As Spring gets started and the American midwest is hopeful that the snow will finally melt away and the Polar vortex of the 2014 winter will be done already, there's a bacterium--Pseudomonas syringae-- that can control the weather, cause frost damage to plants, and it also bears the distinction of being the first GMO approved for release to the environment. In fact, this bacterium might even explain the ice powers of Elsa in Frozen -- maybe she just had the right bacteria at her fingertips.
Snow -- and hail and rain -- can be caused by bacteria. It's not that bacteria are synthesizing the water, but bacteria can serve as the nucleation point, encouraging water molecules to surround them and form a droplet. This means that all those unique snowflakes have a structure something like a Tootsie roll pop. Except instead of a candy center, when you catch a snowflake on your tongue and it melts away, the delicious center is a tiny cluster of bacteria.
Controlling the Weather
Making it rain
This may seem like an idle curiosity, but the implications of this are actually pretty amazing. Bacteria could potentially help us control the weather. Bacteria (or the proteins they use to nucleate water) are already used in snow machines at ski resorts, and have been used in some pilot studies to look at using drones to seed the air around Lake Tahoe with bacteria to encourage more snow to fall. If approaches like this are successful, one could imagine a future where we actually coax rain from clouds to alleviate drought.
The Frost Queen
Snow machines make use of
bacterial technology
The organism responsible for water nucleation is most often Pseudomonas syringae, and it's an organism everyone should know. In order to make it rain, this organism has to survive up in the atmosphere miles above the surface--and it does. How does it get there? Well, it primarily grows on the surfaces of plant leaves, and is accustomed to getting blown around by the wind so that it can colonize new plant leaves. But it's not always benevolent. P. syringae is a pathogen for many species of plant, and it's ability to encourage ice to form means that it is often responsible for frost damage killing plants.
The First GMO
Bacteria encourage frost during cold snaps
P. syringae also bears the distinction of being the first genetically modified organism (GMO) approved to be released into the environment, back in 1987. The idea was to spray crops with mutants that lack the ice-nucleating proteins. It worked, and protected the plants from damage, but as far as I know wasn't cheap enough to go on to wide spread use.
"We are made of Star-stuff"
The Origin of Life is filled with mysteries. As Neil DeGrasse Tyson says on his reboot of Cosmos "We are made of Star-stuff". Pretty cool, but how did that Star-stuff turn into the life we see today? Well, life today can be split along the cellular/acellular divide, with the smallest self-replicating bacterial cells on one side and viruses on the other. Even the recent excavation of a large 34,000 year old virus hasn't challenged this divide.
But there remains a cellular mystery, which is that all life on this planet relies on one of three basic cell architectures: the Bacterial cell, the Archaea cell and the Eukaryotic cell. But one of these things is not like the others. The Eukaryotic cell is weird. It's really, very, VERY strange. It's an energy-guzzling behemoth that somehow broke the rules of surface-to-volume efficiency and bioenergetics to become a success story on planet Earth. In this post, I'll discuss how the Eukaryotic cell came to be -- it's not about the delightful invention of the nucleus, or about benevolently engulfing a bacterium. It's about the pre-cellular network of life, and a predatory bacterium that invaded that network.
-->If you want the short version you can just watch the movie below, or get the full story by reading below.
Life, Getting Started
Before life could get going a couple pieces had to fall into place. There had to be a complex biochemical ecosystem, that included self-replicating macromolecules. There are now several lab examples of how this can occur such as RNA molecules that self-replicate. Recent efforts have shown that self-replicating macromolecules don't break the laws of thermodynamics, but they do bend them a little. It seems there are rules to how and when self-replication can occur, and self-replicating RNAs are just one example of this. In addition, all life today uses the same chemical language, in the form of ribosomes, that gave form to the chaos of that biochemical ecosystem on the early earth.
