The Little Organisms That Could

In Louisiana, the microorganisms that populate the coast help make it one of the most productive ecosystems on the planet

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If you love oysters, you’ve probably heard the “r-month” rule: steer clear of oysters in months that aren’t spelled with the letter “r.” In all the other months, eat as many as possible. On a hot day towards the end of the last non-r-month, I visited molecular biologist Tim McLean in his office at Tulane University, where he gave me the most straightforward explanation I’ve heard of the rule: “The months without ‘r’—May, June, July, and August—are the warmest months of the year, and that’s when Vibrio bacteria are most abundant in the water and you’re more likely to eat a contaminated or an infected oyster.” He explained that as the water heats up, some microorganisms grow more easily, including these toxic bacteria. When the oysters load up on Vibrio, the bacteria can accumulate in high enough levels to make humans sick.

Vibrio bacteria, McLean said, are one tiny piece of a massive network of microscopic drifters that power ocean ecosystems—the algae, bacteria, viruses, and fungi that feed the ocean from the bottom of the food chain. Some are terrifying—like flesh-eating Vibrio—but most are pretty magical. 

In Louisiana, the microorganisms that populate the coast help make it one of the most productive ecosystems on the planet. Much like the flora in our intestines, the microbes in the bayous and bays take the continent’s runoff, break it down, and convert it into nutrients to be reused in the food web. “The marshes don’t get a lot of appreciation or attention for how productive they are,” McLean said. “You’ve got all this plant growth, but very few things actually eat those plants to transfer that productivity up the food chain. So the only way that actually gets to the animals is when all those plants actually die and start to decompose. All the microorganisms are responsible for that decomposition; and as it gets released into the water, other organisms can come in and eat the little bits of plants that are floating in the water—the detritus.” The mud of the wetlands is a twenty-four hour salad bar of this detritus, feeding the oysters, crawfish, crabs, and other bottom-feeding creatures that populate the shallow waters.

But marine microorganisms go beyond making our lives taste better; they make our lives possible in the first place. McLean’s area of expertise is microscopic marine plants called phytoplankton. Using the power of the sun, these organisms absorb carbon dioxide, nitrogen, and phosphorus, producing oxygen in the process. Collectively, he said, phytoplankton produces half of the oxygen in our atmosphere. Other studies estimate that the number is as high as eighty-five percent. 

McLean was an early investigator of the genetic makeup of phytoplankton, looking at the genes of a specific group called dinoflagellates, single-celled swimmers with little tails that whip them through the water. What he found were tiny freaks of nature. 

“Dinoflagellates are different than basically any other organism,” McLean said. “They have a ton of DNA inside their cells—more DNA than we have in our cells, sometimes like ten or fifteen times more DNA ... There was a paper that came out in a scientific journal that said, ‘Dinoflagellates are from outer space’ because they are so weird. We don’t know why or what consequence all that weirdness has.”

But marine microorganisms go beyond making our lives taste better; they make our lives possible in the first place.

One species of dinoflagellate, Karenia brevis, turns the surface of the ocean a cloudy red color when enough of it congregates in a single place. It also produces a neurotoxin that is deadly to marine life. “You see a patch of discolored water, and any fish that swims into it will usually die and float to the surface,” McLean said. The phenomenon is called a red tide, and it occurs in places across the globe. “The blooms are becoming more frequent, they’re covering larger areas, and they’re suddenly happening in places we’ve never seen them before,” said McLean. An unseasonal red tide in December of last year forced the Mississippi Department of Marine Resources to temporarily suspend all oyster harvesting across the state’s coast. Scientists are examining the relationship between rising ocean temperatures and the occurrences of red tides. McLean pointed to a 2014 study that linked increasing Karenia brevis blooms with increasing concentrations of carbon in the ocean.

In a single tablespoon of ocean water, there might be hundreds of thousands of microscopic organisms. Some have been at work for several billion years and represent the oldest life on earth. Since many have yet to be identified or studied, a group of scientists has asked sailors to make simple measurements of water visibility (an indicator of phytoplankton populations) using their smartphones. McLean dreams of establishing a similar practice for the waters of Louisiana since so little is known about the state’s phytoplankton population. 

An effective phytoplankton net can be assembled using panty hose and a mason jar; and a smartphone, outfitted with a plywood and plexiglass extension, can be used as a digital microscope. The microbes in the water are beautiful, McLean said. “Some of these things move; some of them float there; some have all these kinds of weird extensions.” Part Pokémon Go, part citizen science, he wants to build a database to identify the organisms, but he also wants people to “go out, explore, and just see what’s in the water.”