| Chlorodendrales | |
|---|---|
 | |
| Tetraselmis suecica | |
Scientific classification | |
| Kingdom: | Plantae |
| Division: | Chlorophyta |
| Class: | Chlorodendrophyceae Massjuk |
| Order: | Chlorodendrales Melkonian |
| Families | |
Chlorodendrales are an order of green,flagellated,thecate, unicellulareukaryotes, within thegreen algae classChlorodendrophyceae.[1][2]Prasinophyceae are defined by their cellular scales which are composed ofcarbohydrates, and Chlorodendrales are unique within this group due to these scales forming a fused thecal wall.[1] Cells of Chlorodendrales are completely covered in scales, which fuse around the cell body producing the theca, but remain individually separated on the flagella, of which there are typically four per cell.[1] Species within Chlorodendrales live in both marine and fresh water habitats, occupying bothbenthic andplanktonic food webs.[1] Additionally, they arephotoautotrophs, meaning they produce their own food through the conversion of sunlight into chemical energy.
Species within Chlorodendrales are found inmarine andfreshwater ecosystems around the world, including locations such as thesalt plains ofGoa,India.[3] Species occupy niches within planktonic and benthic food webs, in which all species are photoautotrophic and have an ecosystem and trophic role similar toland plants in terrestrial environments.[4] Primary producers are consumed by primary consumers such aszooplankton,invertebratelarvae, andheterotrophicprotists species. Most species within this group are free-living, however some species have become photosyntheticsymbionts in animals; the animal species that can be in symbiosis with Chlorodendrales is limited, due to their need for sunlight for photosynthetic processes.Chlorodendrales populations in natural settings tend to experience extreme fluctuations in population numbers due to seasonal changes inabiotic conditions, such as ambient temperature, the amount of sunlight, and nutrient concentrations.[4] This leads to what has been classified as "blooms," which is the rapid increase in algae numbers during the spring and autumn months because of high light activity and the turnover of nutrients within the water column. It results from the high amount of sunlight, and the mixing of the water layers which resupplies the upper photosynthetic layer with nutrients, allowing for primary productivity to flourish.
Cell shape and size of Chlorodendrales cells varies depending on the species. The cells range greatly in size from species to species, with an upper limit of ~25 μm in length.[3] Cells can be round, ovoid, elliptical, flattened, or compressed; there is great diversity in Chlorodendrales cells.[3]
Chlorodendrales scales are unique from other Prasinophyceae lineages because the scales fuse to form a theca, which acts as an exterior protective casing for the cell.[1] All Prasinophyceae lineages produce these external scales within theGolgi apparatus, and secrete the scales via theendomembrane system.[1]Vesicles budding from the trans-Golgi face carry the scales to the cell surface, and upon fusion release the scales to the external cell face.[1] In Chlorodendrales lineages, these scales fuse after secretion to produce the thecal wall, in which individual scales bind by cross-linking to one another.[1]
The flagellar scales and the cell body scales are structurally and functionally different, the main difference being the fusion of cell-body scales.[1] The scales also differ in size, shape, and macromolecular composition.[1] The Golgi apparatus is only capable of producing one scale-type at a time, thus, species with multiple scale-types must have different phases of thecal development separated by time and space.[1] This also means that cell body and flagellar scale production occurs separately.[1]
The thecae of Chlorodendrales species are often unique, and it is an important character for species identification and classification.[1] Thecate features vary greatly from species to species, in which thecae can vary between 1-5 scale layers, in scale shape, in scale size, and in molecular composition/ultrastructure.[1] These thecate features are genetically determined, and thus, they are a consistent and reliable characteristic that will not be influenced by environmental factors.[1]
There are two genera within the order Chlorodendrales,Tetraselmis andScherffelia. Both species are green, photoautotrophs, flagellated, and thecate.[4] The difference between the two genera is the presence or absence ofpyrenoids;Tetraselmis contains pyrenoids andScherffelia does not.[4] Flagella within the two genera are distinct in composition and morphology from species to species, which can be used to infer taxonomy and identification.[1] Both genera produce three types of flagellar scales, in scale combination, and scale patterning, is unique between species. Scales differ mainly in composition and ultrastructure, and these are the main features examined from flagellar scales when identifying species.[1] The increase in knowledge about the morphological and ultrastructural features of flagellar scales, and the ability to identify species as a result, has been due to the development of advanced microscopy and staining techniques.[1]
Researchers have collected and analyzed molecular data from 13 prasinophyte taxa to better construct thephylogenetic relationship of early branching green alga.[5] Phylogenetic analysis of thesmall subunit ribosomal-RNA sequence was performed using distance,parsimony, andlikelihood statistical tests.[5] The analyses showed four independent prasinophyte groups, in which these lineages represented the earliest divergences among theChlorophyta.[5] The most parsimonious tree created suggests that the Chlorodendrales lineage is a very early diverging group of "core Chlorophytes," of which there are fourclades.[5] However, the diverging order of this group remains unclear.[5]