☔️ Water series: The primordial soup of life: oceans water

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Osmoregulation model in the sea (Gilles Bœuf, Collège de France, 2014).

The source of all our food which is also involved in most of the cooking techniques we use every day is, in fact, this basic molecule essential to life: water (H2O). It is also one of the major environmental concerns, when disputed as a ressource for humans or as a milieu for wild species. How water is the honorable “background” of every form of life on Earth, and how our relationship with it might change while facing global warming, are some of the questions that led us to this “☔️ Water series”, divided in 4 chapters:

  1. ☔️ Water series: The primordial soup of life: oceans water
  2. ☔️ Water series: Interactions of flora with water and climate
  3. ☔️ Water series: How mankind uses water? From food to energy
  4. ☔️ Water series: Future extreme dynamics of water use

Unchanged oceans in the blood

Factor salinity: aquatic organism response (Gilles Bœuf, Collège de France, 2014).
Osmoregulation of a hypo osmotic animal: a seawater fish (Wikimedia).
Osmoregulation of a hyper osmotic animal: a freshwater fish (Wikimedia).

Concentration of salts in cells

Water, and more precisely sea water, is the main ingredient in the recipe of life. Every form of life on Earth is constituted of one primordial yet complex element that we know as “the cell” — wether it contains a core protecting its DNA[1] (eucaryotes like plants and animals) or no core and a free floating DNA[1] (procaryotes like bacterias). What this cells contain and where they generally hang around is generally salted water. As life emerged and thrived in the Earth’s oceans, the chemical concentration of both the inside of cells and most fluids used by living beings (to convey nutrients or waste), are surprisingly close to sea water. Your very own blood osmotic pressure and nutrients concentration[2] is constantly regulated by organs like the kidneys to remain close to a sea water-like concentration. Despite our evolution on land, the very soup we keep inside reminds us where our ancient ancestors come from: the ocean.

Exchange of water inside and outside

This comes as no surprise, as this safe liquid environment has kept a chemical composition amazingly stable since 4B years[3] (closed sea did not). Unlike oceans, the atmosphere chemical composition drastically changed since the first steps of life, from a plant-friendly environnement with high carbon dioxyde (CO2) concentration, to mostly nitrogen (N2) and a higher oxygen (O2) concentration, correlating with the great expansion of terrestrial life.[4] Since then, plenty of life forms challenged their capacity to live in the unfriendly atmosphere, exposed to the burning sun, while keeping their water inside without dying. Animals like jellyfishes seem extra-terrestrial to us mammals; but they are closer to what life was at first, opposing close to no boundaries between their inside and the liquid environment they belong to. Proof is that jellyfish species thrive since 650M years, when we great apes just began our journey 20M years from now. Jellyfish soup is dictated by the osmotic laws that tend to harmonise chemical concentrations and pressure between 2 liquids that can mix together, even through a thin membrane separates what is dead, the ocean, from what is alive — they are called “osmoconformers”.[3][5]

Body salinity strategies

There must be a good reason to keep the same salinity on the inside and the outside, as it asks for energy to regulate your internal salinity instead of letting it depend from the surrounding sea water. On the contrary, vertebrate fishes[6], with their ability to isolate their inside from the outside, conquered new unclaimed areas consisting on fresh waters. Without this skin and regulation tactics, they would simply dilute in rivers, unable to fight the osmotic laws that would push their inside highly salted water outside in the less salted water (a jellyfish in fresh water will dissolve). However, water must be absorbed to run through the fish gills[7] so that fishes can extract the oxygen they need to live. This means that breathing basically disrupts their body salinity, and that’s why fresh water fishes reject large amount of water by peeing to stay salty. On the other hand, sea fishes reject salt filtered by their gills while drinking a lot of sea water to prevent their salinity to rise. We are closer to the first ones, as we get the desired amount of salt from our savoury food, while peeing unsalted water.[3]

Life’s soup recipe in oceans

Illustration of the osmosis process, due to diffusion (GIF made by Popolopo).
The Sodium-Potassium pump involved in energy production in cells (Socratic Q&A).