An origin for viruses
At this early point, isolated cells would only contain a fraction of this diversity and would have been a hindrance to evolution. Instead, there may have been a Woesian pre-cellular network of life that connects nodes for processing information -- such as translating ribosomes -- with self-replicating RNAs that jump from node to node, exchanging information through horizontal gene transfer (HGT). Packing up some fraction of this ecosystem would lead to several possibilities. First, let's consider: (1) packages with no self-replicating elements. These would have little to no impact, as they would transfer material but no information. (2) packages that contain self-replicating elements, let's call this kind of element a virus. Viruses carry information and can carry that information across great distances. This would spread information and bridge physical gaps in a pre-cellular network. If a viral package lands at a processing node, than the information can be amplified and spread further. It is worth noting that today, viruses are still prolific, outnumbering cells on our planet 100:1, so they may well pre-date cellular life as a mechanism of information exchange. Finally, we can consider possibility (3), a package that contains replicating elements (memory) and translating elements (language) would be very special, and have the potential for self-contained replication of the entire package, let's call this a cell.
A Special Package
An origin for bacteria cells
So these special packages, these simple cells, depart from the complexity of the network and start passing their information vertically, from parent to offspring instead of horizontally through the complicated HGT mechanisms of the network. This kind of event may have happened several times, but two lineages of the simplest cellular package survive today - the Bacteria and Archaea. These cells are small, but they are independent. They evolve. They fill niche after niche across the planet, they learn to use sunlight for energy, learn to breath Oxygen and some of them even learn to hunt. When they hunt other bacteria we call them predatory, when they hunt us humans we call them pathogens. These early hunters became specialists at invading the membranes of other living things, stealing their nutrients before moving on to hunt again.
The Predator that crashed the network
Invasion of the host network
So imagine then, that one of these predatory bacteria tries to invade the pre-cellular network, the network that gave it life. And what if the network didn't want to die? Perhaps the network was invaded and plundered many times, but one time the predator was caught and a balance was struck. First nutrients and then information flow back and forth between predator and the host network. As the predator grows and divides it provides more and more energy to the network, let's call this hypothetical element a mitochondrion. The network provides stability, safety and utilizes the energy from the mitochondrion to build structures of greater and greater complexity. Eventually, something like the events that gave rise to the bacteria and archaea cell plans, gives rise to one additional cell plan. This cell plan is bigger, more complex, but still a self-replicating package. This scenario is one possible explanation for how the seeming impossible Eukarya lineage is born.
The Network that caught the predator
An origin for eukaryotic cells
So, why couldn't a eukaryotic cell arise first and then later engulf a bacterium? (1) Genomic evidence indicates that all eukaryotic organisms descend from a progenitor that had mitochondria. (2) The closest bacterial relative to the mitochondrion is Rickettsia, an obligate intracellular pathogen, that can not live on it's own, but must live inside of a host. So, the mitochondrion likely started with a bacterium invading, rather than a large eukaryotic cell engulfing. (3) Energy. This is the most critical issue. The idea that Eukaryotic cells swallowed a bacterium and then domesticated it is a bit like saying that the first American colonists started building houses with wires and light bulbs because they figured an American would discover electricity some day. Of course, the power source had to be available first. And the mitochondria are the power source of the Eukaryotic cell. So it's worth spending some time thinking about the energetics of the Eukarya cell plan.
Consider this: a typical eukaryotic cell needs 1,000 times more energy to replicate than a bacterial or archaeal cell. Specifically, it takes harvesting enough energy to yield 10 trillion ATP for a Yeast cell to make a brand new copy of itself, compared to a mere 10 billion ATP for an E. coli cell to do the same trick. This means that whenever resources are scarce, let's say only enough energy is available to make 9 trillion ATP in the cell, then the Yeast cell doesn't have enough energy to go from 1 cell to 2 cells, while the smaller bacterial (or archaeal) cell type not only goes from 1 cell to 2 cells, it has enough energy to make as many as 900 new cells. This energetic barrier is also (part of) what prevents bacterial and archaeal cell types from growing as large as Eukaryotes, the larger they get, the more inefficient the cells are at utilizing resources and the less they can reproduce.
