The Weird, Whimsical, Watery World of the Phytoplankton
The wondrously diverse shapes of various diatom species. Note the bizarre symmetries and lack of propulsive appendages. Photo via Pixabay.
Every second of every day, half of our planet is inundated with the sun’s rays, powering one of the most important biological processes on Earth: photosynthesis. We usually think of towering forests or verdant grasslands when we hear such a word, but that’s only half the story. A whopping 50% of all photosynthesis is performed by microscopic organisms adrift in our oceans. Technically, you owe every second breath you take to a diverse assemblage of marine microbes called phytoplankton.
While land-based ecosystems are dominated by towering trees and gargantuan flora, the oceans are ruled by organisms too little to be seen by the naked eye. But what exactly are these tiny beings, and how are they able to have such a dramatic impact on our planet? Truth be told, ‘phytoplankton’ is simply an umbrella term that refers to any member of a hugely diverse menagerie of aquatic photosynthesizers. From tiny bacteria to surprisingly large single cells, plankton run the gamut from mundane to downright bizarre.
The most abundant of the plankton are the cyanobacteria, which are also among the smallest. In fact, these tiny bacterial cells are so minute that most oceanic species weren’t discovered until the late 1970s, since they were too small to be captured by conventional sieves. They are quite possibly the single most prolific photosynthesizers on Earth, and they were even responsible for the great oxygenation event that made our atmosphere breathable over 3 billion years ago.
Cyanobacteria colonies of genus Anabaena, arranged in chainlike filaments. The slightly enlarged cells along the chain are called heterocysts, and they are specialized structures for nitrogen fixation. Photo via Flickr.
As if that feat of geoengineering wasn’t enough, cyanobacteria also arguably make all other life in the ocean possible by performing a crucial function referred to as nitrogen fixation. In order to manufacture proteins, DNA, and a host of other important chemicals, life needs nitrogen. However, the element is frustratingly unusable in its gaseous form. As a response to this lack of material, cyanobacteria evolved the ability to convert the gas into usable ammonia; the microbes essentially make their own fertilizer from thin air. Without the bacteria’s ability to draw nitrogen from the atmosphere into the food web, life as we know it could not persist in the open ocean. Annually, they make millions of tons of this vital nutrient available to other marine life.
Of course, the cyanobacteria are far from alone. Benefiting from the fixed nitrogen (and occasionally partnering with the bacteria to help acquire it), the diatoms and the dinoflagellates are two other prominent members of the plankton community. However, unlike the cyanobacteria, these organisms are protists, meaning that they have advanced cellular machinery and a nucleus, like our cells. Diatoms construct elaborate glassy shells out of dissolved silica, and they are a major contributor to the enormous seabed piles of detritus that are referred to as marine ooze.
Dinoflagellates are a bit more mobile than the other two groups, possessing a whirling tail-like appendage called a flagella. Many dinoflagellates are photosynthetic, but a surprising abundance of species are mixotrophic, meaning that they also obtain energy from consuming food. Some particularly sinister species even go so far as to plunder chloroplasts, the photosynthetic machinery, from algal cells and maintain them within their own bodies, effectively allowing a non-photosynthetic “animal” to steal plants’ superpowers.
Diversity of dinoflagellate shapes. Unlike the diatoms, dinoflagellates tend to construct their shells out of calcium carbonate, and use a system of overlapping plates to provide protection. Photo via Flickr.
A simplified diagram of the marine carbon pump. The sedimentation step is where carbon storage takes place, but keep in mind that most of the carbon is recycled back into the system by bacteria and animals that eat and respire. Photo via Wikipedia Commons.
Collectively, the marine plankton has been one of our planet’s most important tools for keeping conditions habitable and stable; namely, they are capable of removing enormous amounts of carbon from the atmosphere and cooling our planet. Phytoplankton photosynthesis powers what is referred to as the ‘biological carbon pump’, which starts when the bodies of dead plankton manage to sink down to the seafloor. Here they accumulate and compress, and ultimately bury their carbon with them, thereby removing it from the system. It has been estimated that marine carbon burial sequesters more gas than all of Earth’s forests and grasslands combined. Marine ecosystems are such potent carbon fixers that some climate experts have even gone so far as to suggest, albeit rather controversially, that fertilizing the ocean with iron will fuel a massive plankton bloom and remove enough CO2 to counter climate change.
In addition to removing carbon and adding nitrogen to marine ecosystems, phytoplankton also make their own clouds. Yes, you read that correctly, clouds. When stressed by too much UV radiation from the sun, these photosynthetic machines, instead of retreating to the depths, release particles called aerosols into the atmosphere. Aerosols are tiny solids that drift about in the air, and water vapor can condense around them to form the basis of sunlight-reflecting clouds, which can offer shade to the microbes below. It has been estimated that phytoplankton can double the rate of cloud formation over a given area of sea, and may contribute greatly to global precipitation cycles in ways that we don’t yet understand. Phytoplankton’s impressive insistence on changing the world around them instead of adapting to prevailing conditions is reminiscent of our own species’ stubbornness.
Unfortunately, as is often the case with such wonders of the natural world, we are inadvertently exterminating phytoplankton with alarming speed. Due to a combination of warming waters and shifting ocean currents, phytoplankton productivity has dropped by 40% since the 1950s. Given that these organisms act as the literal foundation upon which the entirety of the greater marine biosphere rests, and that dozens of industries from fishing to pharmaceuticals depend upon healthy aquatic ecosystems, this is very bad news. Ignoring the uncomfortable ethics behind our complacency toward slowly killing off the biosphere, at the very least we owe it ourselves to try to ensure that these ecosystems remain functional. We simply cannot let these tiny drifters drift away.