Functional and phenotypic flow cytometry characterization of Picochlorum soloecismus

Sub-lethal toxicity of five disinfection by-products on microalgae determined by flow cytometry - Lines of evidence for adverse outcome pathways

Standardised tests are often used to determine the ecotoxicity of chemicals and focus mainly on one or a few generic endpoints (e.g. mortality, growth), but information on the sub-cellular processes leading to these effects remain usually partial or missing. Flow cytometry (FCM) can be a practical tool to study the physiological responses of individual cells (such as microalgae) exposed to a stress via the use of fluorochromes and their morphology and natural autofluorescence. This work aimed to assess the effects of five chlorine-based disinfection by-products (DBPs) taken individually on growth and sub-cellular endpoints of the green microalgae Raphidocelis subcapitata. These five DBPs, characteristic of a chlorinated effluent, are the following monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), bromochloroacetic acid (BCAA) and 1,1-dichloropropan-2-one (1,1-DCP). Results showed that 1,1-DCP had the strongest effect on growth inhibition (EC50 = 1.8 mg.L-1), followed by MCAA, TCAA, BCAA and DCAA (EC50 of 10.1, 15.7, 27.3 and 64.5 mg.L-1 respectively). Neutral lipid content, reactive oxygen species (ROS) formation, red autofluorescence, green autofluorescence, size and intracellular complexity were significantly affected by the exposure to the five DBPs. Only mitochondrial membrane potential did not show any variation. Important cellular damages (>10%) were observed for only two of the chemicals (BCAA and 1,1-DCP) and were probably due to ROS formation. The most sensitive and informative sub-lethal parameter studied was metabolic activity (esterase activity), for which three types of response were observed. Combining all this information, an adverse outcome pathways framework was proposed to explain the effect of the targeted chemicals on R. subcapitata. Based on these results, both FCM sub-cellular analysis and conventional endpoint of algal toxicity were found to be complementary approaches.

Estimation of Microbial Viability Using Flow Cytometry

For microorganisms in particular, viability is a term that is difficult to define and a state consequently difficult to measure. The traditional (and gold standard) usage equates viability and culturability (i.e., the ability to multiply) but the process of determining culturability is often too slow. Flow cytometry provides the opportunity to make rapid and quantitative measurements of dye uptake in large numbers of cells and we can therefore exploit the flow cytometric approach to evaluate so-called viability stains and to develop protocols for more routine assessments of microbial viability. This article provides a commentary and several protocols have been included to ensure that users have a firm basis for attempting these reasonably difficult assays on traditional flow cytometer instruments. What is clear is that each assay must be carefully validated with the particular microorganism of interest before being applied in any research, clinical, or service form. © 2020 The Authors.

Inhibition of DNA Methylation in Picochlorum soloecismus Alters Algae Productivity

Eukaryotic organisms regulate the organization, structure, and accessibility of their genomes through chromatin remodeling that can be inherited as epigenetic modifications. These DNA and histone protein modifications are ultimately responsible for an organism's molecular adaptation to the environment, resulting in distinctive phenotypes. Epigenetic manipulation of algae holds yet untapped potential for the optimization of biofuel production and bioproduct formation; however, epigenetic machinery and modes-of-action have not been well characterized in algae. We sought to determine the extent to which the biofuel platform species Picochlorum soloecismus utilizes DNA methylation to regulate its genome. We found candidate genes with domains for DNA methylation in the P. soloecismus genome. Whole-genome bisulfite sequencing revealed DNA methylation in all three cytosine contexts (CpG, CHH, and CHG). While global DNA methylation is low overall (∼1.15%), it occurs in appreciable quantities (12.1%) in CpG dinucleotides in a bimodal distribution in all genomic contexts, though terminators contain the greatest number of CpG sites per kilobase. The P. soloecismus genome becomes hypomethylated during the growth cycle in response to nitrogen starvation. Algae cultures were treated daily across the growth cycle with 20 μM 5-aza-2'-deoxycytidine (5AZA) to inhibit propagation of DNA methylation in daughter cells. 5AZA treatment significantly increased optical density and forward and side scatter of cells across the growth cycle (16 days). This increase in cell size and complexity correlated with a significant increase (∼66%) in lipid accumulation. Site specific CpG DNA methylation was significantly altered with 5AZA treatment over the time course, though nitrogen starvation itself induced significant hypomethylation in CpG contexts. Genes involved in several biological processes, including fatty acid synthesis, had altered methylation ratios in response to 5AZA; we hypothesize that these changes are potentially responsible for the phenotype of early induction of carbon storage as lipids. This is the first report to utilize epigenetic manipulation strategies to alter algal physiology and phenotype. Collectively, these data suggest these strategies can be utilized to fine-tune metabolic responses, alter growth, and enhance environmental adaption of microalgae for desired outcomes.

High-Quality Draft Genome Sequence of the Green Alga Tetraselmis striata (Chlorophyta) Generated from PacBio Sequencing

A high-quality draft genome sequence of the microalgal species Tetraselmis striata was generated using PacBio sequencing. The assembled genome is 228 Mb, derived from 3,613 polished contigs at 84× coverage depth. This genome contains an average GC content of 57.9% and 48,906 predicted genes.

A high-quality draft genome sequence of the microalgal species Tetraselmis striata was generated using PacBio sequencing. The assembled genome is 228 Mb, derived from 3,613 polished contigs at 84× coverage depth. This genome contains an average GC content of 57.9% and 48,906 predicted genes.