In this little section on The Essentials For Life (As We Know It) so far, we’ve covered water, photosynthesis, and the nitrogen cycle, the last two being microbial in origin. This post brings these three components together in an essay that explores how the world today might be were there no microbes — specifically prokaryotes which I will define shortly — and with a surprise twist at the end!
If not for the deceptively-simple water molecule, all life as we know it would never have come to be. Every living thing on Earth is completely dependent on water’s fluid and solvent properties, with no exceptions. Even an anoxygenic photosynthesiser, which doesn’t use water in photosynthesis, still needs water in its cell for other biochemical reactions to take place. It also needs water’s solvency to make external nutrients available to it.
The origin of water on Earth is a field of study in its own right, and surprisingly controversial. Was it always here, or did it come from outer space? Regardless, it was here by about four billion years ago, well in time for the first life to appear half a billion years later.
Amongst the first to appear would have been the anoxygenic photosynthesisers, able to photosynthesise in the oxygen-free environment of early Earth. Such bacteria are still with us today, 3.5 billion years later. These photoautotrophs (’self-nourishment from light’) were crucial in bootstrapping life, for in harnessing light energy to create food for themselves, they could grow and reproduce (the very definition of a living organism), and set in motion food chains by becoming in turn food for other organisms (the heterotrophs, or ‘other/different nourishment’). Much life to this day depends on photosynthesisers at the beginning of a food chain.
I say ‘much life’ and not ‘all life’, as there are exceptions. On the deep ocean floor where there is plenty of life but no sunlight, chemoautotrophs, or ’self-nourishment from chemicals’, are the photosynthesiser equivalents at the beginning of food chains in that environment. These microbes extract energy not from photons, but from the chemical bonds of substances such as iron, manganese, sulfur and hydrogen — a fascinating area of biochemistry and microbiology here! But both photosynthesis and chemoautotrophy have the same basis: the extraction of energy from abiotic (non-living) components to produce biotic (living) components which then enter a food chain to be consumed by living heterotrophs. It is quite conceivable that chemoautotrophs were also amongst the first autotrophs, and may even have predated photoautotrophs.
Around two billion years ago, cyanobacteria appeared that could produce oxygen from photosynthesis, which eventually oxygenated the planet a billion years later and led to the evolution of complex multicellular organisms. Cyanobacteria too are with us to this day.
These photosynthesisers and chemoautotrophs are prokaryotes (pro-kar-i-oats, with the ‘a’ pronounced as in ‘apple’). Prokaryotes are the unicellular organisms we’d commonly call bacteria. (More accurately, prokaryotes are the Bacteria and the Archaea, but we don’t need to get into that kind of detail here!) These organisms have a single, circular chromosome and no cellular nucleus.
Eukaryotes (yu-kar-i-oats) are everything else that aren’t viruses. Eukaryotic organisms do have a cellular nucleus, and this nucleus harbours linear chromosomes. Plants are eukaryotes, as are we.
All prokaryotes are unicellular but this doesn’t mean all eukaryotes are multicellular — far from it. Fungi and protists (eg algae and amoebae) are both eukaryotic and unicellular, making them microbes as well. (This is why fungal and amoebic diseases are so hard to treat. They are eukaryotes just as we are, and thus what kills them can potentially kill our cells too. Prokaryotes have different cellular structures, and it is these differences which are exploited by antibiotics. Antibiotics have no effect on viruses, which are a different group again. Viruses aren’t even cellular, just RNA or DNA inside a protein coat, and whether they classify as ‘life’ has always been debatable.)
Some of the early chemoautotrophs could convert, or ‘fix’ nitrogen into biologically-available nitrogen compounds, and such organisms still perform this very important function to this day. This brings us to the nitrogen cycle.
Many multicellular eukaryotes have since come along that can photosynthesise just as well as the early prokaryotes — and there’s a very surprising reason for that which we will get to. But the nitrogen cycle is still wholly reliant on microbes (both prokaryotes and eukaryotes) every step of the way, from the nitrogen fixers breaking the very strong triple bond of nitrogen gas (N2) to make nitrogen biologically-available in the first place, to the decomposers recycling and the nitrifiers converting nitrogen-containing compounds, right through to the denitrifiers that recycle that nitrogen back into the atmosphere for the cycle to begin anew.
