Growth of Dunaliella viridis in multiple cycles of reclaimed media after repeated high pH-induced flocculation and harvesting

Enhancing microalgae cell inactivation through hydrodynamic cavitation: Insights from flow cytometry analysis

Hydrodynamic cavitation (HC) has recently emerged as an effective method for disrupting microalgae cells with lower energy consumption compared to traditional mechanical methods like bead milling and ultrasonication. However, the efficiency and energy utilization of HC can vary depending on the microalgae species and operating conditions. To assess the efficacy of HC and quantify the extent of cellular damage at the cell level, we designed a bench-top cavitation system to investigate the time-dependent responses of the microalgae Dunaliella viridis to multiple HC passes. We evaluated cell disruption efficiency by monitoring cell concentration using a cell counter and analyzing cell size distribution and whole cell counts via flow cytometry. Additionally, we assessed cell viability, metabolic activity, and reactive oxygen species (ROS) levels using the fluorescent probes fluorescein diacetate (FDA), erythrosine B (EB), and 2',7'-dichlorofluorescein diacetate (DCFDA). We further analyzed cell chlorophyll autofluorescence and the kinetics of overall cell inactivation. Our results demonstrated that HC effectively disrupted and inactivated cells, approximating pseudo-first-order kinetics. However, the cell inactivation rate and energy utilization efficiency declined rapidly in the early stages, likely due to the accumulation of cell debris. A P-factor model incorporating two first-order rate constants was then developed to better predict the cell inactivation kinetics and further demonstrated that cavitation number alone was insufficient to characterize the dynamic change in the inactivation rate during cavitation. To maintain high inactivation efficiency, it is recommended to keep the cell debris fraction below 10-20 %. HC was found to inactivate cells by rupturing cell membranes, leading to the rapid release of intracellular contents such as esterase and chlorophyll. HC did not affect intracellular esterase activity or chlorophyll content in cells with intact membranes, but the endogenous ROS levels in viable cells were reduced. The mechanisms of cell damage were discussed in detail. For bioproduct harvesting, cell membrane integrity is suggested as a key physiological endpoint for optimizing HC treatment protocols.

Increasing the sustainability of photoautotrophic microalgae production by minimising freshwater requirements

There are now several companies that are producing microalgae such as Arthrospira platensis, Chlorella vulgaris, and Dunaliella salina, among others. They are cultivated mainly in large-scale raceway and tubular photobioreactors. Microalgae production represents a sustainable alternative to conventional biomass production. Microalgae can be used to manufacture agricultural products, animal feed, food and other commercial products. The water requirements for cultivating microalgae are significant, exceeding 1 m3·kg-1. This value varies depending on the production strategy. One of the main reasons for water loss is evaporation, which is influenced by the photobioreactor location, the season, and the operating conditions. Efforts are being made to reduce water requirements and make microalgae production economically viable and more environmentally friendly. Several strategies are being investigated for reducing freshwater use in microalgae cultivation; these include reusing the culture medium and producing microalgae using seawater or wastewater. Such strategies not only reduce water consumption, but also reduce nutrient consumption and costs while increasing sustainability.

Microplate-Based Cell Viability Assay as a Cost-Effective Alternative to Flow Cytometry for Microalgae Analysis

Cell viability is a critical indicator for assessing culture quality in microalgae cultivation for biorefinery and bioremediation. Fluorescent dyes that distinguish viable from nonviable cells can enable viability quantification based on the percentage of live cells. However, fluorescence analysis using the typical flow cytometry method is costly and impractical for industrial applications. To address this, we developed new microplate assays utilizing fluorescein diacetate as a live cell stain and erythrosine B as a dead cell stain. These assays provide a low-cost, simple, and reliable method of assessing cell viability. The proposed microplate assays were successfully applied to monitor the viability of the microalgae Dunaliella viridis under carbon and nitrogen limitation stresses and demonstrated good agreement with flow cytometry measurements. We conducted a systematic investigation of the effects of dye concentration, incubation time, and background fluorescence on the microplate assays' performance. Further, we provide a comprehensive review of commonly used fluorescent dyes for microalgae staining, discuss strategies to enhance assay performance, and offer recommendations for dye selection and protocol development. This study presents a comprehensive new method for microplate-based viability analysis, providing valuable insights for future microalgae viability assessments and applications.