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Spring is, according to many a poem, a season of awakening.

The dreariness of winter fades and the snow melts away to reveal green earth. Animals emerge from their slumber to fill the landscape with their newly birthed young.

And in our most idyllic imaginings, the air carries the sound of chirping birds and the wondrous scents of daffodils, and lilacs, and other fragrant flowers. But nature is a messy business. Spring isn't just sunshine and greenery, it is also rain and mud.

And sometimes, puncturing through that floral perfume are some sour notes. Maybe you've wandered near a pond and caught a strange hint of cucumber. Or maybe that smell has deepened into something more rancid, like cod liver oil.

If you've caught this unpleasant aroma, you might be experiencing spring in the microcosmos. As the weather gets warmer, lakes and ponds begin to thaw, affecting even the tiniest residents. The water around them is still cool, but it turns out that's perfect.

Because when spring comes, flowers aren't the only thing that bloom. Synura are found in freshwater throughout the year, joining together in varied numbers to create spherical colonies. There are around 50 species that are currently recognized from around the world, including in the Arctic.

The only continent they have not been found on yet is Antarctica, but, I mean, who can blame them? These varied species of Synura are distinguished in part by the structure of the complex silica scales that adorn them. We can't see those scales with our microscopes, but scientists with electron microscopes are able to analyze and categorize those structures.

The scales are formed inside specified vesicles and then molded into shape by structures on the endoplasmic reticulum and then they are arranged into overlapping rows along the cell surface. But for all the order and hardness implied by knowledge of this scaly exterior, if you take a wider view, synura kind of look like marigolds tumbling around in the wind, like flowery confetti. Each individual cell contains two flagella, and when they form a colony, those flagella are pointed outward, and the movement of the flagella, of course, is what drives that jumbled swimming.

Aside from their petal-like appearance, Synura also resemble marigolds because of their distinctive yellow color. These organisms are chrysophytes, also known as golden algae. And on the first hearing, "golden algae" might sound a bit strange.

Algae are photosynthetic, and most of the ones we've met in the microcosmos are green because of the pigmented chlorophyll inside of their chloroplasts. Well, each Synura cell has two chloroplasts, just two, and those chloroplasts do contain chlorophyll. But they're also colored with xanthophyll, a yellow-brown accessory pigment that helps the organism process light at other wavelengths for photosynthesis.

The xanthophyll ends up overshadowing the green color of the chlorophyll, and as the colonies of synura begin to accumulate in a bloom, you might see the water they inhabit become brownish in color as well. Synura bloom in autumn as well as spring, but they're still present at low numbers throughout the year. And even when their populations are more modest, their scent might still be apparent.

It only takes about 5-10 colonies in a milliliter of water to produce a cucumber smell. So imagine what it's like to be around a pond where the bloom is in full swing. Even a single drop of water might hold tens of thousands of these colonies.

It's fortunate maybe then that smell-o-vision never took. If it had, we would definitely be confronting you right now with a very unpleasant fish odor. When synura bloom, the water around them ends up filled with a cocktail of alkenes, aldehydes, alcohols, and ketones that are likely the product of their metabolism.

As the colonies accumulate, so too do these compounds. And it's not like all of these chemicals have a particularly bad aroma. Synura have been associated with the release of beta-ionone, which smells like violets, and beta-cyclocitral, which smells like tobacco.

These sound like the fragrance notes you might read on a nice, expensive bottle of perfume. But of course, that's not all that's in the water. Synura also produces trans,cis-deca-2,4-dienal, a compound whose name probably means nothing to you until you happen upon its rancid smell overshadowing all of the other aromas.

And while this may not be the most welcome whiff when you are walking near a pond, the smell is handy if you happen to be microbe hunting. So much of sampling is a mysterious, subtle business, whether you're running a plankton net through a lake or sticking your arm in a mall aquarium tank. You're searching for the invisible, which means that you have to use environmental cues to figure out what you might be collecting.

And you usually will not know what microbes you’ve gathered until later when you get them under the microscope. But as the wise Gandalf once said, "If in doubt, Meriadoc, always follow your nose". And the smell of synura may not be pleasant, but it is honest.

If you want to observe many, many synura, then you can track the smell to its bloom. But with blooms like this, it can be difficult to find other organisms in your sample, so you might end up using the smell as more of a warning to stay away. Synura, of course, are not the only microorganisms to multiply into blooms.

And just as you can't understand the entirety of the microcosmos through any one microbe, you cannot understand algal blooms by studying just one algae. Their causes can vary from seasonal changes to water pollution, and their effects can range as well. Some blooms are not harmful, while others can be toxic to other organisms and damage local ecosystems.

But at their core, algal blooms demonstrate the ingenuity of microbes, their talent for expansion, turning even the tiniest organisms on our planet into a presence that none can ignore. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. If you like this show, right now on the screen are all of the people you should be thanking.

They are our patrons on Patreon. And if you would like to join them, you can head on over to If you want to see more from our Master of Microscopes James, you can check out jam & Germs on Instagram.

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