Microalgal bioinoculants for sustainable agriculture and their interactions with soil biotic and abiotic components: A review

Modern agricultural practices have posed a detrimental impact on the environment due to their intensive use to meet the food demands of an ever-increasing population. In this context, microalgal bioinoculants, specifically cyanobacteria and green microalgae, have emerged as sustainable options for agricultural practices to improve soil organic carbon, nutrient availability, microbial quality, and plant productivity. An overview of current and future perspectives on the use of microalgal bioinoculants in agriculture practices is presented in this review, along with a discussion of their interactions with soil biotic and abiotic factors that affect soil fertility, plant health, and crop productivity. The benefits of microalgal bioinoculants include releasing agronomically important metabolites (exopolymers and phytohormones) as well as solubilizing soil nutrients. Furthermore, they function as biocontrol agents against soil-borne pathogens and facilitate the establishment of rhizosphere communities of agricultural importance. So far, very few studies have explored the basic mechanisms by which microalgal bioinoculants interact with soil biotic and abiotic factors. In recent years, advanced molecular techniques have contributed to a better understanding of these interactions.     

Rhodovulum sulfidophilum,a phototrophic bacterium,grown in palm oil mill effluent improves the larval survival of marble goby Oxyeleotris marmorata (Bleeker)

Survival of marble goby larvae fed either Rhodovulum sulfidophilum, a phototrophic bacterium cultured from palm oil mill effluent (pPB), or microalgae ( Nannochloropsis sp.) was evaluated at two salinities. Larvae directly fed pPB had survival of 0–29% at 5 g L^?1 salinity and 0–19% at 10 g L^?1 salinity, whereas larvae directly fed microalgae suffered complete mortality after 20 days of culture at both salinities. However, larvae indirectly fed pPB or microalgae, i.e. via rotifers (Days 1–30) and Artemia nauplii (Days 21–30) cultured solely from pPB or microalgae, showed improved survival of 35–55% or 44–49% at 5 g L^?1 salinity respectively. In all experiments, fish larvae reared at 5 g L^?1 salinity showed significantly higher (P < 0.01) mean survival than those reared at 10 g L^?1 salinity. The survival of larvae fed the bacterial‐based diet was higher compared with microalgal diet used in previous studies. The pPB had higher total polyunsaturated fatty acids and docosahexaenoic acid (DHA) than the microalgae, which had very high eicosapentaenoic acid (EPA). Larvae with very high ratios of DHA/EPA (>11) or/and ARA (arachidonic acid)/EPA (>5), attributable to their given diet, however suffered the highest mortality.     

Dietary effect of squid and fish meals on growth and survival of Pacific white shrimp Litopenaeus vannamei in the presence or absence of phytoplankton in an indoor tank system

This study was conducted in 36 indoor 40‐L fibreglass aquaria to determine the weight gain and survival of Litopenaeus vannamei fed different dietary levels of fish (FM) and squid meal (SM) and to evaluate the potential of phytoplankton to reduce FM and SM levels in shrimp feeds. Six experimental isonitrogenous (35% protein) and isocaloric (17.5 kJ g^?1) diets were formulated to contain either 5%, 10% or 20% SM combined with either 6.5% or 12% FM. Dietary effects on growth and survival were compared in two systems: a ‘CLEAR water system’ (CWS) without the presence of microalgae and a ‘GREEN water system’ (GWS) with microalgae in the culture water. Shrimp cultured in the GWS had 28–57% greater weight gain than those cultured in the CWS, regardless of dietary treatment. However, survival was not different. Shrimp cultured in the CWS or the GWS, and fed diets containing combinations of FM and SM did not show differences in weight gain and feed conversion ratio. These results suggest that under the conditions existing during this research, 6.5% FM and 5% SM can be used as a cost‐effective combination in commercial feeds for shrimp production and that growth can be enhanced in the presence of primary productivity.     

