Sustainable saline microalgae co-cultivation for biofuel production: A critical review

Microalgae have gained increased attention as a viable, eco-friendly and alternative source of green bioenergy. To compete in the fuel market, saline microalgae cultivation for biofuel production would need to be economically sustainable and co-cultivation of saline microalgae using only saline water and recycled nutrient can potentially be the best solution to reduce the excessive use and prompt downsizing of natural resources like fresh water and fertilizers. This review provides a critical analysis on the selection of potential biofuel producing marine, halotolerant and halophilic microalgae. Here we proposed a microalgae co-cultivation strategy from seawater salinity (35ppt) to salt saturation (300ppt) with biofuel as the main output. We focused that adaptation of a co-cultivation strategy could reduce 95%, 74% and 51% of the overall nutrient waste compared to the monoculture of marine, halotolerant and halophilic microalgae. This paper also highlights a cultivation strategy using both mono and mixed culture over the period of increased saline condition and compares mass industrial-scale biofuel production from microalgae in three sites in Western Australia

The effect of gradual increase in salinity on the biomass productivity and biochemical composition of several marine, halotolerant, and halophilic microalgae

Open ponds are the preferred cultivation system for large-scale microalgal biomass production. To be more sustainable, commercial scale biomass production should rely on seawater, as freshwater is a limiting resource, especially in places with high irradiance. If seawater is used for both pond fill and evaporative volume makeup, salinity of the growth media will rise over time. It is not possible for any species to achieve optimum growth over the whole saline spectrum (from seawater salinity level up to salt saturation state). In this study, we investigated the effects of gradual salinity increase (between 35 and 233 ppt) on biomass productivity and biochemical composition (lipid and carbohydrate) of six marine, two halotolerant, and a halophilic microalgae. A gradual and slow stepped salinity increase was found to expand the salinity tolerance range of tested species. A gradual reduction in biomass productivity and maximum photochemical efficiency was observed as a consequence of increased salinity in all tested species. Among the marine microalgae, Tetraselmis showed highest biomass productivity (32 mg L−1 day−1) with widest salinity tolerance range (35 to 109 ppt). Halotolerant Amphora and Navicula were able to grow from 35 ppt to 129 ppt salinity. Halophilic Dunaliella was the only species capable of growing between 35 and 233 ppt and showed highest lipid content (56.2%) among all tested species. This study showed that it should be possible to maintain high biomass in open outdoor cultivation utilizing seawater by growing Tetraselmis, Amphora, and Dunaliella one after another as salinity increases in the cultivation system

Stepwise culture approach optimizes the biomass productivity of microalgae cultivated using an incremental salinity increase strategy

If our target is to produce low value commodity product, seawater needs to be used to generate sustainable microalgal biomass. In most areas of the world, open ponds are the preferred microalgal cultivation system. If seawater is used for filling up the pond and as make up for evaporative loss, the salinity of the growth media will gradually increase. Every saline microalga produces high biomass only within its optimal salinity range and the optimal salinity ranges of marine, halotolerant and halophilic microalgae are not continuous. The presence of non-optimal salinities reduces the overall biomass productivity under increased salinity. To achieve high biomass at non-optimal salinities, in present study, co-cultivation and stepwise cultivation of marine (Tetraselmis suecica), halotolerant (Amphora sp.) and halophilic (Dunaliella salina) microalgae were tested. Stepwise cultivation using T. suecica and Amphora sp. showed significantly higher biomass and lipid productivities (4.7% and 38.4%, respectively) compared to co-cultivation. Similarly, Amphora sp. and D. salina stepwise culture showed 54.8% extra lipid productivity compared to their co-culture. No significant difference was found between the maximum quantum yield of any of the treatments. Compared to co-cultivation, the outcome of this study confirmed that stepwise cultivation is a better way for maintaining the growth of selected species when salinity is increasing. This is indeed a suitable method for production of microalgal biomass and lipid when grown at suboptimal salinity zone. Further, stepwise process demonstrates the feasibility of a cultivation system based on seawater (to cultivate and replenish the pond), rather than potable fresh water

