The silicoflagellates (order Dictyochales) are a small group of unicellular photosynthetic , or algae, belonging to the supergroup of eukaryotes known as Stramenopiles. They behave as plankton and are present in seawater. 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
The silicoflagellates are unicellular
, 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.
These skeletons form a small component of
, and are well-known
dating as far back as the
early Cretaceous.
The silicoflagellates are considered algae due to being photosynthetic. Their 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.
Ecology
Silicoflagellates are
photosynthetic microscopic
algae present in the upper part of the
marine water 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
, in many parts of Europe. Blooms of silicoflagellates cause
fish mortality because their silica skeletons obstruct and abrade
, leading to asphyxiation and subsequent death. Additionally, some silicoflagellates are thought to produce
, 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.
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.
They were previously classified as
Chrysophyceae (golden algae).
Through morphological and molecular similarities, this group was eventually transferred to the
Dictyochophyceae, which contains three additional orders,
Pedinellales,
Florenciellales and Rhizochromulinales.
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.
Genera
There are four living genera:
Dictyocha,
Octactis,
Stephanocha (earlier known as
Distephanus,
homonymous with the
flowering plant Distephanus)
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.
There are also several extinct genera, but their classification is difficult, since skeletons may show diverse forms within each living species.
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.
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.
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.
