing daughter cells in an energy-dependent manner.

/pdf/cell-wall-lytic-activity-in-chlorella-fusca-56ajn9rtkz.pdf

Doris Meindl

Papers

Cell wall composition of chlorococcal algae

Composition of the cell wall of Chlorella fusca

Glucose excretion by the symbiotic Chlorella of Spongilla fluviatilis.

Cell-wall-bound lytic activity in Chlorella fusca: function and characterization of an endo-mannanase.

Cell wall-lytic activity in Chlorella fusca.