Unicellular Microalgae Research Articles

Brachionus plicatilis culture filtrate promotes sedimentation and harvesting of Chlorella

Harvesting of microalgae has been one of the major bottlenecks for commercial use. Sedimentation of the microalgae cells can facilitate the harvesting process, thus reducing the cost of yielding algae-based industrial products. We investigated the potential of Brachionus plicatilis and Artemia salina culture filtrates for promoting Chlorella sedimentation. B. plicatilis culture filtrate demonstrated the potential for promoting Chlorella sedimenta- tion, the sedimentation rate of Chlorella cells treated with the filtrate increased to 53.4 % within 24 h (20 % higher than control). The secretion of soluble extracellular polymeric substances was significantly increased in the Chlorella culture treated with either filtrate. The PSII quantum efficiency and growth rate in B. plicatilis culture treated Chlorella were not significantly affected. As the presented method is non-toxic and non-pathogenic, it may be an alternative for facilitating the harvesting of unicellular microalgae species used in the pharmaceutical sector and as food supplements.

Improved lipid productivity in Nannochloropsis gaditana in nitrogen-replete conditions by selection of pale green mutants

BackgroundNannochloropsis gaditana is a photosynthetic unicellular microalgae considered one of the most interesting marine algae to produce biofuels and food additive due to its rapid growth rate and high lipid accumulation. Although microalgae are attractive platforms for solar energy bioconversion, the overall efficiency of photosynthesis is reduced due to the steep light gradient in photobioreactors. Moreover, accumulation of lipids in microalgae for biofuels production is usually induced in a two-phase cultivation process by nutrient starvation, with additional time and costs associated. In this work, a biotechnological approach was directed for the isolation of strains with improved light penetration in photobioreactor combined with increased lipids productivity.ResultsMutants of Nannochloropsis gaditana were obtained by chemical mutagenesis and screened for having both a reduced chlorophyll content per cell and increased affinity for Nile red, a fluorescent dye which binds to cellular lipid fraction. Accordingly, one mutant, called e8, was selected and characterized for having a 30% reduction of chlorophyll content per cell and an almost 80% increase of lipid productivity compared to WT in nutrient-replete conditions, with C16:0 and C18:0 fatty acids being more than doubled in the mutant. Whole-genome sequencing revealed mutations in 234 genes in e8 mutant among which there is a non-conservative mutation in the dgd1 synthase gene. This gene encodes for an enzyme involved in the biosynthesis of DGDG, one of the major lipids found in the thylakoid membrane and it is thus involved in chloroplast biogenesis. Lipid biosynthesis is strongly influenced by light availability in several microalgae species, including Nannochloropsis gaditana: reduced chlorophyll content per cell and more homogenous irradiance in photobioreactor is at the base for the increased lipid productivity observed in the e8 mutant.ConclusionsThe results herein obtained presents a promising strategy to produce algal biomass enriched in lipid fraction to be used for biofuel and biodiesel production in a single cultivation process, without the additional complexity of the nutrient starvation phase. Genome sequencing and identification of the mutations introduced in e8 mutant suggest possible genes responsible for the observed phenotypes, identifying putative target for future complementation and biotechnological application.

Efficient accumulation of high-value bioactive substances by carbon to nitrogen ratio regulation in marine microalgae Porphyridium purpureum

An efficient biomass and high-value bioactive substances production strategy was developed for unicellular microalgae Porphyridium purpureum. We studied the optimal culture method and metabolites accumulation under different C/N conditions, and further proposed methods to increase the yield under high C/N ratio. The highest biomass reached 16.24 g/L with ASW medium by mixotrophy. High C/N ratio and mediate C/N can significantly promote the synthesis and secretion of polysaccharides, as well as the accumulation of ω-6 PUFAs; however, inhibit the growth, resulting in lower yield. With the significant increase of C/N ratio, protein degradation was accelerated, providing sufficient nitrogen source for efficient accumulation of carbohydrates (1.66 g/L EPS) and PUFAs (231.24 mg/L ARA). Finally, we reduced the growth inhibition, shortened the culture cycle, and doubled the final biomass to 9.34 g/L under nitrogen deficiency condition. Our exploitation of a cost-effective and feasible culture method for red algae is particularly significant.

Delineation of gamma irradiation (60Co) induced oxidative stress by decrypting antioxidants and biochemical responses of microalga, Chlorella sp.

