As I have argued before, one key to the success of bacteria-as-terraformers is their ability, as single-celled organisms, to network with each other both physiologically and genetically. Well, recent evidence strongly suggests that such connections also include charitable behavior, where certain cells come to the aid of their neighbors:
Humans are capable of great charity, taking hits to their bank accounts and bodies to benefit their peers. But such acts of altruism aren’t limited to us; they can be found in the simple colonies of bacteria too.
Bacteria are famed for their ability to adapt to our toughest antibiotics. But resistance doesn’t spring up evenly across an entire colony. A new study suggests that a small cadre of hero bacteria are responsible for saving their peers. By shouldering the burden of resistance at a personal cost, these charitable cells ensure that the entire colony survives.
Read the rest about this eye-popping study here.
Back in 2001, I proposed that the original cells, used to seed this planet, contained the ability to form “viretes.” The basic idea is that the virete would function something like a gamete, but instead of transmitting genetic information across time to future generations, it would transmit genetic information across space to facilitate the survival of the founding group (see my discussion on cross-talk ) by connecting them. Here is how I put it back in 2001:
Actually, I have been toying with the idea that viruses were designed (keeping in mind that I view viruses as non-living, life-dependent phenomena and not organisms). I would speculate that viruses were originally designed to allow the designed cells to cross-talk extensively. More specifically, I envision cells designed with the program to disperse part of their genetic constitution laterally through a life-cycle-like stage that involved replicating and packaging genetic material for dispersal. In short, I speculate that what we now know as ‘viruses’ were originally a designed sex-like mechanism for unicellular organisms, important for establishing a foothold on a sterile planet (I call them viretes). Possible expressions of this mechanism might include:
a. A cell suicide program coupled to the packaging of genetic material for dispersal.
b. An endospore-like program, where instead of forming a spore around the replicated DNA, the DNA is packaged in virus heads which in turn are packaged into a “release” cell.
c. Controlled exocytotic release.
I would further speculate that such sex-like mechanisms may have been important in the early stages of the designed founder effect allowing the heterogeneous cells to adjust, as a consortium, to an unfriendly environment. During this adjustment phase (analogous to the latent phase in a bacteria growth curve), the cells shuffled their material and hit upon global-adaptive state whereby the importance of such transfer was decreased. We still see “rusty remnants” of this state carried on by the vestiges of transposons, natural transformation, and yes, viruses.
Well, almost 10 years later, it’s looking like I was on to something:
No, not chemical connections.Not genetic connections. Not conceptual connections. How about electrical connections?
Deep on the ocean floor, colonies of bacteria appear to have connected themselves via microscopic power grids that would be the envy of any small town. Much remains unknown about the process, but if confirmed the findings could revolutionize scientists’ understanding of how the world’s smallest ecosystems operate.
Oxygen-breathing bacteria that live on the ocean bottom have a problem. Those sitting atop the sediment have ready access to oxygen in the water but not to the precious mineral nutrients that lie out of reach a centimeter or so below the ground. Meanwhile, those microbes that live in the sediment can access the nutrients, but they lack oxygen. How do both groups survive?
Before looking at a more radical example whereby symbiogenesis with bacteria played a key role in the evolution of a certain metazoan lineage, I thought it a good idea to stress the significance of the terraformers.
The common perception of bacteria is that they are primitive, single-celled organisms. Yet they are not primitive; they are extremely sophisticated in many ways. That’s something most readers of this blog can probably agree with. But I would also argue that, on balance, it is also misleading to think of bacteria as single-celled organisms when, in reality, they are more like cells that are part of superorganism. They form a web of connections. We’ll explore that in future blog entries.
But for now, consider another common perception of bacteria – they are minor players and easy to ignore except when they cause disease. Wrong. Our existence is built on the back of bacteria. Consider a recent survey of the ocean’s biotic diversity:
marine microbes account for up to 90% of all ocean biomass and collectively weigh the equivalent of 240 billion African elephants.
