Καλλιέργεια των πράσινων μακροαλγών Ulva lactuca και Ulvaria fusca για την παραγωγή βιοκαυσίμων

Abstract

Ulva lactuca and Ulvaria fusca are two species of green marine macroalgae that are particularly interesting for biofuels production. These species have high growth rates (0.21 d-1) and high biomass yields (10x corn) which makes them suitable for cultivation. They do not need agricultural land nor fresh water for their growth and that makes them very competitive against agricultural land crops. These green macroalgae are autotrophic photosynthetic organisms. They require light and suitable temperature conditions to maximize their growth and they capture CO2 from their natural habitat as carbon source for the production of energy in the form of sugars. The basic nutrient source for their growth is nitrogen either in the form of NO3 or in the form of NH4. According to the experimental work conducted in this study the maximum growth rate was achieved when culture medium enriched with NO3 (F/2) was used in the case of Ulva lactuca and when culture medium enriched with NH4 was used in the case of Ulvaria fusca cultivation. Ulvaria fusca had approximately 30% lower growth rate than Ulva lactuca when the same culture medium was used. Tests were performed to investigate the effect of bacteria and diatoms during the cultivation process but they showed that addition of antibiotics and GeO2 eliminated bacteria and diatoms but they also inhibited the growth of the macroalgae. Thus pre-treatment of the macroalgae before the cultivation was excluded. The possibility of using digested pig manure (DPM) as nutrient source for the cultivation of the two aforementioned species was investigated and it turned out that Ulva lactuca and Ulvaria fusca can grow in culture medium enriched with DPM with growth rates ~27% lower than the maximum achieved for each species (0.16d-1). Filtration, centrifugation, addition of hydrogen peroxide and autoclave were combined to pre-treat the DPM before the cultivation but they reduced the nitrogen level under detection limits and so pre-treatment of the DPM was unnecessary. There are two categories of products that can be derived from macroalgae. Energy products and non-energy products. In this study anaerobic digestion and the production Σελίδα 10 of biogas as well as ethanol fermentation and the production of ethanol were investigated. Biodiesel was not an option for these green macroalgal species since they contained very low amounts of lipids which are necessary for biodiesel production. Furthermore a review over the non-energy products from Ulva spp. was conducted and showed that there are very useful and interesting polymers that can be extracted, and of course that these algae can be used as food supplements as well. Ulva lactuca and Ulvaria fusca were used as substrate for anaerobic thermophilic digestion with DPM as inoculum. The samples were macerated, and half of the algal fronds were washed with fresh water and the other half were left with the superficial sea water in order to investigate the effect of sea salt in the anaerobic digestion. The average methane yield achieved was 187 mL CH4 / gVS and 214 mL CH4 / gVS for Ulva lactuca and Ulvaria fusca respectively and it was 21-29% higher when the algal fronds were washed before the inoculation (P<0.05). The Na+ ,K+ levels did not exceed 1g/L and sea salt did not seem to be the cause of this disparity. A disadvantage of anaerobic digestion of macroalgae compared to for instance terrestrial energy crops is the high water content of algae. In the present study the TS and VS content of U. lactuca and U. fusca were 3.57% / 2.95% and 5.37% / 4.87%, respectively, which will not allow a high loading rate in a continuously fed system at 15–18 days hydraulic retention time. In addition, the weight specific methane yield of U. lactuca and U. fusca was low (6 m3 t-1 and 10 m3 t-1 respectively) due to the high water content. Therefore in their raw form, U. lactuca and U. fusca cannot be used to boost the methane yield of manure based biogas plants like industrial waste is used today (Angelidaki and Ellegaard, 2003). Furthermore Ulva spp. contains a high amount of carbohydrates (>60 DW%) making them suitable for cellulosic ethanol fermentation. The yeast Saccharomyces cerevisiae was used for ethanol fermentation after the enzymatic hydrolysis of the macerated macroalgal species and the effect of alkaline pre-treatment was also investigated in Σελίδα 11 regard to the potential ethanol production. During fermentation of biomass to ethanol carbon dioxide is released but the growth of the new biomass to maintain a supply of feedstock requires carbon dioxide as well (i.e. plants use CO2 to synthesize cellulose during photosynthesis cycle). Thus, ethanol represents a closed carbon dioxide cycle and no net CO2 is added to the atmosphere, making ethanol an environmentally beneficial energy source when produced in this way (Wyman and Hinman 1990; Chandel et al. 2007). Although U. lactuca and U. fusca have a high content of polysaccharides a large fraction of these sugars are pentoses. These two algal species contain also high amounts of rhamnose and a small amount of xylose which are both pentoses. In consequence the ethanol production did not reflect the overall potential of these macroalgae and a more suitable microorganism or a two-step fermentation could give higher results than the ones obtained. Alkaline/Oxidative pre-treatment did not increase the ethanol production even though it increased the sugar yield slightly. It can be enounced that this pre-treatment method was not suitable for the green macroalgae that were tested. It decreased the ethanol production by 57% and 135% for U.lactuca and U.fusca respectively. The largest and most interesting family of polysaccharides found in Ulva spp. is known as Ulvan and it has been researched for its potential uses and activities. The recent interest for ulvan arises from its unique physicochemical, rheological, and biological properties that are beginning to be described. Its unusual chemical composition and regular structure combining uronic acids, sulfate groups, and rare sugars, such as rhamnose and iduronic acid, are starting to be unraveled. Σελίδα 12 Ulvan’s potential activities are summarized below:  Anticoagulant activities (heparin like)  Antioxidant activities  Antihyperlipidemic activities  Antitumoral activities  Plant defense elicitor  Gelling properties  Plant nitrogen uptake improvement The study was based on the idea of an integrated biorefinery research concept according to which the algae go through the cultivation phase first and then the biomass in utilized as substrate either for enzymatic hydrolysis and ethanol fermentation or anaerobic digestion. Emphasis was given to the recycling of the products of each step thus reducing the need for fertilizers or extra carbon and maximizing the growth of the algae. Also the idea of incorporating algal biomass (e.g. from green tides) in biogas treatment plants in conjunction with organic waste was researched. The effluent digested manure/sludge can then be treated and utilized as fertilizer for further cultivation of the algal cultures. Finally the new biomass can be re-introduced to the system and so on.