Diatoms are algae with hard parts. They make up a major part of the plankton found in fresh and salt water environments. Usually, diatoms exist as single celled free floating organisms, but they can also be colonies of several single cells. Their tiny little ‘shells’ are made up of silica (these shells are called “fustules”).

The fustules have a characteristic shape that goes with each species, and since these are hard (essentially, made of glass) they are often well preserved in sediments. Thus, diatoms actually provide an excellent, even if very tiny, fossil record. In addition, since the silica that makes up their fustules is actually hydrated silicon dioxide, these little organisms preserve a signal of the oxygen isotopic environment in which they live. Indeed, there is a bit of carbon preserved in the fustule as well, as there is a protein template involved in the formation of the fustule, and bits of this end up in the structure, so there is also a record of carbon in diatoms.

A recent paper in PNAS addresses the size difference between fresh water and marine diatoms. Cell size is potentially a very important variable for these little organisms. For instance, larger cells would, on average, sink more frequently and quickly to the bottom of the ocean, thus sequestering carbon (this carbon is the carbon in the living tissue of the diatom). There are presumably ecological reasons why larger vs. smaller cells would evolve.

It turns out that the size range is greater and the maximum size is larger in marine diatoms compared to fresh water diatoms. Why?

The study considered the role of Nitrogen vs. Phosphorus limitation on cell size. Nitrogen and Phosphorus have different patterns of availability in marine vs. fresh water settings. Nitrogen is probably more of a limiting factor in marine environments, and Phosphorus is probably more of a limiting environment in fresh water environments. Also, the range of depths at which diatoms can survive for longish periods of time is greater in marine environments than it is in fresh water environments. For various reasons, both of these relationships suggest that larger diatoms would do well in marine environments.

When Nitrogen is abundant and consistent, fast growth rates of cells is possible. Whenever it is possible, we expect fast growth rate of tiny organisms to be selected for (unless there is some counteracting effect) because in this way the organisms can grow out of the size range for at least some of their predators. The faster rate of growth leads to smaller maximum size.

Although Nitrogen can be limiting in marine environments, it is also very variable in amount over time Variation in the basic food supply for any orgasm can lead, other things being equal, to smaller body size for space limited creatures like elephants on islands, but for these single celled organisms, variation in body size leads to larger size because of the greater potential for food storage in the larger cells.

Another factor is the importance of sinking. For a diatom, sinking too much = death because these organisms get their energy from sunlight. Physiologically active diatoms don’t sink, but when the cell becomes inactive it may start to sink. In diatoms that are physiologically active (healthy, there’s enough sunlight, etc.) size does not affect sinking rages, but in cells that are less active owing to lack of nutrients or sunlight sinking is quicker in larger cells. However, really large diatoms actually sink more slowly than small ones. Therefore, variation in physiological activity selects for a greater range in diatom size. This is what is probably happening, in part, in marine settings.

In contrast to the situation with Nitrogen, variation in Phosphorus seems to select for small sized diatoms, for reasons that are not entirely clear (so we’ll just skip that part…)

The present study gathered data on diatoms and their environments from a wide range of sources, then used all of these data to run simulation studies testing various ESS strategies. The ESS simulations significnatly refined the understanding of diatom evolution and confirmed and provided detail to the idea that Nitrogen and Phosphorus levels, as well as the effective depth at which diatoms operate, explain through Natural Selection theory what we see in nature.

ESS stands for Evolutionary Stable Strategy. This is an interesting and important concept in evolutionary theory. A strategy is pretty much anything that can be thought of adaptively. Body size is a strategy, a certain foraging pattern may be a strategy, etc. A stable strategy is a strategy that is held in place, such that alternative variants are somehow avoided, over time. An Evolutionary Stable Strategy is one in which Natural Selection has in a sense “chosen” among a set of alternative strategies the one strategy that out does all others with respect to fitness. This strategy … this ESS … is expected to remain as the dominant, in place strategy forever. Or until a new ESS comes along, invading the population and replacing the original strategy. Since evolution works with random mutations as the starting point, the “One True ESS” may not be extant in a given population, but then, when it emerges it should spread. In truth, however, since there are lots of mutations and lots of time, most strategies in most populations are already The One True ESS or close to it. What can happen over time, however, is that conditions change and what once was The One True ESS is supplanted by a similar … but importantly different … strategy that was, in a sense, waiting int he wings as part of normal variation.

Litchman, E., Klausmeier, C.A., Yoshiyama, K. (2009). Contrasting size evolution in marine and freshwater diatoms
PNAS, Early Edition

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