Vanadocyte

A vanadocyte is a specialized type of blood cell found in ascidians (tunicates). These cells are notable for their high levels of vanadium (concentrations 107 higher than that of seawater), which is typically a metabolic poison in other contexts.[1][2]

Structure and description

Vanadocytes are specialized blood cells found in marine tunicates. These cells are 8-13 um in size and the cytoplasm contains many acidic globules, termed "vanadophores".[3] These cells are one of several types found in the ascidian circulatory system and are abundant in the blood.[2][1]

The cells are described as highly distinctive, with little variation between species. They have a circular outline that decays into a rosette form upon desiccation, with several inclusions that almost entirely fill the cell. The cell has a greenish color, varying from dark apple to scarcely perceptible, due to their vanadium complexes.[4][5]

These green cells are not, as was initially believed, symbiotic zooxanthellae, although ascidians are known to have such symbiotes elsewhere.[5]

Chemistry

Vanadocytes are of interest to biologists and chemists because they contain high levels of vanadium and vacuole of sulfuric acid with acid mass fraction as high as 9 wt%, both of which are typically toxic to living creatures.[5] Additionally, the vanadium complex itself is unstable, found almost exclusively in the air-oxidizable, +3 oxidation state.[4]

Function

The function of vanadocytes is still unclear.[1] It has been proposed that the vanadocyte transports and processes nutrients, contributes to the polysaccharide external tunic, or serves as a defense mechanism. It is unlikely that the vanadium complex serves as an oxygen transport mechanism because it is unable to reversibly bind oxygen.[4]

Research into their utility as anti-biofouling (allelopathic) mechanisms concluded that the high acidity and high vanadium levels function to significantly reduce epizoic recruitment and predation.[6][7]

References
  1. Michibata, Hitoshi; Uyama, Taro; Ueki, Tatsuya; Kanamori, Kan (15 March 2002). "Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians" (PDF). Microscopy Research and Technique. 56 (6): 421–434. doi:10.1002/jemt.10042. PMID 11921344. S2CID 15127292.
  2. STOECKER, DIANE (December 1978). "Resistance of a Tunicate to Fouling". The Biological Bulletin. 155 (3): 615–626. doi:10.2307/1540795. JSTOR 1540795.
  3. Botte, L.; Scippa, S.; de Vincentiis, M. (September 1979). "Ultrastructural localization of vanadium in the blood cells of Ascidiacea". Experientia. 35 (9): 1228–1230. doi:10.1007/BF01963306. PMID 488290. S2CID 33061777.
  4. Carlson, Robert (1975). "Nuclear Magnetic Resonance Spectrum of Living Tunicate Blood Cells and the Structure of the Native Vanadium Chromogen". PNAS. 72 (6): 2217–2221. Bibcode:1975PNAS...72.2217C. doi:10.1073/pnas.72.6.2217. JSTOR 64680. PMC 432728. PMID 1056026.
  5. Webb, D.A. (1939). "Observations on the blood of certain ascidians, with special reference to the biochemistry of vanadium". Journal of Experimental Biology. 16: 499–523. Retrieved 24 June 2019.
  6. Stoecker, Diane (1980). "Relationships between chemical defense and ecology in benthic ascidians". Marine Ecology Progress Series. 3: 257–265. Bibcode:1980MEPS....3..257S. doi:10.3354/meps003257.
  7. Stoecker, Diane (1980). "Distribution of acid and vanadium in Rhopalaea birkelandi tokioka". Journal of Experimental Marine Biology. 48 (3): 277–281. doi:10.1016/0022-0981(80)90082-9.
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