On-line monitoring of photosynthetic activity based on pH data to assess microalgae cultivation

Characterization of Neochloris oleoabundans under Different Cultivation Modes and First Results on Bioactivity of Its Extracts against HCoV-229E Virus

Microalgae are proposed in several biotechnological fields because of their ability to produce biomass enriched in high-value compounds according to cultivation conditions. Regarding the health sector, an emerging area focuses on natural products exploitable against viruses. This work deals with the characterization of the green microalga Neochloris oleoabundans cultivated under autotrophic and mixotrophic conditions as a source of whole aqueous extracts, tested as antivirals against HCoV-229E (Coronaviridae family). Glucose was employed for mixotrophic cultures. Growth and maximum quantum yield of photosystem II were monitored for both cultivations. Algae extracts for antiviral tests were prepared using cultures harvested at the early stationary phase of growth. Biochemical and morphological analyses of algae indicated a different content of the most important classes of bioactive compounds with antiviral properties (lipids, exo-polysaccharides, and total phenolics, proteins and pigments). To clarify which phase of HCoV-229E infection on MRC-5 fibroblast cells was affected by N. oleoabundans extracts, four conditions were tested. Extracts gave excellent results, mainly against the first steps of virus infection. Notwithstanding the biochemical profile of algae/extracts deserves further investigation, the antiviral effect may have been mainly promoted by the combination of proteins/pigments/phenolics for the extract derived from autotrophic cultures and of proteins/acidic exo-polysaccharides/lipids in the case of mixotrophic ones.

Microalgal Biomass as Feedstock for Bacterial Production of PHA: Advances and Future Prospects

The search for biodegradable plastics has become the focus in combating the global plastic pollution crisis. Polyhydroxyalkanoates (PHAs) are renewable substitutes to petroleum-based plastics with the ability to completely mineralize in soil, compost, and marine environments. The preferred choice of PHA synthesis is from bacteria or archaea. However, microbial production of PHAs faces a major drawback due to high production costs attributed to the high price of organic substrates as compared to synthetic plastics. As such, microalgal biomass presents a low-cost solution as feedstock for PHA synthesis. Photoautotrophic microalgae are ubiquitous in our ecosystem and thrive from utilizing easily accessible light, carbon dioxide and inorganic nutrients. Biomass production from microalgae offers advantages that include high yields, effective carbon dioxide capture, efficient treatment of effluents and the usage of infertile land. Nevertheless, the success of large-scale PHA synthesis using microalgal biomass faces constraints that encompass the entire flow of the microalgal biomass production, i.e., from molecular aspects of the microalgae to cultivation conditions to harvesting and drying microalgal biomass along with the conversion of the biomass into PHA. This review discusses approaches such as optimization of growth conditions, improvement of the microalgal biomass manufacturing technologies as well as the genetic engineering of both microalgae and PHA-producing bacteria with the purpose of refining PHA production from microalgal biomass.

Advanced HRT-Controller Aimed at Optimising Nitrogen Recovery by Microalgae: Application in an Outdoor Flat-Panel Membrane Photobioreactor

A fuzzy knowledge-based controller of hydraulic retention time (HRT) was designed and tested in an outdoor membrane photobioreactor (MPBR) to improve nitrogen recovery from a microalgae cultivation system, maintaining the algae as photosynthetically active as possible and limiting their competition with other microorganisms. The hourly flow of the MPBR system was optimised by adjusting the influent flow rate to the outdoor environmental conditions which microalgae were exposed to at any moment and to the nitrogen uptake capacity of the culture. A semi-empirical photosynthetically active radiation (PAR) prediction model was calibrated using total cloud cover (TCC) forecast. Dissolved oxygen, standardised to 25 °C (DO25), was used as an on-line indicator of microalgae photosynthetic activity. Different indexes, based on suspended solids (SS), DO25, and predicted and real PAR, were used as input variables, while the initial HRT of each operating day (HRT0) and the variation of HRT (ΔHRT) served as output variables. The nitrogen recovery efficiency, measured as nitrogen recovery rate (NRR) per nitrogen loading rate (NLR) in pseudo-steady state conditions, was improved by 45% when the HRT-controller was set in comparison to fixed 1.25-d HRT. Consequently, the average effluent total soluble nitrogen (TSN) concentration in the MPBR was reduced by 47%, accomplishing the discharge requirements of the EU Directive 91/271/EEC.

A fungal immobilization technique for efficient harvesting of oleaginous microalgae: Key parameter optimization, mechanism exploration and spent medium recycling

To confront with energy crisis, microalgae as the promising feedstock have a great potential in exploring renewable energy field, whereas the high costs related to medium preparation and biomass harvesting are the main bottleneck to hinder the development on a large scale. Though cultivation of filamentous fungi for microalgae harvesting is an efficient, sustainable and emerging method, and the studies on specific mechanisms and spent medium recycling for efficiency improvement as well as resource saving through a co-pelletization mode are urgently needed. Hence, in this study, the harvesting process of autotrophic microalgae Chlorella vulgaris by pre-cultured Aspergillus oryzae pellets was investigated systematically. The highest efficiency (99.23%) was obtained within 5 h under the optimized conditions of 30 °C, 130 rpm and fungi:algae ratio of 1:1 on a dry weight basis without demand for pH adjustment (initial value on 9.68). Charge neutralization was not the main mechanisms involved in fungi-algae aggregations, and the functional group changes on cell surfaces as well as secreted metabolites in medium could be mainly responsible for inducing the bioflocculation process. After harvesting, separated water could also effectively support microalgae re-growth. The biomass concentration in medium with 50% recycling was higher than that in fresh medium, while lipid content was increased from 24.37% to 33.97% in fully recycled medium. These results indicated that the pellet-assisted mode for algal harvesting is a promising way to promote biofuel production and resource recycling.

Hydroponic Farm Wastewater Treatment Using an Indigenous Consortium

Hydroponic farms produce wastewater that need to be treated before being released into the environment. A three-step screening process (microplate, batch, and semi-continuous flasks experiments) initially designed to select an efficient microalgae strain allowed the isolation of a consortium that naturally developed in the hydroponic farm wastewater. During the non-optimized semi-continuous experiments, the best performing microalgae strain, Scenedesmus obliquus UTEX393 and the wastewater-born consortium cultures achieved good average linear growth rate (0.186 and 0.198/d, respectively) and high average nitrogen removal rates (23.5 mgN/L/d and 21.9 mgN/L/d, respectively). Phosphorus removal was very high probably due to precipitation. An integrated process was designed to treat the hydroponic farm wastewater using the wastewater-born consortium. Despite relatively low coagulation efficiencies in the preliminary tests, when integrated in a continuous process, chitosan was efficient to harvest the naturally wastewater-born consortium. The process was also efficient for removing nitrate and phosphate in less than seven days (average removal of 98.2 and 87.1% for nitrate and phosphate, respectively). These very promising results will help to define a pre-industrial pilot process.