or not. For example, if there is intense competition for resources, it might be better to use less energy and become more inactive. An example is to be found in the microbe P. aeruginosa, which can live as individual cells, but at a certain concentration of cells, they form a structured biofilm which can allow them to be more resistant to environmental extremes. Quorum sensing is mediated by organic molecules such as very small chains of amino acids (peptides). The prokaryotic ability to engage in rudimentary forms of communication shows us that interactions between cells and complex social dynamics are not just the preserve of multicelled eukaryotes, but communication can also modify and shape prokaryotic populations.
5.11.3 “Multicellularity” in Prokaryotes and Single-Celled Eukaryotes
Prokaryotic cells, and some eukaryotic cells such as certain amoeba, are often described using the term “single-celled” to differentiate them from so-called “multicellular” organisms. I have used these terms in many places in this textbook. It is important to gain some clarity on the use of these terms. In animals and plants, multicellularity refers to the fact that the cells are permanently differentiated, such as, for example, into liver or skin cells. Although each cell generally has a full genome that contains all the information required to make any cell, once cells differentiate into particular types, they tend to stay that way. Furthermore, often cells are dependent on other cells for their continuation. Except in a laboratory, a liver cell, for example, cannot exist in the natural environment on its own, although some cells, such as stem cells, or cuttings from plants, can be used to make a new organism.
However, the terms “single-celled” and “multi-celled” hide the fact that there are many intermediate states. For example, prokaryotes can exhibit a form of multicellularity in that single cells come together to form multicellular structures. For example, different prokaryotes that carry out different metabolisms can come together to form a microbial mat or biofilm, in which each microbe performs a chemical transformation linked to other organisms in the mat. This is explored in greater detail in the next chapter when we see how energy demands in the environment often encourage these associations. The difference between this structure and many plant or animal multicellular eukaryotic cells is that the mat can be dispersed, and the cells can continue to exist as independent entities.
Other remarkable multicellular behaviors are observed in some organisms, such as in slime molds, which are eukaryotic single-celled organisms (Figure 5.25). Cellular slime molds include a wide diversity of species, one of the most common being Physarum polycephalum, which often forms a slimy yellow mass on forest logs. Slime molds exist as single amoeba-like cells that feed on bacteria. The cells are haploid, but can mate with other cells to form a plasmodium, a large mass of nuclei enclosed in a large cell membrane that can reach meters in diameter. The plasmodia form protoplasmic streams that can move rapidly out across the environment in search of food. When food is scarce, the plasmodia contract, and the cells transform into fruiting bodies, another type of cell structure. These bodies are sporangia (singular sporangium) and they mature and release spores that can be blown in the wind to more favorable conditions.
Figure 5.25 Slime molds adopt multicellular structures. (a) Slime mold showing coordinated movement across surfaces.
Source: Reproduced with permission of S.B. Johnny.
(b) The life cycle of slime molds.
Source: Image of Physarum reproduced with permission of Jerry Kirkhart.
Researchers at Hokkaido University in Japan created an artificial map of Tokyo and its surrounding areas where local towns were represented as food (oat flakes), and mountains and other obstructions represented as light areas (the plasmodia avoid light). The slime mold created networks that were similar to the Tokyo rail network. This likely reflects the way in which the plasmodia reconfigures to make the most efficient connections between food, a form of energy minimization, similar to the objective of railway designers. These intriguing experiments, which we will not discuss further here, serve to illustrate that “single-celled” organisms can take part in complex behaviors more akin to complex multicellular organisms.
Although slime molds are eukaryotic, there are examples of cell differentiation in prokaryotes as well, for example, the formation of bacterial spores in Bacillus and Clostridium, and the formation of different motile and non-motile cells in biofilm-forming bacteria such as species of Bacillus and Pseudomonas. Some bacteria, such as the Myxobacteria, engage in similar cellular behaviors as eukaryotic slime molds, forming swarms of cells and specialized fruiting bodies.
The astrobiological significance of these observations is that separating organisms into “single-cellular” and “multi-cellular” is likely to be over-simplified. These observations also suggest that the transition from prokaryotes that are undifferentiated and single-celled to more complex multicellular eukaryotes with irreversible differentiated cellular structures may not be such a radical categorical transition as we like to think. You will often see this depicted as a “one-time” major evolutionary transition. It seems more likely that multicellularity is not a binary feature of life, but that intermediate states of complexity exist.
Discussion Point: When Did Multicellularity Evolve?
The discussion in this chapter should have convinced you that the title of this discussion point is probably too simple. Many prokaryotic and single-celled eukaryotes exhibit behavior that is multicellular, from the swarming activities of slime molds and Myxobacteria to the specialized cells found in some bacteria, such as spores. Nevertheless, it still remains a cogent question to ask ourselves why cells tend to form multicelled structures. Why isn't Earth covered in cells all individually going about their lives? One obvious answer is that energetic needs force collaboration. One microbe's waste can be another one's food, and thus associations will be selected for, as we will see in microbial mats in the next chapter. Cell differentiation, for example the formation of fruiting body cells and swarming cells in Myxobacteria, can be explained in terms of the efficiencies to be gained by specialization, where each particular cell can carry out a function better than a very general cell that must do everything. What about irreversible differentiation, for example in human cells? Imagine a version of yourself where your cells could all break down and spread out looking for food and then regroup? Why did cells give up this versatility? One answer could be that it was energetically favorable for cells to irreversibly commit to certain roles in an organism. Alternatively, perhaps the success of a complex organism like an animal in the environment removes the selection pressure for it to be able to disperse, and those capacities in cells were lost? There are clearly many fascinating questions to be answered. What do you think about the emergence of multicellularity and the steps that led to plants and animals?
5.12 Viruses
This chapter has focused on cellular structures. Insofar as they constitute self-replicating life forms, this is justified, but there are other entities on the planet that are linked to the biosphere and which play an immensely important role in ecosystems and biological interactions. Two of them are viruses and prions, which we now explore. Quite apart from their biological importance, they challenge the neat separation of material into “life” and “non-life,” and are worth considering from this perspective. You might like to explore Chapter 2 again where viruses were discussed in the context of ideas about the definition of life.
Discussion Point: Astrobiology and Viruses
The idea of sending missions to other planets to search for viruses may seem at first a little unlikely. However, consider the fact that viruses may have evolved multiple times and in some environments outnumber cellular life forms. If biospheres of cellular life always produce associated “particles” that replicate in those cells, and those particles are pervasive, then perhaps a search