CREATURE FEATURE. The microbial mindset

What does it take for us to notice a creature in our midst? That depends on the kind of creature — it’s difficult to not notice a rhesus macaque in your vicinity, for example, or a peacock, unless it’s silent and deathly still. But it takes a lot more than simply their presence for us to notice microbes — tiny single-celled organisms that we cannot see without a microscope. As Kabir Das said of god in one of his pithier couplets, we tend to remember microbes only when things go wrong. When the milk goes bad, for example, or when we become ill, we blame the germs that caused our misfortune. To consider microbes at moments beyond those can take a wholesale shift in our perspective of the natural world. Most of us are likely not of such a microbial mindset, and so to bring you a glimpse of what that mindset can reveal, I talked to someone who spends a lot of time pondering the vast swathes of life that are largely invisible to us.

Unlike many of her microbiologist peers, Holly Moeller did not discover her microbial mindset from an early childhood experience of “looking at pond water… and seeing [microbes] cruise around.” But the path she took to studying the evolution and ecology of microscopic creatures was nonetheless reminiscent of a bacterial life. “If you’re a bacterium,” she explained, “you’re not really that good at sensing the world… you just know what’s immediately around you.” To find something they’re looking for, food perhaps, bacteria will start to swim, and “they’ll see whether they’re swimming up or down a gradient of chemical cues that indicate whether a food source is nearby, and if they realise they’re no longer going up the gradient [towards the food], they’ll stop, they’ll spin, and then they’ll pick another direction at random and go. I feel like my career is a lot like that.” Chugging along, doing research she found interesting for a while, Moeller would pause as it got less interesting to her, and then “stop and pick a random direction, and then go off in [that] new direction. It’s been a career that was really influenced by the serendipity of encountering particular people at different times.” This bacteria-like trajectory led Moeller to studying microbes, whose own lives similarly depend on the serendipity of an encounter.

In the case of the microbes, the encounter in question brings about a life-changing shift in these organisms’ “metabolism”— how they “make a living in their environment, how they get their energy and the materials they need to build their bodies and reproduce and survive.” The encounter begins, unremarkably, as a predator meeting its prey, played out on a microscopic scale — the marine plankton Mesodinium rubrum engulfs a tiny algae. But instead of simply digesting this prey, breaking it down into its component molecules and absorbing what it needs, the Mesodinium hijacks the algae’s cellular machinery without damaging it. In particular, the Mesodinium holds on to the algae’s chloroplasts — tiny subcellular organs (charmingly referred to, in biologist-speak, as “organelles”) that allow the algae to photosynthesise. Photosynthesis is the process by which living creaturesturn sunlight into food. By retaining the algae’s chloroplasts instead of digesting them, the Mesodinium becomes capable of photosynthesis too. What’s more, the Mesodinium also carefully saves for itself the parts of the algae that repair and produce more chloroplasts, meaning that once a Mesodinium has eaten some algae, it turns into something like algae itself, living off sunlight for about a month before it needs to eat again.

Upon suddenly being able to photosynthesise, Mesodinium starts to compete for sunlight with the algae that were once its food. It is this shift in their interaction that, every now and then, makes Mesodinium a microbe that is remarkably easy to notice. In the summertime, large stretches of ocean take on the colour of red wine when a population of Mesodinium starts to expand rapidly, in what is called a “bloom.” These blooms, Moeller has discovered, are a consequence of Mesodinium ’s transition from predator to photosynthesiser. If you were a predator that simply digests its prey, “you wind up with your population dynamics tightly coupled to those of your prey, so when your prey go up you go up, when your prey go down you go down.” But by shifting away from relying on prey for energy to hijacking their chloroplasts and then relying on the sun for energy, Mesodinium can “decouple its population dynamics from those of its prey… so the prey has to be there to start, and then Mesodinium rubrum comes in and chows down and stocks up on chloroplasts and then off it goes,” soaking in sunlight and using its adopted chloroplasts to fuel growth and reproduction. Large enough to be visible from space, these Mesodinium blooms bring to our attention a creature that can so easily remain out of sight, smell, touch, and, thus, out of mind.

So the next time “you’re in your kitchen and something has gone bad, or you see a spot of mould on your bread that’s expanding”, think back to the Mesodinium rubrum blooms too, and remember that these are all manifestations of, as Moeller puts it, “how microbes are the awesome guardians of metabolic potential in the planet writ large.”

Ambika Kamath is a behavioural ecologist, currently based at the University of California, Santa Barbara

Email: ambikamath@gmail.com

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