No energy without salts

All that does not answer how osmosis also makes life in water possible and so unique. Well, one clue might well be the facilitated exchange between 2 peculiar elements[8] between the outside and the inside of any cell; an exchange that is primordial to their stability. Cells use small channel proteins at the surface of their semipermeable membrane (skin), that are basically holes able to open or close when the right elements knocks at the cell door (this is “selective permeability”). Thanks to omosis[9], if one element is low inside the cell, the same element would naturally be absorbed by the cell if it opens the door. On the opposite, one element that is high inside would escape from the cell by itself. That’s how cells, by degrading 1 molecule of Adenosine triphosphate (ATP), can import 2 potassium ions (K+) from the outside and export 3 sodium ions (Na+) from the inside. Thus, the ATP degrades in Adenosine diphosphate (ADP) + energy, and that’s why Adenosine is considered as the “energy conveyor” molecule, found in every form of life. This whole process is called “the Sodium-Potassium pump”[9], and might also be an element of neuronal control in the brain — which is constituted with more than 70% water for us humans. To let your brain work seamlessly, you therefore need to absorb a lot of water and salts so that chemical energy can produce thoughts.

Cells emergence in sea water

As the youtube channel Kurtzgesagt says, “a cell is a piece of the dead universe that separated itself from the rest, so it can do its own thing for a while”.[10] Here, “the dead universe” would be sea water, and what separates the cell from it would be its membrane. This “skin” main structure is the “lipid bilayer”, that has been shown to be able to emerge spontaneously, under certain conditions in water containing phospholipids, that are just some kind of fat. If you ever made mayo or other types of sauce, you would know that fats can’t mix with water, and are therefore useful for the cell to separate itself from its watery environment. This is only one of the scientific experiments that tries to demonstrate how life could have emerged in the “primordial soup”, 3.5B years ago. In fact this hypothesis of life emerging in oceans water, called “abiogenesis”[9], is widely accepted by scientists, but its mechanisms stays poorly understood.

Notes

  1. 1.0 1.1 Desoxyribonucleic acid: the molecule carrying genetic informations of one cell.
  2. Both sea water and blood share those characteristics: Total osmotic concentration: 1100 osmol/L; Sodium (Na): 470 osmol/L; Potassium (K): 11 osmol/L; Chlore (Cl): 560 osmol/L; Calcium (Ca): 10 osmol/L. Gilles Bœuf, op. cit. source: https://www.college-de-france.fr/site/gilles-boeuf/course-2014-01-07-11h00.htm
  3. 3.0 3.1 3.2 (FR) Gilles Bœuf, De l’apparition de la vie dans l’océan ancestral à l’émergence de l’homme, Collège de France, 2014. source: https://www.college-de-france.fr/site/gilles-boeuf/course-2014-01-07-11h00.htm
  4. At first, terrestrial life was mostly plants changing CO2 in O2 by photosynthesis: they were the “sun-eaters” that changed the composition of the Earth’s atmosphere. It is humans turn now, as atmospheric carbon dioxyde is on the rise again since the Industrial Revolution.
  5. Like starfish, mussels, scallops, etc. but also sharks and other cartilaginous fishes who have high urea concentration in their body so that water diffuse naturally through their skin to regulate their internal salinity. source: https://en.wikipedia.org/wiki/Osmoconformer
  6. Most fishes that we know and see in our plates are bony fishes, from sardines to trout.
  7. Gills are sort of semi-outer lungs that most marine life uses to filter the sea water and extract dissolved oxygen from it, either by the force of the currents, by moving (ram ventilation) or breathing. source: https://en.wikipedia.org/wiki/Gill
  8. Sea water has an average salinity of about 3.5%, where salt is a mix of Sodium (Na+) and Chloride ions (Cl-), with other salts including Potassium ions (K+) in lower concentration.
  9. 9.0 9.1 9.2 source: Wikipedia pages:
  10. source: https://www.youtube.com/watch?v=QImCld9YubE&t=63s