While some microbes are eukaryotes, it is very plausible that life could have persisted as nothing but prokaryotic to this present day 3.5 billion years later. They have everything covered, in that they can convert abiotic energy into food, as well as eat each other, decompose each other, and recycle all essential elements and compounds along the way. The first complex eukaryotes did not appear until about 600 million years ago — prokaryotes had had the planet to themselves for almost three billion years by then, and were obviously doing just fine!
While prokaryotes can function without needing eukaryotes, some prokaryotes did evolve symbiotic, or close interrelationships with eukaryotes. Some prokaryotes are parasitic and disease-causing, but others are beneficial with both parties benefiting. For example, gut microbes get a consistently warm, moist environment and access to food components, and in return produce essential vitamins and other compounds such as essential amino acids the host cannot make itself.
But these examples are more a case of opportunism than a case of need. Prokaryotes don’t really need us.
But should prokaryotes suddenly disappear, for whatever reason, we’d certainly realise how much we need them. One of the first signs of their absence would show up in ruminants (eg sheep, goats and cattle) and hindgut fermenters (eg horses). These animals are completely reliant on microbes breaking down the cellulose in their diets, as mammals do not have the gene for cellulase, the enzyme that breaks down cellulose. Even termites and other insect wood-decomposers rely on gut microbes to break down the cellulose and other compounds in wood.
And of course, as just mentioned, many animals rely on their own gut microbes (all prokaryotes) to provide essential components to their diets. Malnutrition and deficiencies would appear in the absence of these organisms, but would show up later than in animals with very cellulose-heavy diets, who would simply start wasting away once their body reserves were exhausted.
Fungi — unicellular eukaryotes — are major decomposers in many ecosystems and in the absence of prokaryotes would be vital in recycling the components of dead matter. Some can even nitrify, a crucial stage in the nitrogen cycle that plants depend on. Some fungi can also denitify, but the end product is nitrous oxide (N2O or laughing gas believe it or not), not nitrogen gas. Nitrous oxide has an atmospheric lifetime of 110 years, is a serious pollutant and ozone destroyer, and already of concern in the human impact of the nitrogen cycle. (Artificial fixation of nitrogen became possible with the Haber-Bosch process, first demonstrated in 1909. This has led to the mass production of artifical fertilisers, but an increase of biologically-reactive nitrogen in the ecosystem.)
Most significantly, because fungi do not feature in every aspect of the nitrogen cycle as prokaryotes do, they can neither begin the nitrogen cycle nor end it. The cycle would break in the absence of prokaryotes and nitrogen imbalances would arise.
Remember from earlier how autotrophs (whether photoautotrophs or chemoautotrophs) are at the beginning of all food chains, even now billions of years later? There is a very good reason for this. Let’s take a very simplified food chain whereby a plant is eaten by a herbivore, which is eaten by a carnivore. The plant, herbivore and carnivore each occupy a trophic level. As energy flows up the chain from the plant to the carnivore, less energy becomes available to each level. The energy difference isn’t ‘lost’ (this would be a violation of the First Law of Thermodynamics, or the conservation of energy), but rather is converted to (’wasted as’) heat during biochemical processes at each level — 90% each time!
With so little energy biochemically available to each trophic level, heterotrophic life could not continue were it not for autotrophs continually transferring energy in by converting the energy from photons or chemical bonds into food energy.
Fungi are heterotrophs, and specifically saprotrophs (’nourishment from rotten matter’), organisms that obtain their energy directly from dead and decaying organic matter. (With emphasis on organic matter, or matter containing carbon.) They are not chemoautotrophs, and cannot make available to plants inorganic matter such as iron, magnesium and so on. These are roles only prokaryotes can do, and it would seem that in a world with no prokaryotes, plants would slowly succumb to nutrient deficiencies, with consequences up the food chain.
For example, populations that rely on plants for their iron, calcium, magnesium, etc, needs in turn would succumb to health problems, as would populations that rely on them. Over time these elements could be expected to fall more and more out of circulation and remain inaccessibly locked-up in compounds with no prokaryotes to break them down.