The use of different radiation technologies are becoming more popular in health care (radiotherapy) and food processing. On the other hand, gamma (γ) radiation is a potential ionizing radiation that can generate huge amount of free radicals and Reactive Oxygen Species (ROS) cause a potential damage to important macromolecules in the cell. Therefore, information on biochemical components significantly need attention for their potential usefulness in counteracting gamma radiation-induced oxidative damage. Chlorella is an unicellular microalga enrich with high protein contents and other potential nutritional supplements. However, the information about the change of active biochemical attributes by the effects of gamma radiation on Chlorella is scanty. Hence, this is highly significant to assess the impacts of gamma radiation on microalga Chlorella sp. (BUACC06). The main objectives of this study is to unlock the information of the change of phytochemicals and antioxidants content in response to γ-irradiation on Chlorella sp. (BUACC06). This strain was treated with acute doses of γ-irradiation (0.01 kGy, 0.05 kGy, 0.075 kGy of 60 Co). The effect of higher dose γ-irradiation resulted in a decline in growth, total phenolic, total flavonoid, antioxidant activity, free radical scavenging and alternation of pigmentation; these could be due to the accumulation of radiation-induced ROS in the cell. Further, protein content and antioxidant enzyme activities were increased as increasing of dose and the effective dose was 0.075 kGy. It was concluded that the administration of Chlorella sp. showed radio resistance response to low dose of gamma radiation by synthesize more proteins, carotenoids, and active antioxidant compounds as radio-protective phytochemicals. This gamma radiation could be used as novel effective method to activate biosynthesis of certain bioactive compounds in algal cells to establish improved varieties of microalgae.

Protistal Grazers Increase Grazing on Unicellular Cyanobacteria Diazotroph at Night

In this study, we have for the first time analysed diel microzooplankton grazing selectivity on unicellular cyanobacterial diazotroph (i.e. Crocosphaera watsonii WH8501) and non-diazotrophic unicellular microalga (i.e. Chlorella autotrophica). A mixed diet consisting of these two algae was supplied to 4 species of protistan microzooplankton at daytime and night-time, respectively. C. watsonii fixes nitrogen at night-time and showed a stronger diel pattern of cellular C:N ratio than C. autotrophica. All four grazers ingested more C. watsonii at night-time than in daytime, suggesting the diazotroph became more nutritious (inferred by C:N ratio) and thus a preferred prey for grazers when it fixes nitrogen. In particular, Oxyrrhis marina changed from preferring C. autotrophica in daytime to preferring C. watsonii at night-time. The rest grazers showed species-specific grazing preferences, which could be explained by extracellular polysaccharide production of C. watsonii , feeding mode, cingulum size and cell size of grazers.

Identification of Prostaglandin Pathway in Dinoflagellates by Transcriptome Data Mining.

Dinoflagellates, a major class of marine eukaryote microalgae composing the phytoplankton, are widely recognised as producers of a large variety of toxic molecules, particularly neurotoxins, which can also act as potent bioactive pharmacological mediators. In addition, similarly to other microalgae, they are also good producers of polyunsaturated fatty acids (PUFAs), important precursors of key molecules involved in cell physiology. Among PUFA derivatives are the prostaglandins (Pgs), important physiological mediators in several physiological and pathological processes in humans, also used as “biological” drugs. Their synthesis is very expensive because of the elevated number of reaction steps required, thus the search for new Pgs production methods is of great relevance. One possibility is their extraction from microorganisms (e.g., diatoms), which have been proved to produce the same Pgs as humans. In the present study, we took advantage of the available transcriptomes for dinoflagellates in the iMicrobe database to search for the Pgs biosynthetic pathway using a bioinformatic approach. Here we show that dinoflagellates express nine Pg-metabolism related enzymes involved in both Pgs synthesis and reduction. Not all of the enzymes were expressed simultaneously in all the species analysed and their expression was influenced by culturing conditions, especially salinity of the growth medium. These results confirm the existence of a biosynthetic pathway for these important molecules in unicellular microalgae other than diatoms, suggesting a broad diffusion and conservation of the Pgs pathway, which further strengthen their importance in living organisms.