Co-cultivation and stepwise cultivation of Chaetoceros muelleri and Amphora sp. for fucoxanthin production under gradual salinity increase

In a seawater-based open pond microalgae cultivation system salinity will increase gradually over time due to evaporative loss. Continuous salinity increase would lead to non-optimal salinities which negatively affect the biomass and fucoxanthin productivity. To increase and maintain high overall biomass and fucoxanthin productivity, even in the non-optimal salinity zone, two cultivation methods for marine and halotolerant microalgae were carried out, co-cultivation and stepwise cultivation (sequential cultivation). Two fucoxanthin-producing diatoms, Chaetoceros muelleri (marine) and Amphora sp. (halotolerant), were cultivated at non-optimal salinities between 59 and 65‰. Stepwise cultivation showed approximately 63% higher total biomass and 47% higher fucoxanthin productivity than that of co-culture. The ability to reutilize culture media in the stepwise cultivation increases the sustainability of that method. The use of a stepwise culture regime, coupled with a regimen of gradually increasing salinity, provides the possibility of year round fucoxanthin production from microalgae

How harvesting frequency influence the biomass and lipid productivities of Nannochloropsis sp.

Nannochloropsis is an algal species with a worldwide biotechnological interest. To improve biomass and lipid productivities of this alga while grown in outdoor paddle-wheel driven raceway ponds, different harvesting frequencies were implemented. Four harvesting frequencies (10% harvested, five days (R10F5), 20% harvested, three days (R20F3), 30% harvested, two days (R30F2) and 40% harvested, one day (R40F1) a week) were tested to identify their potential effects on biomass and lipid productivity. The highest biomass productivity of 41.7 ± 1.9 mgL−1d−1 was achieved when Nannochloropsis was harvested five days a week (R10F5). In general, the biomass productivity of Nannochloropsis sp. was R10F5 > R20F3 > R30F2 > R40F1. However, the highest lipid content of 59.7 ± 1.8% was obtained with one day a week harvesting frequency (R40F1). The obtained lipid content was found to be up to 50.7% higher than other treatments (R40F1 > R10F5 = R20F3 = R30F2). Due to significantly higher biomass productivity and lipid content, R10F5 and R40F1, respectively showed higher lipid productivity than other treatments (R10F5 = R40F1 ≥ R20F3 = R30F2). Based on the results, it can be concluded that harvesting frequency influences the biomass productivity and lipid content of Nannochloropsis and proper management of harvesting frequency would help to achieve optimum targeted product (biomass and/or lipid). If the aim is to generate biomass, high frequency of harvest is preferred; however, sacrificing the biomass productivity by lowering the frequency can significantly enhance the lipid content. In contrast, if the ultimate aim is mass production of both biomass and lipid, frequent harvesting is the solution

Antidiabetic plants and formulations used by folk medicinal practitioners of two villages in Narail and Chuadanga districts, Bangladesh

Diabetes is a debilitating disease affecting millions of people worldwide including Bangladesh. Recent years have witnessed a significant rise of diabetes in both urban and rural areas of Bangladesh because of dietary habits and a more sedentary lifestyle. The disease has no known cure in modern allopathic medicine; in fact modern medicine merely attempts to control the symptoms of diabetes like increased blood sugar levels, and tries to mitigate the various other complicated problems that can arise out of diabetes like increased cardiovascular risks, diabetic retinopathy, diabetic neuropathy, and kidney failure, to name only a few. On the other hand, folk medicinal practitioners of Bangladesh have been treating diabetes for centuries and claim to have effective treatments for the disease as well as treatment for mitigating other symptoms arising from diabetes. The folk medicinal practitioners (known usually as Kavirajes) use both simple and complex formulations of medicinal plants for treatment of the disease. It was the objective of the present survey to conduct a survey among the Kavirajes of two randomly selected villages in Narail and Chuadanga districts, Bangladesh to collect information on medicinal plants and formulations used by the Kavirajes for treatment of diabetes. It was observed that the Kavirajes of the two villages surveyed used a total of 24 medicinal plants or plant parts for treatment. While some treatment formulations consisted of only a single medicinal plant, there were also complex formulations involving multiple plant parts. A perusal of the relevant scientific literature indicated that a number of the plants used by the Kavirajes for treatment of diabetes had been reported to possess antidiabetic properties. The plants reported in the present survey thus present considerable potential for further scientific research towards finding novel lead compounds for treatment of diabetes

