Effect of dissolved organic matter on the concentration of marine microalgae Nannochloropsis oceanica

Author: Milene Cacciamani Teixeira Roselet  (Currículo Lattes)
Supervisor: Dr Paulo Cesar Oliveira Vergne de Abreu

Abstract

Marine microalgae of the genus Nannochloropsis are commonly used in aquaculture as live food and are currently recognized as a promising source of biofuel and bioproducts with applications in the pharmaceutical and food industries. However, the large-scale production of microalgae finds difficulties mainly in the collection and concentration of these microorganisms. The process of concentration and sedimentation of cells using flocculants is more efficient and cost effective than others. However, this method still has many technical problems that prevent its full operation. Among the biggest problems, the dissolved organic matter (MOD) produced by microalgae is recognized as a major interferer in the process of concentration of cells,resulting in increased demand for flocculants and even making the process unfeasible. In this Dissertation, the effect of organic matter in the concentration process was evaluated through its quantification and qualification and also by its removal from the culture medium using a "skimmer". The growth of microalgae was monitored on alternate days in a 250 L culture system. Samples of 100 mL of culture were filtered through 0.45 μm pore membrane filters to obtain the MOD and its subsequent quantification and characterization by dissolved organic carbon (COD), absorbance at 254 nm (UV), in addition to protein and carbohydrate concentration. All of these factors increased over the course of cultivation. The concentration efficiency for the microalgae N. oceanica was tested in the particulate fractions (MOP -> 0,7 μm) and dissolved (MOD - <0.45 μm) separated by 0.7 μm and 0.45 μm filters. As a flocculant, the natural polymer Tanfloc was used. The fractions that contained only MOP resulted in concentration efficiency greater than 90%, while the treatments that contained MOD in their composition did not exceed 30% efficiency, demonstrating that MOD is the part of organic matter that most interferes in the process. In a second experiment, when reaching the phases of exponential and stationary growth, 2 liters of culture were collected. The samples were centrifuged to remove microalgae and then filtered through a 0.45 μm membrane filter to collect the MOD, subsequently fractionated by hydrophobicity using XAD 7HP and XAD 4 resins. The concentration efficiency was tested in 3 fractions of dissolved organic matter:hydrophobic (HPO), transfilic (TPH) and hydrophilic (HPI), where N. oceanica cells were resuspended. In the exponential and stationary phases, the 2 worst efficiency was observed in the Positive Control (cultivation), followed by the HPI fraction with the lowest efficiencies among the tested fractions. The stationary phase showed the worst concentration efficiency, both in the Positive Control and in the HPI fraction. UV readings and COD and protein concentrations increased from one phase to another in Positive Control, while the carbohydrate concentration decreased. Thus, it is concluded that the higher concentration of proteins in MOD interferes negatively in flocculation. The efficiency of the concentration was also tested in a third experiment with the removal of MOD by the use of "skimmer" obtaining excellent results (93%),there was no statistical difference in comparison with the Negative Control treatment (97% - without organic matter). The skimmer was efficient in removing mainly proteins from the culture. In addition, it was found that its use did not affect the integrity of N. oceanica cells. The experiments were carried out in triplicate. As the data in the 3 experiments had a normal distribution and were homoscedastic, ANOVA and Tukey's post hoc test (p <0.05) were used. This project was developed at the Microalgae Production Laboratory of the Federal University of Rio Grande - FURG, from October 2015 to February 2017

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