Editing Chlorophyta (section)

== Description ==

Chlorophytes are [[eukaryotic]] organisms composed of cells with a variety of coverings or walls, and usually a single green [[chloroplast]] in each cell.{{Sfn|Adl|Bass|Lane|Lukeš|2019|p=36}} They are structurally diverse: most groups of chlorophytes are [[unicellular]], such as the earliest-diverging [[prasinophyte]]s, but in two major classes ([[Chlorophyceae]] and [[Ulvophyceae]]) there is an evolutionary trend toward various types of complex [[colonial organism|colonies]] and even [[multicellularity]].{{Sfn|Margulis|Chapman|2009|p=200}}

{{Plain image with caption|image=Chlorophyte-cell-diagram.png|align=left|width=300px|caption-position=bottom|caption=Simplified diagram of a chlorophyte cell combining structures seen across the phylum: 1) [[flagellum]]; 2) synistosome with fibers adhering to a pair of basal bodies; 3) [[basal body]]; 4) microtubular rootlet belonging to the "X-2-X-2" arrangement, in this case 4-2-4-2 (only front-facing rootlets are shown); 5) flagellar pit (only two of four flagella are shown); 6) [[rhizoplast]]s; 7) [[Golgi apparatus]]; 8) [[endoplasmic reticulum]] and [[nuclear envelope]]; 9) [[eyespot apparatus]]; 10) [[cell nucleus|nucleus]] with [[nucleolus]]; 11) [[pyrenoid]]; 12) [[mitochondrion]]; 13) [[starch]] granule; 14) [[vacuole]]; 15) outer [[chloroplast]] membrane; 16) inner chloroplast membrane; 17) [[thylakoid]]; 18) [[cell membrane]].}}

=== Chloroplasts ===

Chlorophyte cells contain green chloroplasts surrounded by a double-membrane envelope. These contain [[chlorophyll]]s ''[[chlorophyll a|a]]'' and ''[[chlorophyll b|b]]'', and the [[carotenoid]]s [[carotin]], [[lutein]], [[zeaxanthin]], [[antheraxanthin]], [[violaxanthin]], and [[neoxanthin]], which are also present in the [[leaves]] of [[land plant]]s. Some special carotenoids are present in certain groups, or are synthesized under specific environmental factors, such as [[siphonaxanthin]], [[prasinoxanthin]], [[echinenon]], [[canthaxanthin]], [[loroxanthin]], and [[astaxanthin]]. They accumulate carotenoids under nitrogen deficiency, high irradiance of sunlight, or high salinity.{{Sfn|Solovchenko|Merzlyak|Khozin-Goldberg|Cohen|2010}}{{Sfn|Lee|2018|p=309}} In addition, they store [[starch]] inside the chloroplast as [[carbohydrate]] reserves.{{Sfn|Margulis|Chapman|2009|p=200}} The [[thylakoid]]s can appear single or in stacks.{{Sfn|Adl|Bass|Lane|Lukeš|2019|p=36}} In contrast to other divisions of algae such as [[Ochrophyta]], chlorophytes lack a chloroplast endoplasmic reticulum.{{Sfn|Lee|2018|p=310}}

=== Flagellar apparatus ===

Chlorophytes often form [[flagellate]] cells that generally have two or four [[flagella]] of equal length, although in prasinophytes heteromorphic (i.e. differently shaped) flagella are common because different stages of flagellar maturation are displayed in the same cell.{{Sfn|Graham|Graham|Wilcox|Cook|2022|pp=16-15}} Flagella have been independently lost in some groups, such as the [[Chlorococcales]].{{Sfn|Margulis|Chapman|2009|p=200}} Flagellate chlorophyte cells have symmetrical cross-shaped ('cruciate') root systems, in which [[ciliary rootlet]]s with a variable high number of [[microtubule]]s alternate with rootlets composed of just two microtubules; this forms an arrangement known as the "X-2-X-2" arrangement, unique to chlorophytes.{{Sfn|Lewis|McCourt|2004|p=1537}} They are also distinguished from [[streptophyte]]s by the place where their flagella are inserted: directly at the cell apex, whereas streptophyte flagella are inserted at the sides of the cell apex (sub-apically).{{Sfn|Graham|Graham|Wilcox|Cook|2022|pp=16–10}}