Halo-adapted microalgae for fucoxanthin production: Effect of incremental increase in salinity

In order to commercially exploit microalgae for the production of fucoxanthin, species must remain productive in the increasingly saline environment typical in outdoor cultivation ponds. To this end, this study investigated the salinity range, growth, fucoxanthin content and productivity of two halotolerant and four marine microalgae under salinity increase condition. The semi-continuous cultivation followed by gradual salinity increase and slow adaptation helped saline microalgae to extend the salinity range up to 55%. Tested species showed about 12% to 90% more fucoxanthin content at their optimal salinity compared to when grown at non-optimal salinities. Fucoxanthin productivity was found directly linked to biomass productivity. Marine microalgae performed best at salinities 55‰. Among marine species, the highest fucoxanthin content and productivity was observed in Chaetoceros muelleri which was 2.92 mg g− 1 and 0.072 mg L− 1 d− 1 of ash free dry weight (AFDW), respectively, at 45‰ and fucoxanthin content was relatively consistent over the range of salinity between 35 and 55‰. Between two halotolerants, Amphora sp. showed the highest content and productivity of fucoxanthin which was 1.2 mg g− 1 and 0.053 mg L− 1 d− 1 of AFDW, respectively at 85‰ salinity. The results indicate that it is most likely possible to achieve continuous production of fucoxanthin by cultivating marine and halotolerant species one after another when salinity rises due to evaporation. Details of fucoxanthin content and productivity for Chrysotila carterae, Chaetoceros muelleri, Amphora sp. and Navicula sp. are reported for the first time

Medicinal plants used for preventive medicinal purposes: A survey in Muktipara village, Chuadanga district, Bangladesh

Bangladesh has over 5,000 floral species, approximately a fifth of which are considered as medicinal plants. Such plants form the major base of formulations prescribed by folk medicinal practitioners, otherwise known as Kavirajes, for treatment of diverse ailments. Kavirajes are present in almost every villages of the around 86,000 villages of the country and play a major role in providing primary health care to the rural residents. In our various ethnomedicinal surveys, we noticed that Kavirajes prescribe plants for not only curative but also for preventive purposes. Since such plants can play a major role in avoidance of diseases and so save health-care costs, the objective of the present survey was to document the various plants that were advised by the practicing Kaviraj of Muktipara village in Chuadanga district of Bangladesh. A total of 11 plants distributed into 11 families were found to be advised by the Kaviraj to village residents to be taken at different times of the year towards prevention of several common ailments. The various ailments against which the plants were prescribed included respiratory tract infections like coughs, colds and mucus, syndromes produced by excessive exposure to sun's heat, hemorrhoids, skin disorders, chicken pox, and flatulency or bloating, i.e. formation of excessive gas in the stomach. Interviews with the rural households of the village indicated that the plants do give positive effects for which the plants are advised to be taken, indicating that these plants can form as good sources of preventive medicine. Since prevention of disease is much more desirable than occurrence of a disease followed by its cure, the plants merit considerable potential for studies to establish them as functional foods

A review of scientific literature on anti-diabetic activity in medicinal plants used by folk medicinal practitioners of two villages in Narail and Chuadanga districts, Bangladesh for treatment of diabetes