鸡粪发酵液培养的小球藻水热液化制备生物原油及其特性

为探索沼液资源再利用,以鸡粪沼气发酵液培养的小球藻为原料,采用水热液化技术制备生物原油。采取正交试验,在温度250~330℃、时间30~90 min及含固量15%~25%下,探讨了水热反应后各相产物特性及元素回收效率。生物原油产率为13.23%~23.83%,最高产油率在330℃、60 min、15%时取得。生物原油中碳、氢及氮回收率分别是16.13%~31.14%、19.18%~34.89%及5.97%~14.32%,最高碳回收率及最低氮回收率分别在330℃、60 min、15%及250℃、30 min、15%时获得。水热液化各相产物中,碳、氢及氮回收率在水相中占主导地位,分别为48.74%~60.43%、46.81%~62.13%及74.84%~82.67%。热重分析暗示生物原油可能适合制备润滑油。此外,GC-MS分析表明生物原油中烃类物质质量分数为16.14%~24.91%,主要为低碳链烃类,如甲苯及二氢茚等。     

3种海洋微藻ω-3脂肪酸去饱和酶基因表达影响因素分析

研究了温度、光照和酸碱度对三角褐指藻、小球藻和球等鞭金藻-ω3脂肪酸去饱和酶基因表达的影响。实时定量PCR结果表明,10和15℃的低温处理,降低了3种微藻ω-3脂肪酸去饱和酶基因的表达丰度,提高处理温度至25和30℃(三角褐指藻除外),则促进该基因的转录;在5 000 lx的光照强度下处理48和72 h,可以提高3种藻类的ω-3脂肪酸去饱和酶基因的表达;酸碱度对3种微藻-ω3脂肪酸去饱和酶基因的表达影响与不同的物种有关,pH 5~9的培养结果显示,酸碱度对三角褐指藻影响较大,球等鞭金藻次之,而对小球藻影响较小。     

紫外诱变热带微藻选育高油脂藻株

为选育高含油量微藻藻种,本试验以优势藻株La4-37为原材料,采用紫外线辐射法对其进行诱变处理,获得了296株藻株。利用尼罗红荧光检测法对获得的藻株进行荧光检测,筛选获得相对含油量最大的诱变藻株M077和M040。通过对诱变株生长及脂荧光强度动态跟踪发现,诱变株生长周期和油脂积累时期基本一样,当达到平稳期时,诱变株油脂积累能力均有较大提高,脂荧光强度分别是原始藻株的6.2倍和1.7倍。     

Microalgal bacterial flocs originating from aquaculture wastewater treatment as diet ingredient for Litopenaeus vannamei (Boone)

Microalgal bacterial flocs (MaB‐flocs) in sequencing batch reactors are a novel and promising technology to treat aquaculture wastewater. To improve the economics of this technology, the harvested MaB‐flocs should be valorized. Therefore, we investigated if MaB‐flocs from an outdoor pilot reactor treating pikeperch (Sander lucioperca L.) wastewater can be used as a colour enhancing ingredient for shrimp diets. Considering the nutritional composition and high ash content of MaB‐flocs, five iso‐nitrogenous and iso‐lipidic shrimp diets were formulated by replacing 2–8% of the basal diet ingredients by MaB‐flocs. These diets were continuously fed to juvenile Pacific white shrimps Litopenaeus vannamei (Boone) cultured in a hybrid recirculating aquaculture system. The addition of these relatively low amounts of MaB‐flocs to shrimp diets significantly increased the pigmentation (redness and yellowness) of cooked shrimp tails. The diet modifications did not affect the shrimp survival, weight gain, size distribution and food conversion rate, nor did they affect the proximate composition and fatty acid profile of the raw shrimp muscle. This shows that MaB‐flocs originating from treatment of pikeperch wastewater can substitute 8% of diet ingredients of Pacific white shrimp while enhancing its pigmentation.     

