https://orcid.org/0000-0002-9614-1371
![Sample our Environment & Agriculture journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5934/TOC-Subject-Sample-2021-Environment-Agriculture.jpg"})
Abstract
Programmed cell death (PCD) plays a key role in unicellular microalgal ecology. However, the methodologies for detecting PCD are problematic. Clearly, to interpret the empirical data, clarity on how to measure microalgal PCD is essential. Here, we critically review the current measurements of PCD and provide suggestions for future methodological developments and interpretations. We review the traditional measures of PCD and associated cellular responses in microalgae and provide assessments of their frequencies of use and true positive rates. Traditional physiological measurements of photosynthetic activity, change in gene regulation, measurements of reactive oxygen species and terminal deoxynucleotidyl transferase dUTP nick end labelling are highly sensitive assays. They provide important measures of cellular physiological responses but are not unique to PCD. Both caspase-like and metacaspase activity reveal useful information about stress responses and demonstrate high (94% and 100%, respectively) positivity rates, however, they can play a role in cell activities other than death. Furthermore, the controversy surrounding positive caspase assays, even though microalgae encode metacaspases rather than orthologous caspases, is highlighted. DNA laddering had the lowest true positive rate (64%) and was not reported in diatoms while phosphatidylserine externalization was consistently positive in all taxa except dinoflagellates. These data illustrate the limitations of some PCD markers across different taxa. Ultrastructural alterations (transmission electron microscopy) were highly correlated with PCD across all microalgal taxa (true positive rate of 94%) and seem essential for the initial assessments of whether a cell is dying in an organized, ‘programmed’ way. However, in the face of the complexity of PCD phenotypes and the non-specific nature of the methodologies, no single indicator can be used to diagnose PCD. Here, we highlight the importance of employing a time-sensitive multi-assay approach to detect PCD in the eukaryotic microalgae before any ecological or evolutionary interpretations can be made.
Highlights
Measurements of PCD have different specificities and sensitivities.
TEM appears essential as part of an initial investigation.
Complementary markers provide information about cell stress and death responses.
Acknowledgements
We are thankful to Prof. Odete Rocha, from the Department of Ecology and Evolutionary Biology, Federal University of Sao Carlos, and Prof. John Berges from Department of Biological Sciences, University of Wisconsin-Milwaukee for their valuable comments and suggestions. We also thank Karen van Niekerk from the University of Witwatersrand for very helpful comments on the caspase/metacaspases section. Two anonymous reviewers provided very constructive criticisms which significantly improved this paper.
Disclosure statement
No potential conflict of interest was reported by the authors.
Supplementary information
The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2022.2041731
Supplementary text 1. Overview of methodologies.
Supplementary data S1. Reports of potential programmed cell death processes in microalgae which were not included in supplementary data S2.
Supplementary data S2. Raw data.
Author contributions
MMBF led the writing of the manuscript. MMBF, ILB and PMD contributed equally to the data collection and analysis.
Data availability statement
The authors confirm that the data supporting the findings of this study are available within the article or its supplementary materials.