Characterization of CO2 fixation on algal biofilms with an infrared gas analyzer and importance of a space-rich structure on the surface

An attached microalgal culture system, or a biofilm system, which can reduce the cost of biomass recovery, is an attractive approach to solve the CO2 problem. In this study, we developed a method for precisely measuring CO2 fixation on algal biofilms, and optimized the photosynthetic capacity of a unicellular microalga, Chlorella. The CO2 concentration in the sample gas, measured with an infrared gas analyzer, was corrected by considering the difference in H2O concentration between the sample gas and the reference gas. The corrected CO2 concentration coincided with that of the reference gas, for which there were no cells. Using the established method, the photosynthetic capacity of Chlorella attached to a glass fiber membrane filter was characterized. Interestingly, the CO2 fixation rate per chlorophyll was almost the same as in liquid, at cell densities of up to 120 mg chlorophyll m−2. The light-dependency on the filter was very similar to that in liquid, while the photosynthetic rate was more markedly limited under low CO2 conditions than in liquid, suggesting that CO2 is a more severe limiting factor than light on the filter. The cell concentration-dependency of photosynthesis and microscopic observations suggest that photosynthesis is only performed by cells in a few top layers on the filter. To improve the CO2 supply, the solid surface was changed to a towel, which increased the photosynthetic rate in spite of its low optical transparency. The maximum rate of CO2 capture per area was estimated to be over 110 tons ha−1 year−1. These findings suggest that the Chlorella solid-surface culture system is a promising system for carbon capture, and that it can be further improved if the solid surface is converted into a steric structure with a high optical transparency.

Occurrence and light response of residual plastid genes in a Euglena gracilis bleached mutant strain OflB2

Euglena gracilis is a unicellular green eukaryotic microalga that features characteristics of both plants and animals. The photosynthetic function of its chloroplast is easily lost under stress resulting in bleached mutants, while the physiological role of their residual plastid DNAs remains unclear. In this study, we obtained five bleached mutants by ofloxacin (Ofl) treatment, identified 12 residual plastid genes in five bleached mutants, and determined the mRNA levels in the wild type E. gracilis (WT) and one bleached mutant (OflB2) under dark and light stimulation conditions by quantitative reverse transcribed PCR (qRT-PCR). Results show that the expression of all selected plastid genes in both WT and OflB2 mutant did not change significantly in darkness, while their responses to light stimulation were different. Under the light stimulation conditions, half of the genes did not change significantly, while most of the other genes were down-regulated in OflB2 mutant and up-regulated in WT. Therefore, the bleached mutant retains part of the plastid genome and the plastid relic is responsive to light. Our research will help to understand the functions of residual plastid DNA and evolution of chloroplasts.

CHARACTERIZATION OF MICROALGAE -BASED FEED SUPPLEMENTAND THEIR POSSIBLE INFLUENCE ON CATTLE RUMEN MICROBIAL ECOSYSTEM

Rumen is a complex ecosystem where feed consumed by animals is digested by the help of rumen protozoa. They plays an important role in contributing nutrients to the host animal. This study aimed to investigate the effects of dietary inclusion of unicellular microalgae Chlorella vulgaris spp. on total count and generic composition of protozoa in cows rumen. In this study, dietary treatments were tested during three 21-d experimental periods. Each of period content different amount of microalgae supplement. In the first experimental period in cows feed was added 30g (3,14g/kg of DM) of Chlorella, in second period-90g (9,6g/kg of DM) of Chlorella and in third period-170g (18,7 g/kg of DM). As a result of this study, ciliates of 10 genera were detected and identified. The number of this genera and total amount of ciliates were counted. The analysis of rumen protozoa population in tested cows shows visible effect of the presence of microalgae in cows dietary. In particular, the density of ciliates protozoa of the cows treated with 90 and 170 g of algal supplement was visibly higher compare to results from the control diet. Microalgae-based supplement diet had stimulative effect on ruminal protozoa population and caused increasing of many protozoa genera such as Isotricha, Dasytricha, Charonina, Buetschlia, Ostracodinium, Ophryoscolex.