The Origin of Life
An origin for life based on the Woesian tree
Is this origin story correct? It's hard to say with complete certainty what events exactly transpired across the vast expanse of time that led to life today. But we do have the ability to construct testable hypotheses, which have already led to the resounding proof that all life today descends from the same common ancestor, and that the Eukaryotic cell plan had mitochondria from the beginning. The energetics of the Eukaryotic cell, and the intimate symbiosis with mitochondria both imply that this special symbiosis may have developed before the first Eukaryotic cell left the pre-cellular network.
The Fate of the Network
And what became of this pre-cellular network? Was it permanently crashed by the predator that became the mitochondrion? Was it gobbled up later by all the large, multi-cellular organisms of the Eukarya lineage? Or is it still thriving in some nook or cranny of the Earth today? I'm not sure....what do you think?
According to Wikipedia, “the singularity, is a hypothetical moment in time
when artificial
intelligence will have progressed to the point of a greater-than-human
intelligence, radically changing civilization.” Whether a technological singularity
is going to happen in the near future or the distant future is unknown and it may
in fact, never happen. But the concept that an evolving system such as artificial
intelligence, could produce something that is greater than the sum of its parts
is already evident in nature. For instance, humans have language systems that
are far more complex than the communications of our closest relatives such as
chimps, bonobos and orangutans. Language provides a new platform for development,
as information can be exchanged on a level that goes beyond genetic transfer of
information from parent to offspring.
Cellular life
The three cell architectures
Another example of a singularity-like event is at the very
beginning of cellular life. And this singularity -- the invention of the cell
-- might have happened three separate times.
All life that we know of is related and descended from a
common ancestor. The last universal common ancestor (L.U.C.A.) gave rise to all 3 domains of life we have today: Bacteria, Eukarya, and Archaea. LUCA might have been similar to a modern bacterial
cell plan, just more simple -- a so-called protocell, that once able to grow
and divide gave rise to all life on planet Earth. But another intriguing possibility is that the
LUCA was not simple and it was not cellular.
Network life
This idea comes from Carl Woese, who suggested that a
pre-cellular network spawned the 3 known cell architectures of Bacteria,
Eukarya, and Archaea – and spawned these 3 cell types at different times. The
idea is intriguing, because instead of relying on all of the complexity of life
coming from a single, simple package of replicating material, it implies that
pre-cellular life was a complex network of biochemical ideas. In this model,
there was an ecosystem of biochemical exchange, with a tremendous diversity
small packets of information exchange through what we now call horizontal gene transfer (HGT). There were no parent cells, no offspring and no linear form of
descent.
LUCA was also likely to rely heavily on RNA for information
and activity, with the invention of the language of translation (RNA ->
Protein) a critical first step in the origin of life. But the next critical
step was to spawn a cell from this network – the first bacterial cell that was
also the first isolated, self-replicating form of life. Woese argues that
bacteria, archaea and eukaryotic cells are so different from one another, that
they must’ve spawned from the pre-cellular living network at different times.
Fungal mycelia are a modern form of networked life
Is there non-cellular, network life today?
There is no evidence that the type of biochemically diverse network that Woese
describes giving rise to cellular life still exists today. But biochemists can perform in vitro evolution experiments to test how chemical systems can evolve outside of cells--and they do. And interestingly, many
forms of life today mix cellular life with a network of material exchange: Mycorrhizal connections allow metabolic exchange between fungi and plants, Plasmodial slime molds can forgo the limitations of cell division, forming extensive branching structures with a shared cytoplasm. And many bacteria form tubes and vesicles that connect neighboring cells. This
cellular/network mix is also found in some of the thermophilic organisms that
may be most similar to the very first cells.
s of cells when nutrients are scare, and form long, branching cytoplasmic networks without cellular separation and finally many
bacteria form networks of membrane extensions through
