Algae are a heterogeneous assemblage of phytosynthetic organisms, one among the foremost vast and diverse groups of ancient photoautotrophic pro- and eukaryotic organisms (about 30,000 known species). They are micro- and macroscopic, unicellular, colonial, or multicellular, mobile and immobile, attached and free-living. Algae are widespread in water and soil habitats, at different geographic latitudes, and on all continents. They occur in waters with different degrees of salinity, trophicity, organic matter, and hydrogen ions, and at various temperatures. They include planktonic, periphytonic and benthic organisms. Algae are unique model organisms in evolutionary biology and are also utilized in various genetic, physiological, biochemical, cytological, and other investigations. Algae have practical significance as edible or poisonous plants, as indicator organisms within the monitoring of ecological systems, as agents of self-purification of polluted waters and within the purification of sewage, because the primary producers within the trophic chains of hydrobionts in marine and freshwater, and also as organisms for biotechnology. Marine-algae open access articles are the summary of current state of understanding on a particular research topic. Systematic reviews determine an objective list of criteria, and find all previously published original research papers that meet the criteria. They then compare the results presented in these papers.