Mixotrophic Cultivation of Dunaliella salina on Crude Glycerol Obtained from Calcinated Fatty Acid Production Process

Dunaliella salina is a unicellular microalga with the potential of industrial applications. Commercial-scale cultivation of D. salina requires extensive efforts on cost reduction. This can be achieved by minimization of nutrient cost and maximization of specific growth rate and productivity. To this end, in this study, mixotrophic cultivation of D. salina on crude glycerol (by-product of calcinated-fatty acid (CaFA) production process) was investigated in shake flasks and the influences on protein, carotenoid and chlorophyll contents of the cells were studied. The highest maximum specific growth rate and biomass productivity was achieved by cultivation on 2.5 g/L crude glycerol. The protein content was more than two folds and the carotenoid content was more than 60% higher than that of autotrophic cultivation. Subsequently, continuous cultivation of D. salina on crude glycerol in a helical photobioreactor was performed at the dilution rate of 0.52/d. The biomass productivity of 2.3 × 106 cell/(mL d) was achieved which was more than 75% of the values for previously reported autotrophic cultivations. Conclusively, continuous cultivation of D. salina on by-product of CaFA production process could be a potential process technology for production of D. Salina.

Nitrate starvation associated with strong light induces astaxanthin accumulation in a new green microalga Haematococcus pluvialis isolated from Algerian freshwater

The unicellular microalgae Haematococcus pluvialis is of great interest because it is an exclusive and the richest source of natural astaxanthin (3, 3’- dihydroxy-β-carotene-4, 4’-dione). We carried out an isolation and screening of a new H. pluvialis strains with high astaxanthin production potential from Algerian freshwater. The isolated strains were assessed for their growth and astaxanthin accumulation under different stress conditions associated with high light intensity. The main goal of the present study was to select highly astaxanthin-producing H. pluvialis strains from the Algerian environment. Eighteen microalgae strains in total were isolated on a Bold’s basal medium from 48 aseptically collected freshwater samples from 5 different regions of Algeria. The identification of H. pluvialis was made according to morphological criteria. Growth tests have led to the selection of the best H. pluvialis strain that will be grown under stress (nutrient starvation and variable salinity) associated to high light intensity (200 µmol photons.m-2.s-1) to determine the optimal conditions of astaxanthin accumulation. Optimal nitrate concentration giving rise to maximum biomass was 0.50 g/L. Nitrate starvation associated with high light irradiance was most effective in induction of astaxanthin accumulation which reached 67.25 ± 2.28 mg/L in H. pluvialis cells. H. pluvialis morphology under such stress has shown the occurrence of mature aplanospores with thick cell wall. These results clearly show that the isolated H. pluvialis strain has a good performance to synthesize astaxanthin under nitrate starvation associated with high light irradiance suggesting its possible industrial culture for the production of this pigment.

De novo transcriptome sequencing of Monodopsis subterranea CCALA 830 and identification of genes involved in the biosynthesis of eicosapentanoic acid and triacylglycerol

Monodopsis subterranea, a unicellular yellow-green freshwater microalga, is widely known for its ability to produce high amount of therapeutically beneficial ω-3 polyunsaturated fatty acid eicosapentanoic acid (EPA; C20:5Δ5,8,11,14,17). Currently, no genomic information is available on M. subterranea despite its nutraceutical and commercial applications. Analysis on fatty acid methyl esters from M. subterranea strain CCALA 830 demonstrated accumulation of 28% EPA. In order to obtain better understanding of EPA biosynthesis and to identify genes involved in the process of lipid metabolism and accumulation in this alga, de novo transcriptome sequencing and assembly was performed using Illumina Hiseq 2000 sequencing. A total of 35,954 transcripts were obtained through final transcriptome assembly with an average transcript length of 769.36 bp. BLAST similarity searches for assembled transcripts were performed followed by annotation using Gene ontology and Kyoto Encyclopedia of Genes and Genomes orthology identifiers. Transcripts involved in lipid biosynthesis including various fatty acid desaturases and elongases involved in PUFAs biosynthesis were identified during the study. In addition, sequences for several transcription factors and a number of simple sequence repeats were also ascertained which can be used as powerful genetic markers for further genetic analysis. This study would provide a useful resource for future research on M. subterranea genome.

