Continuous Production of Lipids with Microchloropsis salina in Open Thin-Layer Cascade Photobioreactors on a Pilot Scale

Abstract

Studies on microalgal lipid production as a sustainable feedstock for biofuels and chemicals are scarce, particularly those on applying open thin-layer cascade (TLC) photobioreactors under dynamic diurnal conditions. Continuous lipid production with Microchloropsis salina was studied in scalable TLC photobioreactors at 50 m2 pilot scale, applying a physically simulated Mediterranean summer climate. A cascade of two serially connected TLC reactors was applied, promoting biomass growth under nutrient-replete conditions in the first reactor, while inducing the accumulation of lipids via nitrogen limitation in the second reactor. Up to 4.1 g L−1 of lipids were continuously produced at productivities of up to 0.27 g L−1 d−1 (1.8 g m2 d−1) at a mean hydraulic residence time of 2.5 d in the first reactor and 20 d in the second reactor. Coupling mass balances with the kinetics of microalgal growth and lipid formation enabled the simulation of phototrophic process performances of M. salina in TLC reactors in batch and continuous operation at the climate conditions studied. This study demonstrates the scalability of continuous microalgal lipid production in TLC reactors with M. salina and provides a TLC reactor model for the realistic simulation of microalgae lipid production processes after re-identification of the model parameters if other microalgae and/or varying climate conditions are applied.

A Modeled High-Density Fed-Batch Culture Improves Biomass Growth and β-Glucans Accumulation in Microchloropsis salina

“…This phenomenon has also been observed in other types of microalgae, such as Chlorella zofingiensis , where fed-batch cultures are used to improve specific growth rates and biomass yield by overcoming inhibitory effects [ 42 ]. For example, in the study of Xu et al (2004) [ 21 ], they increased the dry biomass production of Nannochloropsis sp. by 40% by adding glucose to the culture medium, reaching 1.1 g/L of biomass over 10 days in fed-batch cultures versus 0.8 g/L in batch cultures.…”

“…Previous studies showed that Microchloropsis salina can accumulate more biomass, lipids, and β-glucans in a nitrogen-rich media than in a nitrogen-limited media [ 19 , 20 , 21 , 22 ]. In addition, the best growth rates and production of lipids, and β-glucans are achieved with the addition of a source of inorganic carbon, such as carbon dioxide at 2.0% ( v / v ) [ 21 , 23 , 24 , 25 ].…”

“…Previous studies showed that Microchloropsis salina can accumulate more biomass, lipids, and β-glucans in a nitrogen-rich media than in a nitrogen-limited media [ 19 , 20 , 21 , 22 ]. In addition, the best growth rates and production of lipids, and β-glucans are achieved with the addition of a source of inorganic carbon, such as carbon dioxide at 2.0% ( v / v ) [ 21 , 23 , 24 , 25 ]. In the case of lipid production, it has been shown that, in high-density batch cultures, the growth of the microorganism is inhibited due to high concentrations of substrate (nitrate and phosphate) [ 26 ].…”

“…and none of these improve its production parameters, either at a volumetric or specific level [ 12 ]. Despite the benefits that β-glucans have and could bring to the industry, M. salina has been used mainly to produce lipids for biofuel elaboration [ 21 ]. In summary, the novelty of the strategy of using a two-stage fed-batch culture lies in producing large amounts of carbohydrates.…”

Utilizing a CHP Power Plant’s Energy and CO2 Emissions for the Manufacture of Affordable and Carbon Neutral Algae Bioplastic for Re-Useable Packaging

“…For instance, at the base scale of 15 TLC reactors (121,360.97 kg Biomass AFW /year) for the mid-case performance, in scenarios 1, 2, and 3, the actual CO 2 sequestration was approximately 0.21% of the CHP plant’s CO 2 emissions (∼105.4 million kg/year) and it would only increase to roughly 2.1% if the algae biorefinery’s increased the number of TLC reactors to 150, which is a scale financially not sustainable for scenarios 2 and 3. In reality, algae CO 2 sequestration is limited by its biochemistry and environmental factors to around 2 g CO2 /g biomass . , Yet, the CO 2 conversion efficiency of TLCs compared to OPRs (13.0–20.0%) is much higher in comparison at approximately 76.0% to above 90.0% in certain conditions. ,− …”