240 billion African elephants. And it’s safe to say that the majority of this microbial biomass is bacterial. Consider this from “Prokaryotes: The unseen majority” by William B. Whitman, David C. Coleman, and William J. Wiebe:
Thus, the total amount of prokaryotic carbon is 60–100% of the estimated total carbon in plants, and inclusion of prokaryotic carbon in global models will almost double estimates of the amount of carbon stored in living organisms. In addition, the earth’s prokaryotes contain 85–130 Pg of N and 9–14 Pg of P, or about 10-fold more of these nutrients than do plants, and represent the largest pool of these nutrients in living organisms.
In fact, how many bacterial cells exist on the planet? Answer – 5 x 10^30
And as any microbiologist will tell you, these cells are found everywhere we find life, including places where bacteria are the only life forms.
So why is all this significant?
Over a year and half ago, I laid out the four general expectations that arise from the hypothesis of front-loading evolution. One such expectation was that “front-loading would be linked to terraforming.” As I explained,
So if we are to front-load the existence of mice-like creatures into the genomes of single-celled organisms, we also need to ensure the Earth will be prepared, at some point, to receive the mice. And it is the preparation of a receptive Earth that we can call terraforming
Bacteria are easily viewed as the terraformers, where one of their most glorious successes was to draw from the ancient Earth’s abundant supplies of water and use this to oxygenate the atmosphere which in turn would facilitate the evolutionary emergence of eukaryotes, then metazoan. Yet there is much more to bacteria.
When it became clear that the genome of a single-celled eukaryotic organism did not need to be radically retooled to transition to the multicellular state of an organism like Volvox, one of Jerry Coyne’s colleagues commented, “Maybe all the hard work was done by bacteria.”
Indeed. Not only have bacteria terraformed our planet, but they probably facilitated metazoan evolution itself. In fact, they may have assisted metazoan evolution such that nothing like a metazoan would have emerged had bacteria not existed.
As a tease for this shift in thinking, consider some recent research:
Here’s a small excerpt from a Science Daily article that might help you better visualize the relation between front-loading and terraforming:
During the Proterozoic, oxygen levels in the atmosphere rose in two phases: first ranging from 2.5 to 2 billion years ago, called the Great Oxidation Event, when atmospheric oxygen rose from trace amounts to about 10% of the present-day value. Single-celled organisms grew larger during this time and acquired cell structures called mitochondria, the so-called “powerhouses” of cells, which burn oxygen to yield energy. The second phase of oxygen rise occurred between about 1 billion and 540 million years ago and brought oxygen levels to near present levels. This time intervals is marked by the earliest fossils of multi-celled organisms and climaxed with the spectacular increase of fossil diversity known as the “Cambrian Explosion.”
Notice that the two most significant evolutionary events – the origin of eukarya and origin of metazoa – were both coupled to rising oxygen levels. Oxygen, of course, is a biological output. This understanding can lead us to a tantalizing shift in perspective – the evolution of the complex eukaryotic cell, needed to make something as complex as animal, and the evolution of metazoa itself, were not the gradual accumulation of incremental complexity stretched out over deep time. Instead, the origin of these two novel states was nudged into existence, as least in part, due to rising oxygen levels which in turn had a biological origin.
Hey, let’s connect cell-cell communication with terraforming:
In this newly discovered mechanism, bacteria coalesce on tiny particles of carbon-rich detritus sinking in the ocean. They send out chemical signals to discern if other bacteria are in the neighborhood. If enough of their compadres are nearby, the bacteria en masse commence sending out enzymes that break up the particles into more digestible bits (see interactive below).
As a result, a substantial amount of carbon does not sink to the depths, which affects both the marine food web and the planet’s climate. The re-released carbon can be reused by marine plants, and less carbon dioxide, a heat-trapping greenhouse gas, is drawn out of the air into the ocean. In addition, less carbon is effectively transferred to the bottom of the ocean, from where it cannot easily return to the atmosphere.
The finding represents the first evidence that bacterial communication plays a crucial role in Earth’s carbon cycle.
Let’s return to Lake’s new hypothesis about the origin of double-membrane bacteria:
Here, by analysing the flows of protein families, I present evidence that the double-membrane, Gram-negative prokaryotes were formed as the result of a symbiosis between an ancient actinobacterium and an ancient clostridium.
Since actinobacteria and clostridia might represent cells very similar to the original cells, let’s have a look at them. First, consider actinobacteria.