Identification of proteins in the adhesive trails of the diatom Amphora coffeaeformis

Throughout all kingdoms of life, a large number of adhesive biomolecules have evolved to allow organisms to adhere to surfaces underwater. Proteins play an important role in the adhesion of numerous marine invertebrates (e.g. mussels, sea stars, sea urchins) whereas much less is known about the biological adhesives from marine plants, including the diatoms. Diatoms are unicellular microalgae that together with bacteria dominate marine biofilms in sunlit habitats. In this study we present the first proteomics analyses of the diatom adhesive material isolated from the tenacious fouling species Amphora coffeaeformis. We identified 21 proteins, of which 13 are diatom-specific. Ten of these proteins share a conserved C-terminal domain, termed GDPH domain, which is widespread yet not ubiquitously present in all diatom classes. Immunofluorescence localization of a GDPH domain bearing protein (Ac629) as well as two other proteins identified in this study (Ac1442, Ac9617) demonstrated that these are components of the adhesive trails that are secreted by cells that glide on surfaces. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Nanoporous Silica of Some Egyptian Diatom Frustules as a Promising Natural Material

Background:Diatoms are a large unique group of unicellular microalgae with a significant ornamented cell wall made of hydrated silica, which is called "frustule". Their ornamented siliceous shells are usually composed of multi-layer structures with multi-scale porosity, which can be used as a promising source to obtain hierarchical macro and mesoporous silica microparticles. The present study is one step forward through a long road seeking for green nanofabrication techniques of such porous materials that will be economically more efficient with large design flexibility.Materials and Methods:For seeking different porosity scales, architectures, and distribution patterns, a total of 237 diatom species and varieties belonging to 68 genera were identified from 59 samples, which were collected from different Egyptian habitats and environments. Of these eight species were selected to study their frustules' ultra-structures in details, including; Aulacoseira granulata (Ehrenberg) Simonsen, Actinocyclus octonarius Ehrenberg, Cyclotella meneghiniana Kützing, Pleurosira laevis (Ehrenberg) Compére, Synedra ulna (Nitzsch) Ehrenberg, Achnanthes brevipes Agardh, Nitzschia amphibia Grunow and Nitzschia palea (Kützing) W. Smith. Nitzschia palea (Kützing) W. Smith was also isolated and cultivated.Results:The ultrastructure and porosity of all studied species had been revealed. The porosity scale was ranged from 5 to 500 nm in diameter.Conclusion:The obtained results showed the potential of diatom frustules in nanotechnology as a source of natural silica microparticles with macro and mesoporous structures could be of a large interest for applications including ultra, micro and nanofiltration, drug delivery systems, optoelectronics, or other novel nanotechnology applications.

Development of an integrated process of membrane filtration for harvesting carotenoid-rich Dunaliella salina at laboratory and pilot scales

Dunaliella salina is an unicellular microalga, well-known for the production of carotenoids, especially β-carotene. This microalga is very fragile and shear sensitive due to the absence of a rigid cell wall and may easily rupture when shear stress conditions are applied. This work proposes to study the pre-concentration of carotenoid-rich (“orange”) Dunaliella salina culture by membrane filtration, preceding a final concentration using low-shear centrifugation. The goal is to minimise the total capital and energy costs, ensuring a low cell integrity loss and high permeate fluxes.The harvesting process was performed under controlled permeate flux conditions at laboratory and pilot scales. At pilot scale, the harvesting of Dunaliella salina attained a final concentration factor of 10, at a permeate volumetric flux of 21 L/(m2 h). When applying the two-step approach of membrane + centrifugation, the reduction of Total Cost of Ownership (TCO) and energy consumption were 70 and 76%, respectively, compared to harvesting with a one-step centrifugation approach.

Effects of Burkholderia thailandensis rhamnolipids on the unicellular algae Dunaliella tertiolecta

The effects of rhamnolipids (RLs) produced and further purified from Burkholderia thailandensis, on the unicellular microalgae Dunaliella tertiolecta were investigated, in terms of RLs ability to affect algal growth, photosynthetic apparatus structure and energy flux, round and through photosystems II and I. Specifically, 24–48 h RLs-treated algae (RLs at concentrations ranged from 5 to 50 mg L−1) showed significantly decreased levels of growth rate, while increased levels of Chl a and b were obtained only in 72–96 h RLs-treated algae. Similarly, although no changes were obtained in the Chl a/b ratio and almost all chlorophyll fluorescence parameters over time, yields of electron transport (ϕR0, ϕE0) and respective performance index (PItotal) were negatively affected at 72 and 96 h. Based on those findings, it seems that the inhibitory effect of RLs on the algae growth rate after 24 and 48 h and the gradual attenuation of the phenomenon (after 72 h of exposure), may indicate the initial response of the organism, as well as algae ability to overcome, since RLs showed no effects on algae photosynthetic ability. Those findings reveal for the first time that RLs from Burkholderia thailandensis are not harmful for Dunaliella tertiolecta. However, further studies with the use of more aquatic species could be essential for assessing the RLs-mediated effects on aquatic biota.

