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Caulerpa and the secret of life

Caulerpa is deeply, deeply weird. It looks like a multicellular plant, but it isn’t. Each one of these colonies is actually a single cell. It is a plant, specifically a type of green alga (Caulerpaceae), but it is not multicellular; everything you can see (each colony) is literally one single cell. One large, 'visible-to-the-naked-eye' cell, with different regions that are specialized for capturing light, for spreading across the seabed or for taking up nutrients from the substrate, but all one cell that is continuous and undivided inside. Actually, this is not so weird – we are surrounded by examples of large cells that are macroscopic (or that we can see without a microscope), although we usually don’t realise it. For example, hen’s eggs are complex structures, but the yellow yolk of an unfertilized egg is actually a single cell called an ovum, and most eggs bought in the supermarket are not fertilized by a cockerel, so they don’t become a multicellular embryo. So, an egg yolk is literally a single, macroscopic cell. In the case of ostrich eggs, the ovum can take up almost half a litre of volume (that’s roughly around 14 fluid ounces or almost a pint), and most definitely something we don’t need a microscope to see. And some extinct birds and non-avian dinosaurs had even larger eggs. So some cells can be absolutely enormous. So how can a large cell such as Calerpa live and grow? Usually plants grow by adding cells, and each cell then follows instructions from DNA found within that cell, inside a nucleus. This is important because if the cell does something, like photosynthesis to make food, these physiological processes need instructions that must be close by and accessible. And the larger the cell, the greater the distance between the physiology and the DNA instructions, which could slow everything down. Large plants also have vascular systems (constructed from multiple cells) for transporting substances between different parts of the plant. So how can Calerpa live, and what can this tell us about how life happens inside cells? So Calerpa has a number of problems: firstly, the physiology needs DNA instructions close by, secondly, there must be some kind of internal transport system, third, if it doesn’t grow by adding cells, how can it get bigger? For the first problem, Calerpa is a single cell, but it doesn’t have just a single nucleus. It has many nuclei distributed throughout the cell, each one managing a local volume of living material. In technical terms it’s a “shared cell”, or Coenocyte, in which the cell is shared between many nuclei. For the second problem, transport of resources around the plant, Calerpa uses a system that all cells use, which is something called a cell-skeleton, or cytoskeleton. This sounds like a supporting skeleton, and it does help to shape and support the cell, but it is more akin to a railway network that plants use to transport substances around inside themselves. In this particular case, Calerpa keeps everything moving constantly to mix the liquid cytoplasm, ensuring that resources moving to where they are needed, and that waste products can get to the cell surface to be expelled. And the third problem, how it grows without adding cells, actually depends on the other two problems. Being a plant, the cell grows within a relatively solid cell wall made mainly of a network of cellulose fibres. This is rigid, but the cytoskeleton can bring and add more cellulose into it to widen it out, expanding the cell wall and so the volume that can be occupied by the cell. As the cell expands the nuclei closest to this point of growth will divide, making sure that the instructions for growth and for life are available nearby, and the cytoskeleton is elongated and also grows into the new volume of cell. I personally found this plant, and the way it grows, to be an inspiration for considering what life is and the mechanism of life, because although it may seem to be doing something different and strange, it is actually just a larger scale version of what all cells do. All living cells can grow, and this is managed by the cytoskeleton under the instructions of genetic material. Different cells may do this in slightly different ways, perhaps with the DNA protected inside a nucleus or free in the cytoplasm, or perhaps as the cells enlarge, they also divide. But the basis of life remains the same: a network of molecular machines that work together and regulate themselves, each component changing shape rhythmically and cyclically to drive processes that organise sparse resources into consolidated material structure. Based on the peer-reviewed paper: Pierce S (2023) Life’s Mechanism. Life, 13(8):1750. https://doi.org/10.3390/life13081750 Part of a series on what life is:    • What is life and how does it work? The mec...      • What life actually is | a biologist explains      • Caulerpa and the secret of life      • If life happens in cells, how can large or...      • Extraterrestrial biospheres | the kind of ...      • Astrobiology's n=1 problem is unscientific   This channel is not monetized.