Skeletonema costatum is a cosmopolitan centric diatom that belongs to the genus Skeletonema. It was first described by R. K. Greville, who originally named it Melosira costata, in 1866. It was later renamed by Cleve in 1873 and was more narrowly defined by Zingone et al. and Sarno et al. Skeletonema costatum is the most well known species of the genus Skeletonema and is often one of the dominant species responsible for red tide events. The diatom S. costatum is known for its carbon acquisition mechanisms, and it has been used in the production of biofuel and as a feed for aquaculture. The organism is appealing for commercial use due to its high photosynthetic efficiency, high tolerance to pH, temperature, and salinity changes, high lipid and fatty acid content, and rapid growth rate.
Structure and morphology
Cells belonging to S. costatum are single-celled but exist as long chains of about 6 to 24 cells but can be up to 60 cells in length. As with all diatoms, the siliceous cell wall (frustule) consists of two interlocking components ("like two halves of a petri dish"), the hypotheca and the epitheca. Each cell is approximately 8 to 12 μm in diameter and about 3.5 to 11.5 μm apart from each other. The cells are connected by long straight fultoportula processes and contain up to 2 chloroplasts per cell. Processes are tube-like silicified projections that protrude from the valve wall. Fultoportula processes, also known as strutted processes, protrude through the valve wall with 2 or more satellite pores surrounding them. Fultoportula processes are only found in the centric order Thalassiosirales. Each fultoportula process in S. costatum has 3 satellite pores and terminal fultoportulae processes with claw-like tips. Cells belonging to S. costatum are cylindrically-shaped cells and have a ring of long flattened intercalary fultoportula processes protruding from the periphery of each valve, each closed along their entire length. Each intercalary fultoportula has a longitudinal suture extending from an external pore at its base to its tip. Intercalary fultoportula processes of adjacent valves are connected at a 1:2 junction, where each process interlocks with two more, creating a "zigzag appearance". This 1:2 junction is a distinctive feature of S. costatum. Their intercalary rimoportula are positioned marginally, they have long terminal rimoportula, and their girdle band features rows of pores. Each valve has one of their fultoportulae replaced with a rimoportula, identified by its longer external process and the "spoutlike teapot"-shaped tip of the terminal rimoportulae.
Morphologically similar species Among species in the genus Skeletonema, S. costatum is most morphologically similar to S. subsalsum, with both species exhibiting rows of small pores between the parallel rows of transverse branching ribs on their girdle bands. They are also the only two Skeletonema species with long intercalary rimoportula processes. Diatoms in the species S. costatum can be identified by the persistent presence of a 1:2 junction, and the closed tubules of its intercalary fultoportulae processes. Skeletonema subsalsum will sometimes have a 1:1 junction.
Morphological variation The morphological plasticity of S. costatum cells has been extensively studied. Castillo (1995) attributes significant variations in morphological features, such as cell diameter, number of cells per chain, and the length of intercalary processes, with variations in environmental conditions, most notably, salinity. If cultured in freshwater, S. costatum develops short intercellular processes and is observed to seemingly not have space between sibling valves at 1 psu.
Taxonomy As of 2021, 21 species in the genus Skeletonema were "identified and taxonomically accepted", with S. costatum being one of them. Skeletonema costatum was first described by R. K. Greville in 1866 when it was originally called Melosira costata. It was later renamed by Cleve in 1873. Skeletonema costatum has since been more narrowly defined, with numerous species previously attributed to S. costatum identified as distinct species. The species originally described by Greville is often referred to as S. costatum sensu lato (s. l.), which represents multiple different species with similar morphological traits. The species granted the original epithet, costatum, was the species more narrowly described by Zingone et al. (2005) after reexamination of the type materials of S. costatum using electron microscopy and molecular analysis of rDNA. Zingone et al. (2005) identified two distinct morphologies within the type material, describing the less abundant morphology as S. grevillei and the more abundant morphology as the original epithet, costatum. The latter was assigned the original epithet due to its closer similarity to the specimen originally described by Greville, which he had conveniently marked. The two morphologies differed in their frustule ultrastructures, including "the shape of FPPs [fultoportula processes], the type of interlocking between IFPPs [intercalary fultoportula processes] of sibling valves, and the cingular band ornamentation". S. costatum sensu stricto (s. s.) can be used to describe the more narrowly defined S. costatum species to differentiate it from S. costatum sensu lato. S. costatum sensu stricto (s. s.) has also been referred to as S. costatum (Greville) Cleve emend. Zingone and Sarno.
Distribution and habitat Skeletonema costatum is widely distributed geographically, apart from the Antarctic Ocean. It is found around the world, including off the coasts of Hong Kong Island, Florida, USA, Uruguay, Brazil, Northern Queensland, Australia, China, and the Sea of Japan. Skeletonema costatum primarily resides in the neritic zone and is commonly found in brackish waters as opposed to the more oceanic, S. tropicum. Skeletonema costatum is frequently the dominant phytoplankton species in coastal waters and the dominant species responsible for red tide events. Although the dominant species in red tide events at a given area can change over time, there are studies on red tide events in China in which S. costatum was dominant. This include the Yangtze River estuary in the interior of China, Hongsha Bay of Sanya in the South China Sea, and in Fujian coastal waters.
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