Arsenic Removal from Aqueous Media using Scenedesmus obliquus: the Promoting Impact of Microalgae-Bacteria Consortium

The present study was conducted to investigate efficacy of unicellular microalga Scenedesmus obliquus either as solo culture or in combination with the bacterium Shewanella sp in removal of arsenic from aqueous media. For this purpose, pure culture of the microalga was fed with arsenic solution and removal rate was monitored. Moreover, impact of pH, temperature and initial biomass was studied. In the second phase, a consortium of S. obliquus- Shewanella was prepared and used to investigate the effect of bacterium presence in terms of removal enhancement. The results showed that pH 7.0, temperature of 29 oC and initial biomass of 0.8 g/L caused the highest rate of arsenic removal. Comparison of pure culture of S. obliquus and S. obliquus- Shewanella consortium revealed that the latter significantly enhanced removal efficacy.

Exploring the niche of a highly effective biocalcifier: calcification of the eukaryotic microalga Oocardium stratum N\xe4geli 1849 in a spring stream of the Eastern Alps

Microbially mediated calcification is a major process of carbonate production, yet little is known about eukaryotic microalgal calcifiers. We describe calcification and propagation of the unicellular microalga Oocardium stratum in an Alpine spring stream. The spring sheds Ca–Mg–HCO3 water with a temperature of 8–11 °C. The biota is dominated by O. stratum and diatoms; mosses, cyanobacteria, and filamentous eukaryotic algae are accessories. O. stratum colonize various substrates within the stream throughout the year. When colonizing, single cells attached to mucilage, then induced precipitation of a rim of calcite, and underwent a first division. A mature clone of O. stratum typically consists of single cells each housed within a calcite tube precipitated by the microalga. Upon cell division, the tubes branch, too, under retention of the optical orientation of the calcite. Continued growth, cell division, and calcification result in laminae of Oocardium calcite (OC) concordant with substrate shape. O. stratum accelerates but seems not to control calcite precipitation. A maximum vertical calcification rate of 5 mm/a was documented for a site ~ 25 m downstream of the spring. ‘Crystal-skeletal’ OC characterized low calcite supersaturation, whereas higher supersaturation corresponded with rhombohedral OC. Abiotic precipitation downward of the upper tips of growing calcite tubes resulted in compact spar crystals, irrespective of initial crystal habit. Diatoms that thrived on OC benefit from a large differentiated habitat. Our study confirms previous works that identify O. stratum, not cyanobacteria, as major biocalcifiers in some hardwater springs. Diagenetically mature Oocardium tufa, however, may be confused with spring limestones of other origins.

Bioremediation and Biomass Production with the Green Microalga Chlorococcum humicola and Textile Mill Effluent (TE)

The study aimed at possible bioremediation of textile mill effluent (TE) and simultaneous production of useful biomass. The green, unicellular microalga, Chlorococcum humicola was grown in TE. Even in undiluted effluent, this alga reduced several important effluent parameters from 9 to 94% in 6 days and produced increased biomass with improved cell contents as compared to control. Strikingly, NO3 and NO2–nitrogen went below detectable limits within 3 days. In a comparative growth experiment, the growth rate of the alga was maximum in 10% dilution of effluent with supplementation of 0.15% NaNO3. This alga produced extracellular H2O2, even in dark which was limited by nitrogen. TE-grown cells accumulated 32% more lipids and 25% more starch as compared to control. Further, two allelopathic extracellular free fatty acids, namely palmitic and linoleic acids, were detected.

State Estimation of Microalgae Photobioreactors: Applications to Haematococcus Pluvialis

Haematococcus Pluvialis is a type of freshwater green unicellular microalgae that can be used as economical natural source of astaxanthin. To guarantee optimal algae production and the proper conditions for the obtainment of high yield of astaxanthin, the growth process should be carefully monitored and controlled. When on-line measurements are not available, state estimators can provide a useful tool for process monitoring. In this paper a geometric observer is designed based on an extended version of the Droop model for microalgae growth, identified on the basis of batch experiments. The observer functioning and convergence are tested using experimental data obtained from a laboratory scale bioreactor.