However, all eukaryotes would have bigger problems than anything described above should prokaryotes really disappear from the world! This is because there is one vital dependency not yet mentioned that all eukaryotes — even the microbial ones — have on prokaryotes, that would have every last eukaryote dead in minutes should prokaryotes simply vanish. Yes, literally in minutes, if that.
With some exceptions, such as our red blood cells, each and every eukaryotic cell on the planet, whether a single-celled amoeba or one of millions in a tree, contains within it at least one prokaryote. Photosynthesising eukaryotic cells contain two! But what are they and how can this be?
Some time in the distant past a eukaryote engulfed a prokaryote. Remarkably, a beneficial relationship arose and the engulfed cell became a mitochondrion (the energy factory of a eukaryotic cell; plural is mitochondria). Some time later again, a photosynthesising prokaryote was also engulfed, and another beneficial relationship developed. That prokaryote became a chloroplast (still photosynthesising to this day). There are good illustrations of these here and here, and the former mentions some of the evidence for this.
Incredible, isn’t it? That our very cells contain prokaryotes in a symbiotic relationship. Prokaryotes are everywhere, and so important in all aspects of life that our own cells cannot survive without them.
A plant would slowly starve should its chloroplasts cease to exist, as it is completely dependent on these to make its food. But the disappearance of mitochondria would lead to an even quicker death for all eukaryotes including plants, for without mitochondria, cellular respiration cannot occur. In somewhat simple terms, cellular respiration is the collection of processes that combine oxygen with particles derived from food to create energy — even plants need cellular respiration, and mitochondria, to break down the food they made for themselves. In less simple terms: this.
An organism without mitochondria dies exactly the same as if it had inhaled hydrogen cyanide gas (HCN). HCN is a molecule formed when one hydrogen ion (H+) joins with a cyanide ion. A cyanide ion, also called a cyano group, is a molecule of one carbon atom triple-bonded to a hydrogen atom with an overall negative charge (CN-, or -C≡N). The cyano group binds irreversibly to the iron atom in a mitochondrial enzyme called cytochrome C oxidase, stopping cellular respiration and thus energy production. Either way, a cell with no functioning mitochondria quicky dies. When enough key cells die, such as in the heart, the organism dies.
Fun story: Cyanide is associated with a bitter almond smell, but not much is known about the taste (can’t imagine why…) except that potassium cyanide is perhaps acrid and burning. Well, many moons ago in the PhD days, I was pushing a lot of soil samples over countless days and nights through a high-performance liquid chromatograph (HPLC). Though when I used it it was called a high-pressure liquid chromatograph. Ours took up most of a bench compared to the more compact-looking ones on that page.
Acetonitrile was one of the solvents used. Its formula is CH3CN, and you may be able to recognise the cyano group in that formula?
Anyway, every night, for weeks on end, I went home routinely with the distinct taste of bitter almonds in my mouth! But no other symptoms. I never felt in any danger, though in these days of OH&S some busybody would probably have a fit! The lab was open and well-ventilated, and I did (as one should) all handling and preparation in a fume hood as much as was possible. Once the samples were loaded in the HPLC, everything was automated and I only needed to be around every few hours to load another carousel with samples and to top up the solvents. It wasn’t like I was hanging around sucking in fumes for jollies!
Acetonitrile in small doses is only mildly toxic apparently — yes it is metabolised into hydrogen cyanide by the body, but slowly enough that a body can keep up with it. I wasn’t drinking it or using it as a bodywash! Probably what was going on was that the metaobolites were being exhaled, and because taste and smell are so closely attuned, I was tasting the vapours. <shrug> It’s worth mentioning that it was only me in that lab that ever experienced this. Apart from the fact that not everyone needed to use acetonitrile for their samples, I was pumping through lots of samples and all at once — I was the HPLC hog! So much so that I adopted a nightshift routine so others could have a go during normal hours.
For the simple and unassuming organisms they appear to be, prokaryotes truly are the ultimate life-enablers and recyclers. No eukaryote can fix nitrogen, photosynthesise, break down cellulose, or even exist without prokaryotes around. A world without prokaryotes would be a lifeless one without question.
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