An ethnomedicinal survey conducted amongst folk medicinal practitioners (Kavirajes) of two villages in Narail and Chuadanga districts of Bangladesh revealed the use of twenty four medicinal plant species for treatment of diabetes or alleviating diabetic symptoms in human patients. Perusal of the available scientific literature on the anti-diabetic plant species showed that out of the twenty four medicinal plants used, fifteen plant species have been reported in the scientific literature to possess considerable anti-diabetic properties directly in the form of ability to reduce blood sugar following administration, or possess properties, which can alleviate diabetic symptoms or reduce the risk factors for diabetes, including anti-oxidant and hypolipidemic properties. Overall, it can be concluded that the considerable expertise gained by the Kavirajes through longterm practice as well as passage of accumulated knowledge from generation to generation has practical validity and is not based on superstitions or myths about these plants. The close coincidences between the use of medicinal plants by the Kavirajes and their real time scientific validity studies suggest that these plants (some of which are yet to be scientifically studied) have considerable potential for discovery of novel anti-diabetic agents. Diabetes is a debilitating disease affecting millions of people worldwide including Bangladesh, and for which allopathic medicine has no known cure. As such, discovery of novel anti-diabetic agents, which even though they may not cause complete cure, but can alleviate various diabetic symptoms in a more efficient manner or reduce the risk factors leading to diabetes will be of considerable benefit to mankind. As such, these medicinal plants used by the Kavirajes need to be scientifically studied in a more rigorous manner, and efforts should be made to collect information from other Kavirajes in other villages and towns of Bangladesh on anti-diabetic plants and formulations for their uses

Plants prescribed for both preventive and therapeutic purposes by the traditional healers of the bede community residing by the turag river, dhaka district

The Bede community people are an indigenous community of Bangladesh and because of their preference for living and travelling on boats are also known as the river gypsies. They seldom reside for long at any given place but travel constantly on the waterways to ply their trade at various riverside villages. The community keeps mostly to themselves except when interacting with village people to sell sundry items, catch snakes, and practice their traditional medicinal system, which consists of cupping (drawing blood) and selling medicinal products containing plant, animal or fish parts. Medicinal plants form the chief item in their formulations for treatment of diverse ailments. The community has various professional groups within them based on their profession, among which one such group being the Dhanantari Vaidyas, who supposedly are able to cure all diseases. In recent times, the Government is trying to settle them by various riversides so as to enable them to get access to education and modern health care. The older generation, however, still prefer their traditional medicines to allopathic medicines. One such Bede group was located by the Turag River in Kamar Para village on the outskirts of Dhaka city in Dhaka district, Bangladesh. During the course of an ethnomedicinal survey among this community, it was observed that the Dhanantari Vaidyas advocated consumption of a number of plants in the cooked form for not only therapeutic but also preventive purposes. It was the objective of the present study to document the use of these plants. It was observed that 32 plant species were prescribed by the Vaidyas for regular consumption (preventive) or consumption during times when a particular disease occurred (therapeutic). These species were distributed into 20 families; the Fabaceae family contributed 4 species, while the Amaranthaceae, Asteraceae, and Cucurbitaceae families provided 3 plants each. Most of the plants were leafy vegetables with their leaves and stems prescribed for consumption following cooking; however, a few of them had their roots or fruits prescribed for consumption without cooking. Four plants (Allium cepa, Allium sativum, Capsicum frutescens and Coriandrum sativum) were used as accessories when cooking other plants. Among these four plants, Allium sativum was also used by itself for prevention and treatment of any type of dysentery and stomach problems, and Coriandrum sativum used by itself for prevention and treatment of gastrointestinal disorders and to increase appetite. Only two plant (Eclipta alba and Curcuma longa) parts were administered topically. Consumption of plants or plant parts for preventive purposes can be a cost-effective form of health care. As such, the plants prescribed for consumption by the Dhantari Vaidyas of the Bedes merit further scientific studies to assess their full curative potentials