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Have you ever wondered what seasons look like to a microbe? How they navigate the highs, the lows, and all the muddy, slushy in-betweens?

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SOURCES:
https://vertexaquaticsolutions.com/seasonal-changes-in-ponds-and-lakes/
https://www.climate-policy-watcher.org/lake-ecosystems/ecological-zonation-in-lakes.html
https://royalsocietypublishing.org/doi/10.1098/rstb.2016.0408
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Go to squarespace.com/microcosmos to get a   free trial and 10% off your first  purchase of a website or domain. Have you ever wondered what  seasons look like to a microbe?

How they navigate the highs, the lows,  and all the muddy, slushy in-betweens? The answers are as varied as the  number of climates in this vast world. So today, let’s travel to a pond  in Poland and see the way its own   tiny universe changes over the course of a year.

James, our master of microscopes,  has been collecting samples from   the same ponds in Warsaw over and  over again for the past six years. And you might think that would get dull and  repetitive, but over that time, he’s found   unique and rare organisms that have taught us  so much about how the microbial world works. And even when things do get repetitive,  there’s still something exciting happening.

James has noticed   that some microbes make appearances in these  ponds at around the same time each year. For example, summer is a great  time to find green things. With all the light coming in from  the sun, photosynthetic organisms   like algae and euglenids thrive  in the areas James frequents.

And as filamentous algae like spirogyra  start expanding their populations with   the warmer temperatures, the landscape shifts. These algae create branches that tangle together. And as they carry out photosynthesis,  they also produce little bubbles of   air that get stuck in the masses of their  own bodies, raising them to the surface.

The tangles of filamentous algae look like  giant blobs just floating on top of the water. But they also become prime real estate   for organisms like stentors that  latch themselves onto the algae. The proximity to the surface means  that the stentors will have access   to both oxygen-rich water, and to plenty of food.

And like stentors, hydras will attach  themselves to the floating algae,   extending themselves to try and catch  water fleas swimming in the pond. But summer doesn’t last forever, and in Warsaw,  the transition from season to season is marked   by changes in temperature that shape the  ponds and lakes these organisms live in.  In the summer, when temperatures are high,   the warmer surface waters are lower in density  than the colder waters towards the bottom. This creates two layers that  don’t mix together much,   and where the bottom layer becomes  more and more depleted of oxygen.

But as temperatures cool down,  those waters begin to mix again,   circulating oxygen throughout these layers. Fall is really a period of transition in the  microcosmos, a bridge from summer to winter. The green of summer shifts to muddier hues,   thanks to golden algae like synura that  thrive in the increasingly cold waters.

Their population booms so much that  the water they live in will turn brown. Another organism that seems to fare  well in fall are dinoflagellates. But as winter approaches,   these dinoflagellates prepare for the  microcosmos equivalent of hibernation: they turn into cysts that  drop to the bottom sediments,   waiting for spring and better conditions.

And of course, winter means  more changes in the water,   as the colder, darker days settle  in and ice covers the surface. There’s less oxygen and less light, so  aquatic plants and algae start dying,   and that decaying organic matter  consumes oxygen under the ice. So you might think that winter is a  terrible time to look for microbes.

But for James, winter samples are his favorite,   even if dealing with the cold  makes it very unpleasant. And it must be really fascinating to see  what creatures are able to handle the winter. One organism that seems to  continue on as normal are diatoms,   which might be surprising given  that diatoms are photosynthetic.

During a season marked by darkness and cold,   it seems like diatoms should be as miserable as  the rest of their photosynthetic counterparts. But instead, they survive, thanks  in part to oil droplets in their   bodies that store nutrients for times like these. Then there are these green stentors, which  James hasn’t yet been able to identify.

But one thing to note is that  they are filled with algae. In fact, one of the surprising details of winter  pond life is that it doesn’t completely lose its   greenery, as endosymbiotic algae cash in on  their relationship with their ciliate hosts. During the summer months, the algae can provide  some of its photosynthetic products to the host.

And during the winter, the ciliate can provide  nutrients and protection for the algae. In fact, James has noticed that these  ciliates filled with endosymbionts   become way more common in winter than  summer, which he thinks might be because   they don’t have as many predators  to worry about during this season. Eventually though, spring  makes its presence known.

The temperatures rise, the ice melts,  and the days fill with more sunlight. And with the shift in seasons,   James starts to see hardy algae like  the trachelomonas appear in his samples. In the same way that fall was a  transition from summer to winter,   spring is a transition in the reverse direction.

The dinoflagellate cysts that lay dormant through   winter awaken as the sediments  fill with diatoms and algae. And meanwhile, invertebrates like  snails begin increasing in number,   munching away at the fragments of filamentous  algae left behind from the previous fall. And this cycles back, of course, to  summer, when those filamentous algae   will come to dominate the pond and make a  structure for other organisms to live in.

It’s like this whole world  is constantly in motion,   changing in response to the larger forces  around it, and creating a steady rhythm that   builds over the course of a year, but  takes even longer to fully appreciate. But even these cycles can go  through their own disruptions. Sometimes these changes are small, like  instead of Synura blooming in the fall,   James might see a different golden algae species.

Those seemingly minor shifts are  fascinating in their own right,   a reminder that these little niches can  be filled with so many different species. But then there are the big changes. Over the recent years, James has observed  more and more cyanobacteria blooms in   the summer, as excess nutrients send  populations of cyanobacteria soaring.

And while that might sound  great for the cyanobacteria,   it can doom the rest of the pond by  blocking sunlight and consuming all   of the nutrients and oxygen that  other organisms need to survive. In fact, last summer, one of James’ favorite ponds  was so overwhelmed with a cyanobacteria bloom that   it didn’t provide the abundance of other microbes  he usually associates with that time and place. So the way the microcosmos experiences  seasons shifts as well, which makes sense.

After all, seasons are more than just  hot and cold, sunshine and clouds. They’re also the landscape,  and the life that fills it. If there’s a little puddle of  water outside your door right now,   it can give you a snapshot of the season that’s  as vivid as an autumn leaf or a spring rain.

And each time someone takes a  peek at one of those snapshots— whether it’s a master of microscopes dedicated  to his task or you watching one of these videos— the images fill in a story about this planet,   one that is ever-changing even  when it's at its most familiar. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us. And thank you to Squarespace  for sponsoring this episode.

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So go to Squarespace.com to sign up for a  free trial, and when you’re ready to launch,   go to squarespace.com/microcosmos to save 10%  off your first purchase of a website or domain. We’d also like to say thank you to  each and every one of our Patrons. Some of their names are on the screen  right now, and these are the people   that make this channel, and videos like  this possible and we are so so grateful.

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