Dictyochales

The silicoflagellates (order Dictyochales) are a small group of unicellular photosynthetic protists, or algae, belonging to the supergroup of eukaryotes known as Stramenopiles. They behave as plankton and are present in oceanic waters. They are well-known from harmful algal blooms that cause high mortality of fish. Additionally, they compose a rich fossil record represented by their silica skeletons.

Morphology

File:The silicoflagellate Dictyocha fibula.png

Dictyocha fibula

The silicoflagellates are unicellular protists, composed of cells with one emergent flagellum and a siliceous skeleton constructed from a network of hollow rods outside of the cytoplasm. The morphology of the skeleton can vary greatly, from a simple ring, an ellipse or triangle, to a more complex and complete arrangement of rods. For example, in Dictyocha fibula (pictured) the skeleton rods are arranged in a series of peripheral polygons surrounding a central hexagon.<ref name="Lee Dictyochophyceae"/> These skeletons form a small component of marine sediments, and are well-known microfossils dating as far back as the early Cretaceous.<ref name="Gleserocha"/>

The silicoflagellates are considered algae due to being photosynthetic. Their chloroplasts are usually present inside the numerous cytoplasmic processes that extend from the central mass of the cytoplasm, where the cell nucleus is located. There is a possibility that silicoflagellate chloroplasts are derived from haptophyte algae through tertiary endosymbiosis.<ref name="Lee Dictyochophyceae"/>

Ecology

Silicoflagellates are photosynthetic microscopic algae present in the upper part of the marine water column, as plankton. They are adapted to both warm and cold waters. Similarly to diatoms, silicoflagellates are most productive where high levels of silica and nutrients are present in near-surface waters. They are known to cause harmful algal blooms, or red tides, in many parts of Europe. Blooms of silicoflagellates cause fish mortality because their silica skeletons obstruct and abrade fish gills, leading to asphyxiation and subsequent death. Additionally, some silicoflagellates are thought to produce ichthyotoxins, i.e. substances toxic to fish, although their effect is debated. Apart from physical damage to fish, the depletion of dissolved oxygen in water due to their cellular respiration during bloom growth has also caused fish mortality in fisheries and aquaculture.<ref name="Ajuzie 2015"/>

Systematics

Taxonomy

In biological classification, silicoflagellates compose the family Dictyochaceae (in botanical nomenclature) or Dictyochidae (in zoological nomenclature), contained within the order Dictyochales or Silicoflagellata.<ref name="AIGP Silicoflagellata"/><ref name="Lee Dictyochophyceae"/> They were previously classified as Chrysophyceae (golden algae).<ref name="AIGP Silicoflagellata"/> Through morphological and molecular similarities, this group was eventually transferred to the Dictyochophyceae, which contains three additional orders, Pedinellales, Florenciellales and Rhizochromulinales.<ref name="Florenciellales"/> After decades of phylogenetic analyses, silicoflagellates and the rest of Dictyochophyceae are accepted as a class of the phylum Ochrophyta, within the eukaryotic supergroup Stramenopiles.<ref name="Adl 2019"/>

Genera

There are four living genera: Dictyocha, Octactis, Stephanocha (earlier known as Distephanus,<ref name="Extant Distephanus"/> homonymous with the flowering plant Distephanus)<ref name="Stephanocha"/> and Vicicitus, with 11 recognised living species. Dictyocha was previously the only member of the order, until Vicicitus was created for a species previously assigned to the raphidophyte Chattonella but later proven to be a Dictyochophyceae through phylogenetic analyses.<ref name="Vicicitus"/><ref name="Revision Dictyochales"/>

• Dictyocha Template:Au — 4 spp.<ref name="Hernández-Becerril 2001"/>
• Octactis Template:Au — 2 spp.<ref name="Revision Dictyochales"/>
• Stephanocha Template:Au (=Distephanus Template:Au)<ref name="Stephanocha"/> — 4 spp.
• Vicicitus Template:Au — 1 sp.<ref name="Vicicitus"/>

There are also several extinct genera, but their classification is difficult, since skeletons may show diverse forms within each living species.<ref name="McCartney 2011b"/><ref name="Gleserocha"/>

• †Arctyocha Template:Au — 3 spp.
• †Corbisema Template:Au — 5 spp.
• †Cornua Template:Au — 6 spp.<ref name="McCartney 2011b"/><ref name="Gleserocha"/>
• †Gleserocha Template:Au — 3 spp.<ref name="Gleserocha"/>
• †Lyramula Template:Au — 2 spp.
• †Schulzyocha Template:Au — 5 spp.<ref name="McCartney 2011b"/>
• †Umpiocha Template:Au — 2 spp.<ref name="McCartney 2011b"/>
• †Vallacerta Template:Au — 5 spp.
• †Variramus Template:Au — 2 spp.<ref name="McCartney 2011b"/>

Evolution

The fossil record of silicoflagellates extends back to early Albian times, in the Early Cretaceous, around 115 million years ago. However, data on Late Cretaceous and Paleocene silicoflagellate evolution is sparse, and they are best known from the Eocene to Recent era.<ref name="Gleserocha"/> Silicoflagellate skeletons from the Cretaceous are markedly different from Cenozoic ones: before the Santonian (around 85 million years ago) they presented branched, non-ringed shapes.<ref name="McCartney 2022"/> During the Cenozoic, silicoflagellates show an evolutionary trend towards more complicated apical and basal structures, resulting in a design that resembles a hemisphere. As a consequence, after mitosis, the pair of daughter cell skeletons resembles a near-sphere.<ref name="McCartney 2022"/>