Chloroplastida

Chloroplastida is a major lineage of eukaryotes that comprises the green plants and their closest green algal relatives. It is defined by plastids derived from a single primary endosymbiotic event with a cyanobacterium and is a central part of the larger Archaeplastida grouping. In many classifications, Chloroplastida is synonymous with Viridiplantae, though naming conventions can vary.

Taxonomy and composition: Chloroplastida includes two principal subdivisions: Chlorophyta, the green algae, and Streptophyta, which encompasses certain green algae (including the charophytes) and the land plants (embryophytes). This clade covers organisms ranging from unicellular algae to complex terrestrial plants.

Characteristics: Members possess plastids with chlorophylls a and b and store photosynthetic products as starch. Plastids are typically bounded by two membranes, reflecting their primary endosymbiotic origin. Cell walls are often cellulose-based, supporting a wide diversity of forms from simple unicells to complex multicellular structures. Life cycles among Chloroplastida vary, with many exhibiting an alternation of generations between haploid and diploid phases.

Evolution and ecology: The Chloroplastida lineage originated through primary endosymbiosis roughly 1.5 to 2 billion years ago. They are primary producers in freshwater, marine, and, in the case of land plants, terrestrial ecosystems, contributing substantially to global carbon fixation and oxygen production.

Examples and significance: Notable green algae include Chlamydomonas, Volvox, and Ulva, while land plants span bryophytes, ferns, gymnosperms, and flowering plants. The group is central to studies of photosynthesis, plant evolution, and ecological productivity. Chloroplastida is distinct from red algae and glaucophytes, which are placed in separate lineages within Archaeplastida.