Diatoms, a group of phytoplankton, are amazing on a variety of levels. For one thing, their cell walls are made of silica meaning each one lives in a little glass house, and an elaborate one at that (Image 1). Those ornate ‘holes’ do have a function. One of the biggest hurdles phytoplankton face is avoiding sinking. Since silica is dense compared to seawater, this honeycomb-esque pattern reduces the weight of diatom, thus still allowing it to benefit from the protection provided by a ‘glass’ wall without causing it to sink rapidly. They also use other mechanisms to reduce sinking such as carbohydrate ballasting or air bubble entrapment among setae (spines).
Diatoms are also one of the most diverse groups of phytoplankton. They can exist as single cells or as chains (Image 2) and you can find them just about anywhere: some are planktonic, while others are benthic, epilithic, epiphytic, epizoic, endozoic, and even epi-ice-ic (okay, I made that last one up…is there a word for living on/in ice?).
Broadly diatoms fall into two categories: pennate and centric. The diatom cell wall is made up of two valves (or frustules or tests or theca), where the, slightly smaller, hypovalve sits within the epivalve (think petri dish). The overall shape of the valve can be described as pennate or centric: centric meaning it is more or less round with radial symmetry, and pennate meaning it is more oblong with bilateral symmetry. The ‘side’ of a diatom where the valves overlap is known as the girdle band. Thus, when you view a diatom under the microscope you will either be seeing it in valve (top) or girdle (side) view.
Personally, I find diatom reproduction to be one of the most unique and interesting thing about this group of organisms. Most of the reproduction taking place among diatoms is asexual (Image 3), and because of the size difference between the valves, when the parent valves become the new epivalves of the daughter cell, one cell gets smaller; the daughter cell with the parent hypovalve acting as its epivalve will be smaller than the parent cell, while the daughter cell with the parent epivalve acting as its own epivalve will remain the size as the parent cell (Image 4). Over many cycles of reproduction this size differentiation becomes so dramatic, that the diatom struggles to fit all of its cellular material within the ever decreasing valves. At this point the diatom must switch from asexual to sexual reproduction and those [tiny] diatom cells divide into either eggs or sperm. Turgor pressure then causes the valves to slide apart, pop open, and release the gametes into the water column where they come together and fertilization occurs. The fertilized egg undergoes mitotic divisions without cleavage to increase in size and then begins to secrete a new, large siliceous cell wall, so the whole process can start all over again.
I could tell you much more about the wonders of diatoms, but I fear the explanatory images would take over, so I’ll save the rest for another post.
Elizabeth Sargent is currently a PhD student at the National Oceanography Centre, Southampton studying nitrogen fixation and its role in fluxes of carbon and nitrogen to the deep sea. She is also a regular contributor for Words in mOcean.