# What lives in a drop of water? An alga with dozens of branches is a single cell. A tiny rotifer has organs. Look closer: pond water can hold them both. ## Green alga: This entire shape is a single cell. Every branch belongs to the same alga. Its two halves meet at a narrow waist. The green comes from chloroplasts, the parts of the cell that capture light for photosynthesis. Follow the edge out to its smallest branches. Then compare the top half with the bottom: they are very similar, without being perfectly identical. ![A single green Micrasterias cell with two broad halves and deeply branching edges.](https://mariuscomper.uk/lumea-dintr-un-strop/assets/micrasterias.webp) A single-celled green alga photographed under a light microscope. Anatoly Mikhaltsov · [Source](https://commons.wikimedia.org/wiki/File:Micrasterias_rotata.jpg) · [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/) ## Volvox: Inside the big sphere, smaller spheres are growing. The green dots on the surface are cells. Together, they form a Volvox colony. The larger spheres inside are daughter colonies, developing before they are released. Distinguish the tiny surface dots from the green spheres inside. The outer cells have fine filaments called flagella, whose beating propels the colony. ![A transparent sphere dotted with tiny green cells, containing several smaller green spheres.](https://mariuscomper.uk/lumea-dintr-un-strop/assets/volvox.webp) A colony containing daughter colonies. The photograph combines several focus planes. Matthew Herron · [Source](https://commons.wikimedia.org/wiki/File:Volvox_aureus.jpg) · [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/) ## Rotifer: An entire animal, almost transparent. It has a mouth, a digestive tract and muscles. A rotifer is made of many cells, although it fits in the same microscopic field as single-celled organisms. In many rotifers, cilia around the mouth move water and draw in food particles. At the broad end on the right, look for very fine filaments. Beating one after another, they can look like turning wheels. Further down, you can explore how this illusion works. ![An elongated rotifer against black, with a transparent blue outline, reddish interior and fine cilia on the right.](https://mariuscomper.uk/lumea-dintr-un-strop/assets/rotifer.webp) A rotifer photographed by Frank Fox. The image identification does not specify a species. Frank Fox · [Source](https://commons.wikimedia.org/wiki/File:Mikrofoto.de-Raedertier-14.jpg) · [CC BY-SA 3.0 DE](https://creativecommons.org/licenses/by-sa/3.0/de/) · https://www.mikro-foto.de ## Stentor: A cell that can rebuild its missing parts. Each of the two shapes in the photograph is one cell. In the species Stentor coeruleus, researchers have observed something remarkable: after a cell is cut in two, each half can regenerate its missing parts if it retains part of the large nucleus. Find the broad end, fringed with cilia. Their beating produces a current that brings in food. The shape can change as the cell contracts. ![Two violet, trumpet-shaped Stentor cells with small golden inclusions.](https://mariuscomper.uk/lumea-dintr-un-strop/assets/stentor.webp) Two Stentor cells photographed using polarized light and dark-field illumination. Marek Miś · [Source](https://commons.wikimedia.org/wiki/File:Stentory.jpg) · [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/) ## Diatom: An alga inside a silica shell. Diatoms build their cell walls from silica, a material also used to make glass. The transparent shell reveals the golden-brown interior of this living cell. Follow the long outline, curved into a gentle S. Inside are the chloroplasts that help the alga use light. The full photograph retains the author’s scale bar. ![A long, golden Gyrosigma cell, slightly curved, enclosed in a transparent wall.](https://mariuscomper.uk/lumea-dintr-un-strop/assets/diatom.webp) A living diatom photographed using differential interference contrast. Anatoly Mikhaltsov · [Source](https://commons.wikimedia.org/wiki/File:Диатомовая_водоросль_Gyrosigma_acuminatum.jpg) · [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/) ## A single cell can do all this. Micrasterias uses light. Stentor feeds and can rebuild parts of its body. Through a microscope, an organism’s outline cannot tell you how many cells it contains. In Micrasterias, the whole branching shape belongs to one cell. In Volvox, many cells form a sphere together. In a rotifer, cells form tissues and organs. ## Why do cilia look like turning wheels? Cilia are fine filaments that bend one after another. Together, their movements look like rotation. The name “rotifer” comes from this impression of a turning wheel. Films on plants, clumps of algae and sediments can hold very different organisms. They do not all appear in the same sample. What you find depends on where you collect the water and when you look at it. ## How we know - [USGS](https://www.usgs.gov/media/images/a-natural-marvel-a-single-cell): A single-celled freshwater alga and its green chloroplasts. - [Neustupa & Šťastný, PeerJ (2018)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6287601/): Micrasterias semicells, branching and departures from symmetry. - [University of New Hampshire · Phycokey](https://www.cfb.unh.edu/phycokey/Choices/Chlorophyceae/colonies/colonies_flagellated/VOLVOX/Volvox_key.html): The spherical colony, surface cells and daughter colonies inside. - [University of Michigan · Animal Diversity Web](https://animaldiversity.org/accounts/Rotifera/): Animal anatomy, the ciliary corona and feeding currents. - [Sood et al., eLife (2022)](https://doi.org/10.7554/eLife.80778): Bisection and regeneration experiments in Stentor coeruleus. - [Marshall, Frontiers (2021)](https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2021.753625/full): The macronucleus in cell regeneration and ciliary feeding currents. - [Diatoms of North America](https://diatoms.org/what-are-diatoms): Single-celled algae, photosynthesis and silica walls. - [Diatoms of North America](https://diatoms.org/species/49992/gyrosigma_acuminatum): Species identification, chloroplasts and freshwater occurrence. - [Quekett Microscopical Club](https://www.quekett.org/wp-content/uploads/2022/09/pond-life-leaflet-2022.pdf): An illustrated introduction to microscopic pond life. These photographs come from separate observations. Illumination and microscopy techniques affect how colours and outlines appear. Enlargement here brings the photograph closer; it does not represent a microscope’s optical magnification. Images have been resized for the page and framed in circles in the explorer. The full photograph retains the author’s composition and scale bars. Photo credits and licences are listed with each specimen.