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There are many unique and fascinating plants in the world and you can only argue about which one is the most special one. However, plants of the genus Caulerpa have a good reason to be one of the top candidates.


Caulerpa, which resembles ordinary sea grass, but is actually classified as an algae, consists of only one single cell. This makes it one of the largest single cell organisms in the world. As of now, a total of 101 species have been classified, with most of them showing different growth forms. What's truly captivating is that these algae are composed of only one single cell, yet they manage to attain staggering sizes, often spanning several meters in length, as well as complex structures that can resemble 'real' leaves. This is remarkable, especially if you keep in mind that the human body contains an average of 30-40 trillion cells.


The biology


There is no denying that this group of algae is challenging our understanding of plant biology by showing complex structures and sizes despite being just one single cell.


In the world of Caulerpa, the conventional boundaries of what a single cell can achieve are blurred. Caulerpa is a coenocytic. An organism that exists as a single cell, but contains multiple nuclei is called coenocytic and definitely defies the norm we associate with multicellular organisms. Caulerpa thrives as a single cell entity with a shared cell wall, housing a multitude of protoplasts within.


Caulerpa’s growth pattern resembles higher plants on land: It develops a horizontal stem that extends into a network of branches, all adorned with leaf-like structures. How can a solitary cell grow to such impressive sizes and develop structures akin to leaves, stems, and even roots? Other organisms, that show a similar complexity in structure, require the coordination of thousands of cells.

 

Excursus


What is a protoplast?


Let’s start with: What is a cell wall? A cell wall is a distinctive feature of plant cells. It's a rigid structure located outside the cell membrane that provides support, protection, and shape to plant cells. The cell wall is primarily composed of cellulose (a complex carbohydrate), along with other substances like lignin and pectin. This structure gives plants their structural integrity and allows them to stand upright. Humans do not have cell walls like plant cells do. Human cells are enclosed by a cell membrane, also known as the plasma membrane, which serves a similar purpose of protecting and maintaining the integrity of the cell.


In plant biology, a protoplast refers specifically to a plant cell whose cell wall has been removed, leaving the plasma membrane and its enclosed contents. Do we (humans) have a protoplast? In animal cells, the equivalent to a protoplast would be the cytoplasm, which consists of the fluid and organelles contained within the cell membrane.


A plant cell (above) and an animal cell (below)
 

So, how does this work?


While Caulerpa is a single cell, its protoplasm is not uniform. Different regions of the protoplasm can have variations in composition and concentrations of cellular components.


This differentiation is comparable to specialized zones within a multicellular organism, each responsible for specific functions. Caulerpa's coenocytic structure means it has multiple nuclei within its single cell. These nuclei can specialize and direct different functions within the cell. Each nucleus contains genetic information that controls the production of specific proteins and molecules. These proteins play roles in regulating growth and development, similar to how genes control growth and differentiation in multicellular organisms.


Just as multicellular plants use hormones to regulate growth and development, Caulerpa may have molecular signals that influence different parts of the cell to take on specific roles. These signals could be chemical cues that trigger the growth of certain structures.


Did you know? The largest cell in the human body is the ovum - the female egg cell. It can reach up to 100 micrometers in diameter and is visible to the naked eye. The second largest cell is a Purkinje cell. This neuron is found in the brain and is responsible for movement and balance. Its size ranges from 20-100 micrometers in diameter. Purkinje cells can have dendrites that are several millimeters in lengths. Dendrites are responsible for receiving signals from other neurons or sensory receptors.


Are there any other organisms like Caulerpa?


Apart from Caulerpa, other algae also exhibit coenocytic growth. For instance, certain species of the green algae genus Valonia are also coenocytic. These large, spherical cells can be found in marine environments and are sometimes referred to as "bubble algae." There are also fungi and molds that are coenocytic, such as those in the Zygomycota group.


A fun fact: Some species, such as C. lentillifera or C. racemosa, are edible and are cultivated in Southeast Asia, Oceania, and East Asia.


A not so fun fact: These algae are an invasive species in the Mediterranean sea, threatening the ecosystem. Because of that, they are sometimes referred to as the killer algae.


An interested fact: Research is currently investigating extracts from C. taxifolia as a possible source for anti-cancer drugs. CTE (C. taxifolia extract) has been shown to arrest breast and lung cancer cell growth in a recent study (2019).

 

Selected references


Ranjan A, Townsley BT, Ichihashi Y, Sinha NR, Chitwood DH (2015) An Intracellular Transcriptomic Atlas of the Giant Coenocyte Caulerpa taxifolia. PLoS Genet 11(1): e1004900.


Mehra, R., Bhushan, S., Yadav, U. P., Bast, F., & Singh, S. (2019). Caulerpa taxifolia inhibits cell proliferation and induces oxidative stress in breast cancer cells. Biologia, 74(2), 187-193.


Klein, J., & Verlaque, M. (2008). The Caulerpa racemosa invasion: a critical review. Marine pollution bulletin, 56(2), 205-225.


Jacobs, W. P. (1994). Caulerpa. Scientific American, 271(6), 100-105.


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