Biology and physiology of photosynthetic microorganisms of industrial interest
Contact: Graziella Chini Zittelli (Unit of Firenze)
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Photosynthetic microorganisms, such as microalgae and cyanobacteria, can be used to produce high-value pharmacological or nutraceutical products, biomass for human or animal consumption, and for bioenergy production and waste-water treatment. All these applications will be developed through the search for, isolation and cultivation of appropriate microalgal strains endowed with interesting activities or able to produce more efficiently or in higher amounts the desired compound. The main goal of the research group is the study of biology and physiology of selected microorganisms in order to optimize their growth performance and understand the cellular/molecular mechanisms that trigger the synthesis of the compound, and allow to completely exploit their potential. The study of the biology and physiology of selected photosynthetic microorganisms is also of great importance for their outdoor mass cultivation. Outdoors, in fact, the cultures are exposed to continuous stress caused by rapid changes in lighting, temperature, pH, which can fluctuate (especially light) from the minimum values for growth to inhibitory values. On the other hand the induction and accumulation of high value-added products, in particular carotenoid pigments (astaxanthin, zeaxanthin, lutein), and the production of bioenergy through the induction of synthesis of lipids (biodiesel) and / or hydrogen are associated with nutritional stress conditions. In addition, the potential use of microalgae for the treatment of wastewater is also associated with stress caused by the presence of substances that inhibit growth (e.g. polyphenols in vegetable water) and heavy metals Knowledge of the stress factors that influence the growth is a very important point to better address the selection of strains capable of competing with other potentially competing organisms in outdoor cultures and optimize productivity.
Research
1. Selection and laboratory cultivation of microalgae strains promising for various applications.
The purpose is to identify strains not only capable to produce high amount of interesting products but also with good ability to massive growth. In Fig. 1 are schematized the different steps of microalgae cultivation adopted: maintaining stock cultures (tubes or plates on solid medium), preparing starter cultures (flasks of increased volume), preparing inocula (from small bubble tubes to carboys) for outdoor mass cultivation that is carried out in different types of photobioreactors (PBR).
2. Study of microalgae growth kinetics and stress phenomena that limit productivity and photosynthetic efficiency.
Isolating strains with faster growth rates than strains currently available in order to improve biomass production without the need for genetic modification. Photosynthesis is one of the physiological processes in photosynthetic organisms that is severely affected by stress and changes of photosynthetic activity are considered a valid parameter to monitor the health and vitality under stress conditions. The use of fluorescence chlorophyll technique represents a non invasive and useful tool to measure the photosynthetic activity, giving a rapid quantification of the physiological stresses in photosynthetic organisms and is a measurement routinely adopted in our research activity.
3. Study of stress conditions that favor the accumulation of high value-added products (e.g. carotenoid pigments) or allow to produce energy through the induction of synthesis of lipids and / or biohydrogen.
In particular, assessing the effect of nutrient deficiencies (N, P, S) in both laboratory and outdoor. Under stress conditions, such as high irradiance and nitrogen deficiency the microalga Haematococcus pluvialis synthesizes large quantities of ketocarotenoids, such as cantaxanthin and astaxanthin. During the transition from the green to the red stage, the exposure to high irradiance under nitrogen starvation, induced the astaxanthin accumulation up to 5% of dry weight (Fig. 2).
Several microalgal strains have been screened in the laboratory for their biomass productivity and lipid content in order to evaluate their oil production potential. As expected, lipid-rich strains showed in general lower biomass productivity. Among the strains that showed a good combination of the two parameters the eustigmatophyte Nannochloropsis has showed to respond to nutrient stress with a significant increase of lipid synthesis (up to 60% dry wt).
4. Selection and cultivation of microalgae rich in polyunsaturated fatty acids (PUFA).
Some PUFA-rich microalgal strains have been selected and cultivated (Table 1). Factors affecting culture productivity and PUFAs synthesis have been investigated under laboratory conditions and outdoors.
Maintenance of cyanobacteria and microalgae collection of more than 500 strains and selection of new strains, particularly from extreme environments, often with interesting metabolic characteristics for applications, and their identification by means of a microscopic and molecular combined approach.
| PUFA content (% d.wt) | ||||
| Microalgal species | GLA C18:3 g | ARA C20:4 n-6 | EPA C20:5 n-3 | DHA C22:6 n-3 |
| Arthrospira platensis | 1-1.5 | - | - | - |
| Nannochloropsis sp. | - | 0.6 | 3-4 | - |
| Isochrysis sp. (T-ISO) | - | - | 0.1 | 1.7 |
| Pavlova lutheri | - | 0.1 | 2.2 | 1.0 |
| Monodus subterraneous | - | 0.3-0.5 | 3-3.5 | - |
| Phaeodactylum tricornutum | - | - | 2-2.5 | 0.1 |
| Tetraselmis suecica | - | 0.05 | 0.40 | - |
Skills
- Experience in the growth of photosynthetic microorganisms (photosynthetic bacteria, microalgae and cyanobacteria) both under laboratory conditions or outdoor
- Assessment of photosynthetic performance by measurement of variable chlorophyll fluorescence (Fv/Fm : maximum quantum yield; DF/F’m: effective quantum yield; ETR: electron transport rate; NPQ: non photochemical quenching) and chlorophyll fluorescence transient.
- Isolation of strains from samples of water and soil, their purification, maintenance and preservation
- Morphological analysis by light microscopy
- Taxonomic identification by the molecular techniques PCR and sequence analysis
- Biochemical analysis: general composition, extraction and quantification of lipid, extraction, separation and identification of pigments.