Algae can be considered the lowest of the low, the most basal level of the ecosystem. Called pond scum and otherwise considered green and slimy, algae may hold a place at the bottom of the food chain, but it would be a mistake to think they were weak and defenseless. Some algae have elaborate coats of armor made of either calcite or silica to give them a rigid structure in their aquatic environment.
Coccolithophores are algae with golden-brown chloroplasts because of the presence of the pigments diadinoxanthin and fucoxanthin. Emiliania huxleyi is a well-studied species of coccolithophore, which is abundant in the world’s oceans and also forms massive blooms under favorable conditions. These organisms make an exoskeleton of calcite plates that looks like a collection of armored discs over the cell surface. These coccolith scales are continually shed into the ocean as the algae grow and divide. Of course, this coat of armor must be translucent to allow for visible light to penetrate to the chloroplasts where photosynthesis occurs. They must also be somewhat porous to allow for the uptake of carbon dioxide and other nutrients. They may look like alien spacecraft, but you may be more familiar with these organisms than you might expect, as they are the main components of chalk and chalk-based rock formations.
Other algae prefer to make an armor of silica. A thin layer of glass may not seem like an appropriate substance for fortification, but it is sturdier than the typical biological membrane. Diatoms use this strategy in seemingly infinite variations in form.* While the shapes may change, the basic architecture is the same; in that, these glass houses are comprised of two halves or frustules (science word of the day) that fit together like a pill box. The top half (called the epitheca) is slightly larger and fits over the bottom half (called the hypotheca) so that the algal cell is completely surrounded by rigid silica, but with some flexibility. These two halves allow for the algae inside to divide without breaking their protective shell. Upon cell division, each new algal daughter cell takes one of the parent frustules to be its epitheca valve. The new daughter cells create their own hypotheca within about 10 – 20 minutes. For those of you paying attention, this means that over the course of several cell divisions, the daughter cells that inherit the parental hypotheca (which then becomes their epitheca) will be smaller than those that inherited the larger epitheca from the parent cell. At some point the size of the frustule must be restored to its larger typical size and the algae produce auxospores, which lack the silica frustules. During this part of the life cycle, the algal cells swell to a larger size and create a new frustule of maximum size. Each of the frustules also contains numerous pores to allow for the exchange of gases and nutrients at the cell membrane because the single slit where the two halves come together is insufficient.
These coats of armor come at a cost to the algae because it makes the organisms denser than the surrounding aqueous environment. In other words, they are constantly sinking in the water column, just as anyone else wearing a coat of armor would do. This is a problem when they still require access to sunlight to fuel their photosynthetic lifestyle. Consequently, they are at the mercy of water turbulence created by the wind to remain within the appropriate zone in the water column. This leads to a cycle of bloom and bust for their reproduction and existence in their environments. The algae have some strategies to keep their coats of armor from completely weighing them down into the dark abyss and extinction. As mentioned above, they can undergo elaborate life cycles, parts of which abandon their armor during which time they would float higher in the water column. The fanciful shapes of some diatoms also optimize the lift they receive from wave action and water mixing to increase their chances of getting closer to the sunlight. Others can modify their lipid content or ionic content of their central vacuoles to create more buoyant cells within their heavy shells to slow their sinking.
Even those autotrophs on the small end of the size spectrum have elaborate defense systems. Diatoms and coccoliths prove that even Medieval-style fortification can be compatible with the photosynthetic lifestyle.
*For more beautiful images of diatoms, check out the Flickr feed of the California Academy of Sciences.
This post was written for the February edition of Berry Go Round, Botanical Warfare: The Parasites, Stranglers, Chemists and Thieves. Click here if you’d like to read more about botanical battle tactics.
References and Links:
For more diatomaceous microscopy art: http://deepbluehome.blogspot.com/2011/01/psychedelic-diatoms.html
Diatoms may also be a useful measure of environmental health: http://www.scientificamerican.com/article/algae-in-glass-cases-could-determine-frackinge28099s-toll/?utm_source=twitterfeed&utm_medium=twitter