Below the flagellar apparatus of prasinophytes are [[rhizoplast]]s, contractile muscle-like structures that sometimes connect with the chloroplast or the cell membrane.{{Sfn|Graham|Graham|Wilcox|Cook|2022|pp=16–15}} In core chlorophytes, this structure connects directly with the surface of the nucleus.{{Sfn|Yamashita|Baluška|2023|p=2}}

The surface of flagella lacks microtubular hairs, but some genera present scales or fibrillar hairs.{{Sfn|Lee|2018|p=309}} The earliest-branching groups have flagella often covered in at least one layer of scales, if not naked.{{Sfn|Graham|Graham|Wilcox|Cook|2022|pp=16–15}}

=== Metabolism ===

Chlorophytes and streptophytes differ in the enzymes and organelles involved in [[photorespiration]]. Chlorophyte algae use a [[dehydrogenase]] inside the [[mitochondria]] to process [[glycolate]] during photorespiration. In contrast, streptophytes (including land plants) use [[peroxisome]]s that contain [[glycolate oxidase]], which converts glycolate to [[glycoxylate]], and the hydrogen peroxide created as a subproduct is reduced by [[catalase]]s located in the same organelles.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=16-11}}

=== Reproduction and life cycle ===

[[Asexual reproduction]] is widely observed in chlorophytes. Among core chlorophytes, both unicellular groups can reproduce asexually through [[autospore]]s,{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=17-8}} wall-less zoospores,{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=17-11}} fragmentation, plain cell division, and exceptionally budding.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=17-9}} Multicellular thalli can reproduce asexually through motile zoospores,{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=18-8}} non-motile [[aplanospore]]s, autospores, filament fragmentation,{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=19-3}} differentiated resting cells,{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=18-19}} and even unmated gametes.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=18-29}} Colonial groups can reproduce asexually through the formation of autocolonies, where each cell divides to form a colony with the same number and arrangement of cells as the parent colony.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=19-14}}

Many chlorophytes exclusively conduct asexual reproduction, but some display [[sexual reproduction]], which may be [[isogamy|isogamous]] (i.e., [[gamete]]s of both sexes are identical), [[anisogamy|anisogamous]] (gametes are different) or [[oogamy|oogamous]] (gametes are sperm and egg cells), with an evolutionary tendency towards oogamy. Their gametes are usually specialized cells differentiated from [[vegetative cell]]s, although in unicellular [[Volvocales]] the vegetative cells can function simultaneously as gametes. Most chlorophytes have a [[Biological_life_cycle#Diplontic_life_cycle|diplontic life cycle]] (also known as zygotic), where the gametes fuse into a [[zygote]] which germinates, grows and eventually undergoes [[meiosis]] to produce [[haploid]] [[spore]]s (gametes), similarly to [[ochrophyte]]s and [[animal]]s. Some exceptions display a [[Biological_life_cycle#Haplodiplontic_life_cycle|haplodiplontic life cycle]], where there is an alternation of generations, similarly to land plants.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=16-13}} These generations can be isomorphic (i.e., of similar shape and size) or heteromorphic.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=18-14}} The formation of reproductive cells usually does not occur in specialized cells,{{Sfn|Lee|2018|p=318}} but some [[Ulvophyceae]] have specialized reproductive structures: gametangia, to produce gametes, and sporangia, to produce spores.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=18-14}}

The earliest-diverging unicellular chlorophytes (prasinophytes) produce walled resistant stages called [[microbial cyst|cysts]] or 'phycoma' stages before reproduction; in some groups the cysts are as large as 230 μm in diameter. To develop them, the flagellate cells form an inner wall by discharging mucilage vesicles to the outside, increase the level of lipids in the cytoplasm to enhance [[buoyancy]], and finally develop an outer wall. Inside the cysts, the nucleus and cytoplasm undergo [[cell division|division]] into numerous flagellate cells that are released by rupturing the wall. In some species these daughter cells have been confirmed to be gametes; otherwise, sexual reproduction is unknown in prasinophytes.{{Sfn|Graham|Graham|Wilcox|Cook|2022|p=16-17}}