A partial substitution of microalgae with single cell detritus produced from seaweed (Porphyra haitanensis) for the nursery culture of tropical oyster (Crassostrea belcheri)

Oyster seed production in hatchery and nursery systems has relied on the production of microalgae, which is cost effective, but often unpredictable. The development of a satisfactory substitute diet would greatly reduce production costs of hatchery operations. Single cell detritus (SCD) production from seaweed (Porphyra haitanensis) may have great potential as a partial substitute for microalgae. In this experiment, a technique for producing SCD was developed and the product was used as a partial substitute for microalgae for the nursery culture of juvenile tropical oysters (Crassostrea belcheri) in a hatchery setting. A reverse osmosis technique was used on ground seaweed (particle size <200 μm) immersed in freshwater and placed on a shaker at 100 rpm for 2 h before being returned to seawater. This technique was shown to be highly effective for SCD production with a density of 33.7 ± 7.0 × 10⁴ cell mL^?1 and the percentage of SCD particle diameter <20 μm was 89.2%. A partial replacement of microalgae with SCD was found to be unsuitable for nursing oyster larvae. For juvenile oysters (shell width 1.85 ± 0.03 mm and shell length 1.78 ± 0.06 mm) substituting 75% of microalgae with SCD showed lower absolute shell growth, and lower daily yields and survival rates when compared to rates substituting 50% or lower substitution with SCD, or 100% microalgae (P < 0.05). It was found that substituting 50% of the traditional microalgae with SCD produced from seaweed (P. haitanensis) can be used as a partial microalgae substitute for the nursery culture of the juvenile tropical oyster.     

Nutrient efficacy of microalgae as aquafeed additives for the adult black tiger prawn,Penaeus monodon

This study used a small amount of several lyophilized microalgae (5% of dry weight in aquafeed) to feed adult black tiger prawns Penaeus monodon for 4 weeks, aiming to promote prawn health and nutritional level. Apart from slight increases in ash content, chlorophyll and β‐carotene in the microalgae‐added feeds, there was no difference from the commercial diet in their biochemical composition. After 28 days of feeding, prawn body length, body weight, condition index and also total amount of fatty acids and amino acids of the flesh (2nd abdominal segment) were not affected by microalgae additives as compared with the control commercial diet‐fed group (P > 0.05). Prawns fed the microalgae diets, however, had a relatively better survival rate than those fed the commercial control diet, although no statistical significance was detected in any except for the diatom Melosira sp. The elevated survival rate was concomitant with a higher phagocytosis rate and superoxide dismutase (SOD) activity in the immune response analysis, and significantly a higher astaxanthin content detected in the tail muscle of microalgae diet‐fed prawns (5.36–10.78 ng of astaxanthin per gram of muscle DW). These findings demonstrated that low‐inclusion microalgal additives in prawn diets over a short feeding period (4 weeks) can result in healthier adult prawn, and hence could contribute to the development of an optimized feeding strategy for prawn aquaculture close to harvesting.     

Microalgae as Contributors to Produce Biopolymers

Biopolymers are very favorable materials produced by living organisms, with interesting properties such as biodegradability, renewability, and biocompatibility. Biopolymers have been recently considered to compete with fossil-based polymeric materials, which rase several environmental concerns. Biobased plastics are receiving growing interest for many applications including electronics, medical devices, food packaging, and energy. Biopolymers can be produced from biological sources such as plants, animals, agricultural wastes, and microbes. Studies suggest that microalgae and cyanobacteria are two of the promising sources of polyhydroxyalkanoates (PHAs), cellulose, carbohydrates (particularly starch), and proteins, as the major components of microalgae (and of certain cyanobacteria) for producing bioplastics. This review aims to summarize the potential of microalgal PHAs, polysaccharides, and proteins for bioplastic production. The findings of this review give insight into current knowledge and future direction in microalgal-based bioplastic production considering a circular economy approach. The current review is divided into three main topics, namely (i) the analysis of the main types and properties of bioplastic monomers, blends, and composites; (ii) the cultivation process to optimize the microalgae growth and accumulation of important biobased compounds to produce bioplastics; and (iii) a critical analysis of the future perspectives on the field.