Nutrition

    Diatoms are mostly phototrophic organism with a few species that are mixatrophic and even fewer that are solely heterotrophic. With nutrients and growth factors, they require all the same ones that other eukaryotic organisms do but on top of that they also require greater amounts silicon, for their frustules. They obtain this silicon from the environment where it is dissolved in the form of silicic acid.
 
    The oceans usually sufficient silicic acid concentration for diatoms to grow effectively but there is rather large areas where iron concentrations are low which leads to limited growth in diatoms. This usually only occurs in open waters.
 
    Unlike with land plants the carbon that diatoms fix almost never goes into long term storage. Instead it ends up being used for cell division and reproduction or they get eaten as a strong base for marine and aquatic ecosystems.
 
    Some diatoms aren’t sole photoautotrophic. The species that can be heterotrophic are mostly pennale species that live in the benthic regions. In polar waters they have been found in a confusing symbiosis with sponges. The symbiosis a mutualism and a parasitism part to it. The mutualism part of the symbiosis occurs when the diatoms are able to have enough light to produce the energy to cover their metabolic demand that; well, the parasitic part occurs when the diatoms are unable to produce enough energy to cover their metabolic demand they shift to eating the sponge, so in low light to no light conditions. During the mutualism part of the symbiosis the sponges provides protection to the diatoms from predation and the diatoms provide energy in the form of extracellular polysaccharides, which most diatoms produce, and they assist as support for the sponge. But during the parasitic part of the symbiosis the diatoms being unable to fulfill their metabolic demand begin to consume the sponge to prevent them from becoming vegetative and allowing them to continue to divide. The diatoms in these situations end up being facultative parasite and facultative mutualism depending on time of year and if the area in which the host sponge is growing has sufficient light to prevent the diatoms from becoming parasitic. (Also can be seen in Interactions) (For more indepth information http://www.biolbull.org/cgi/reprint/198/1/29.pdf).

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