Obligate mutualisms- We NEED them.
Obligate mutualisms are evidence of a long and beneficial history of interaction between two organisms with one typically being “host” to the other. The association is so entrenched that the two cannot live separately under normal conditions, but that doesn’t mean the solo situation can’t happen.
For example, a serious environmental and global problem of today is coral bleaching. When corals experience certain stresses, they expel the mutualist photosynthetic algae (zooxanthellae) embedded in the coral tissue and the tissue becomes transparent or “bleached”. All of the color from the algae is lost and the white of the coral skeleton becomes visible. Without the zooxanthellae to provide the corals with food in the form of carbohydrates (sugar) from photosynthesis, the corals will slowly starve unless the ocean waters are rich in small organisms that can be captured as food.
There are several reasons why the zooxanthellae are expelled from the coral, but all are environmental stresses such as abnormally-high water temperatures, pollution, pathogens, sedimentation, and wildly fluctuating environmental conditions. Without the algae in their tissues, corals are dependent on their own ability to capture food, which they are fully capable of doing, but tropical coral reef zones have a reputation for clear water and this is precisely because of the very low abundance of tiny living things. It’s for this reason that the mutualism between corals and algae formed in the first place - as a mechanism for living in an otherwise ideal habitat that had low food supplies.
Humans are faced with similar problems. A rich and diverse digestive ecosystem is a healthy condition. Many species of bacteria have likely been associated with us for so long they have assumed some of the functions that humans used to have or they have provided functions that we didn’t have, but that were highly beneficial to us. At some point, the bacteria became indispensable.
The current understanding is that one role of the microbiome is to stimulate our immune system and this may have been an evolving role for bacteria in the gut. In very primitive systems, if a bacterium took up residence, the system would have to protect itself from the presence and possible negative effects of the bacterium. That is, a healthy system should respond to the presence of the bacterium and be stimulated to defend itself whenever the bacteria are detected, but not when the bacteria are not detected. In a long-lived organism with different life stages, the presence of certain bacteria could evolve to be the trigger indicating that “it’s time” to produce defensive functions.
For example, a newborn infant has no defensive abilities, but the colonization of the colon by bacteria provided by the mother acts as a signal that the baby’s protective functions should be activated or that further maturation and development of the immune system is necessary. In other words, if the development of our immune system is coincident with the colonization by certain bacteria in the colon, then it’s very likely that over a very long period of time the presence of those bacteria has become a necessary signal for our timely and normal development.
What if the bacteria that stimulate the development of the immune system never arrive or are killed off? Current research is strongly suggesting that underdeveloped immune systems may be a long-term consequence of the bacterial conditions we experience in infancy. If vaginal birth and breastfeeding provide the first contact with bacteria to the sterile body and digestive system of the infant, the very earliest developmental pathways may be affected.
The development of the immune and digestive systems seems to be a rapid, but drawn out process; it begins very early, but doesn’t happen overnight. The early inoculation of the baby’s microbiome by the mother appears to be very important, but a delay in that inoculation will not stop the systems from developing more or less normally. In contrast, preventing the colonization of beneficial bacteria for an extended period may prevent normal development during the appropriate window of opportunity.
And so, vaginal vs caesarean birth, breast milk vs formula, and the use of antibiotics at birth and during infancy are all linked to changes to physiological and even physical development as infants move through different growth stages. The key to understanding this damage obviously lies in understanding exactly when some of the interactions between the baby and the new microbiome are occurring, but it is very likely this is not an exact calculation.
While obligate bacterial mutualists may be very important for certain developmental processes in the human body and even for ongoing functions and processes, we also know very little about our innate capacity in the absence of the microbiome. That is, if some important bacteria are missing, do we still have the genetic ability to perform the functions ourselves? If we receive the appropriate stimulus in some other way, can we form and perform the needed functions?
Research on the interaction between the microbiome and the immune system will help answer those and many more questions, but it is important to remember two things: First, we are BIGS and the bacteria are BUGS. They can respond to changes in the environment very quickly and can draw on millions of genes that humans do not possess. That doesn’t prevent us from possessing and maintaining an ability for proper development when the bacteria are absent, but we have only 20,000 genes to draw from. However, it is certainly possible that the speed and efficiency of those functions may be impaired by long-term non-use (that is, over the previous hundreds of generations).
Second, the environment changes constantly. If there is a need for a new function or a need for an enhanced function, we will be very unlikely to produce it ourselves. And it’s also very possible that new or enhanced functions might be needed at a range of intensities and not merely as an on/off switch. Our microbiome’s genome is far better suited to handle such demands than our genome is. Given the millennia that the microbiome has been involved in our development, digestion, and immunity, it is possible that we never had the need to adapt to a changing environment in the traditional sense.
The fact that humans, as BIGS, are as physiological adjustable as we are, that we have expanded our population to cover the globe, and that we are as omnivorous as we are, suggests that we have ceded a great deal of our evolution to the BUGS that we are associated with. And in return, despite being BIGS, we gained the abilities of BUGS to respond to important and regular changes in the environment, such as seasonal changes in food quality and diseases.
Third, any loss of obligate mutualists in our personal ecosystem must have a cost. All recent research points to the microbiome as being a required component of childhood development and our adult lives. The loss of obligate bacteria will likely be directly related to loss of function, protection, stimulus, and capacity. There is no doubt: we are not individuals, we are ecosystems. As ecosystems, we depend on diversity for stability, functionality, resilience, resistance, and in other ways. Losing obligate mutualists in our microbiome would be like losing a limb that cannot be regrown; it is something we should avoid.