Not Your Average Drifters – Plankton, Part I

Have you ever accidentally swallowed a mouthful of seawater at the beach? You probably didn’t think much of it, other than “well that was pretty gross.” But you might be surprised to find out just how much you ate in that liquid refreshment! The broad term for the microscopic plants and animals that you’re chowing down on is “plankton,” a group of organisms that we’ve talked about here before, but they definitely deserve a closer look.

Calcareous phytoplankton, SEM

“Plankton” is a term that comes from the Greek meaning “wanderer” and was coined to describe any organism that doesn’t have the ability to swim against the water current. So, technically, even some very large animals like jellies are members of the plankton, but most planktonic organisms are very small, and as the title suggests, the best things come in small packages.

Unlike our favorite New England ice cream, plankton basically come in only two flavors: phytoplankton (plants – the subject of this blog) and zooplankton (animals – I’ll talk about these next time). Both the plant and animal type contain a dizzying array of form and function, and their beauty may be unrivaled in the sea.

Why should we care about phytoplankton? Well, we owe our lives to the horde of single-celled plants that float around in the ocean. Literally – they produce at least half of the oxygen on our planet, and perhaps as much as 80%! Just think about those numbers for a second; amazing production from something so small. It’s obvious, then, that the phytoplankton’s strength is in numbers, which is how they also form the base of the marine food chain.

Phytoplankton provide sustenance to a wide variety of herbivores (including most of the zooplankton), some of which are of great commercial importance, like mussels, oysters, and scallops. As these herbivores are eaten, the productivity of the phytoplankton is transferred up the food chain, ultimately to us.

Those are some pretty good reasons to love plankton, but I’m not done yet. You know all of that carbon that we’re pumping into our atmosphere? Well, phytoplankton take much of that carbon out of the atmosphere through photosynthesis. And when they sink to the bottom, phytoplankton sequester a massive amount of that carbon to the deep sea. Even when they’re long gone they’re important, because their hard bits are preserved in the fossil record, helping scientists to decipher everything from the age of rocks to past environmental changes.

Their importance might be matched by their looks; even in a place renowned for its beauty, phytoplankton stand out. Take the diatoms, for example, which make breathtakingly beautiful skeletons made from silica (the compound used to make glass). Or the dinoflagellates, which are normally harmless but can occasionally bloom and release toxins that form the sinister red tides. There are cyanobacteria, better known as “blue-green algae” that are actually ancient photosynthesizing bacteria. But my personal favorite, and maybe the most curious, has to be the coccolithophores, which cover themselves in buttons made of calcium carbonate. Why would they do such a thing? Scientists aren’t really sure, and debate abounds, but what they are surer of is that more acidic seawater will not be good for the coccolithophores.

In fact, when we look at the big picture, global phytoplankton concentrations have been on the decline for the last century. This is a scary trend, given their importance, and some researchers have even proposed fertilizing large areas of the ocean to cause phytoplankton blooms. Sounds promising, but one of the challenges associated with such large-scale interventions is predicting the unintended consequences. For example, what effect would those blooms have on the zooplankton? Stay tuned, we’ll take a look at those little guys in Part II.

Casey Diederich is a 5th year PhD candidate in Tuft’s University’s Biology Department, and is conducting his research on slipper-shell snails. We are thrilled to have Casey guest blogging for us about some of the more fascinating animals in our ocean. Watch for his close-up look at plankton, Part II, coming soon. You might be surprised at how interesting and important these little guys are! – Ed.

Feature image via Wikimedia Commons

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