Marine Biology

Diatoms (Class : Bacillariophyceae)

Diatoms with their characteristic yellow-brown pigment that masks their green chlorophyll are also called golden algae. They are unicellular and either solitary or chain forming.  The cell contents are enclosed in a unique glass (pill box), which is called as frustules and have no visible means of locomotion. The frustule is made of two parts, much like a petridish /petriplate, one valve fitting over another. The upper part (largest part) is called as epitheca and the smaller part is called as the cell wall (frustule) is made of silicon dioxide.  The valves or the frustules are highly ornamented with species-specific designs, pits and perforations, which make the frustule a lot lighter in weight and also provide a place for materials to move in and out of the cell. 

       Diatoms may occur singly or they may occur in chains of various kinds.  Many species have flotation mechanisms (spines, internal oil droplets or disc shaped).  Some of these are holoplanktonic and some are not planktonic at all i.e benthic.  When conditions are bad they dieand sink. The cell decomposes and the frustule breakes up and mixes with sand and mud.  This combination of sediments and glass frustules makes the siliceous ooze called diatomaceous earth.  It is mined by human beings and used as filtering as well as insulating materials.

Diatoms belong to two orders viz. Centrales and Pennales.  Some bloom forming species of diatoms are known to produce harmful chemicals (i.e domoic acid) which got concentrated in animals which feed on such plankton (i.e. filter feeders).  Large amounts of domoic acid consumed by mammals such as seals, sea lions and human beings by eating the shell fishes caught from the polthese may exhibit erratic behaviour and finally  die. , If the accumulation of domoic acid exceeds certain level in human beings  it is known to cause amnesia and this poisoning is called Amnesic Shellfish Poisoning (ASP).

ASP can be a life-threatening syndrome and  is characterized by both gastrointestinal and neurological problems. Gastroenteritis usually develops within 24 hours of the consumption of toxic shellfish; symptoms include nausea, vomiting, abdominal cramps, and diarrhea. In severe cases, neurological symptoms also appear, normally within 48 hours of toxic shellfish consumption. These symptoms include dizziness, headache, seizures, disorientation of focus, short-term memory-loss, respiratory difficulty, and coma. Diatoms species such as Pseudonitzschia sp. And  Nitzscia pungens produce this acid.. This type of biotoxin is not destroyed even after cooking and can cause illness in human beings  who have consumed sea food which has accumuted     excess levels of this acid.

  Some species of diatoms can be mass cultured  under controlled conditions and used as live food for larvae in shrimp and finfish hatcheries. E.g. Skeletonema  costatum, Chaetoceros spp., etc.