Largest cells on Earth

(ORDO NEWS) — We talk about the champions among unicellular organisms – bacteria the size of a fingernail and algae the size of a ball – as well as the main threat to the existence of these giants of the microcosm.

We used to think that in order to see a living cell, you need a microscope. But it is not always the case. For example, the largest cell in the human body, the egg, reaches a diameter of 0.1 mm and is quite visible to the naked eye.

The largest cell is generally called an ostrich egg, although this is “cheating”: there is really only one cell in it – the embryo of a future bird – but the lion’s volume of the egg falls on nutrient reserves. But in nature there are real giants, full-fledged living cells that can not only be seen, but simply taken, feeling the heaviness in your palm.

Largest cells on Earth 2Typical sizes of cells, organelles, viruses and molecules

Bacteria: cells up to 2 cm

Unlike fungi, plants, and animals, bacterial cells do not contain nuclei and their DNA resides directly in the cytoplasm. In addition, they are much smaller, usually no more than a few microns – as separate organelles that work in eukaryotic cells.

But recently, while studying the microflora of mangroves in the Caribbean, biologists have discovered extremely unusual bacteria, gaining an average of about a centimeter in length. Outwardly, Thiomargarita magnifica looks like small mold fungi: at one end they are attached to the substrate – the fallen leaves of mangrove plants – and gradually lengthen.

The largest specimens grow up to 2 cm with a diameter of 0.15 mm, like thin threads. Toward the end, bridges gradually form on them, from which new bacterial cells “bud” from time to time.

All this is strongly reminiscent of the last prokaryotic record holder, Thiomargarita namibiensis , found in bottom sediments off the coast of Namibia and reaching 0.75 mm in length. Both those and others accumulate sulfur crystals, due to which their filamentous cells shimmer in the light with all the colors of the rainbow.

Thanks to this effect, they received the generic name Thiomargarita – “sulfur pearl”. Interestingly, these bacteria contain something like nuclei – membrane structures that contain DNA.

A cell can have about 700 of these “pseudo-nuclei” and carry about 11 thousand genes, which is also quite a lot for prokaryotes. However, these giants of the microbial world still cannot be compared with single-celled eukaryotes.

Largest cells on Earth 3Each thread of Thiomargarita namibiensis is a single bacterial cell

Algae: cells up to 20 cm

In tropical seas, the pot-bellied valonia ( Valonia ventricosa ) is quite widespread , it is also the “sailor’s eyeball” – an algae, the spherical cells of which reach 5 cm in diameter. But if you go to a great depth where sunlight does not reach, you can find even more exotic creatures.

Local xenophyophores can grow from a small ball – up to 20 cm – while remaining one and completely independent cell. Xenophyophores are found at depths up to 10 km, covering entire areas of the bottom, extremely poor in nutrients and life, with real “meadows” of cells of various variable forms.

Xenophyophores feed by filtering organic matter from the water that descends from the surface. Like other foraminifera, a fragile “skeleton” of limestone bound with organic adhesive adhesives is deposited on their surface.

The cell branches, filling each section of its own labyrinth. And small fish Paraliparis from the family of sea slugs lay their eggs here.

These fish have a special ovipositor, long and hollow, like a syringe needle. It allows them to bring eggs to the most unexpected places: inside motionless sponges or shells of mollusks, on the gills of crustaceans, and under the xenophyophore exoskeleton.

Largest cells on Earth 4Bottom “carpet” of unicellular xenophyophores interspersed with manganese nodules

In defense of unicellular

Other deep-sea inhabitants also huddle here, finding shelter side by side with giant unicellular: crustaceans and molluscs, polychaete worms and sponges. A single xenophyophore cell can coexist with hundreds of such smaller multicellular organisms, and not all of these animals have been described by science.

It is not surprising that in the International Information System on Agriculture and Related Industries (AGRIS), which is being developed under the auspices of the UN, xenophyophores are named one of the indicators of the sustainability of marine ecosystems, along with corals.

Their damage and the destruction of “oases of unicellular” at the bottom endangers entire groups of local organisms.

Meanwhile, the most luxurious xenophyophore benthic gardens that have so far been found are just in places full of risk – for example, on the abyssal plains of the Clipperton Fault Zone, east of the Hawaiian Islands in the Pacific Ocean.

Here they cover millions of square kilometers, and on each square meter there is room for 12 giant unicellular organisms. The threat is right there, in the form of heavy spheres – nodules of useful minerals that slowly crystallize from the water. Such formations are composed of valuable metals such as manganese and attract great interest.

A number of companies, such as Nautilus Minerals, are building the technology and robotics to exploit these deepwater deposits. Such projects pose a huge threat to all benthic life, and especially to the large but sensitive single-celled xenophyophores on which many neighbors depend.


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