Abstract
Algae are a heterogeneous group of protists with multiple phylogenetic origins (i.e., they are polyphyletic) that include all photosynthetic eukaryotes except land plants. Despite being a nonnatural assemblage, the term alga as referring to aquatic primary producers is valuable in an ecological context and has wide usage in biology. Recent genomic studies in algae have accelerated considerably our understanding of their ecology and evolution. The origin and phylogenetic interrelationships of different algal groups are being clarified using these molecular sequence data. In addition, the genomic age has broadened our understanding of the endosymbiotic origin of photosynthetic organelles (plastid) and their spread among eukaryotes. For example, recent genome-level analyses indicate that dominant marine algal groups, such as diatoms, haptophytes, and dinoflagellates, are descendants of a putative common ancestor that acquired a plastid from a red alga by secondary endosymbiosis, whereby one eukaryote enslaves the plastid of another. Genome data also have provided countless insights into algal metabolism, primary production, symbiosis, and algal ‘bloom’ formation in the world’s oceans. Additionally, algae offer exceptional models for understanding biodiversity, basic cell biological processes, the ecology of aquatic ecosystems, and genome evolution that spans the eukaryotic tree of life.