Homing pigeons return home from unfamiliar release points. Sea turtles and salmon cross oceans unerringly to feeding grounds and back home again through murky sea water. Butterflies cross thousands of miles to specific trees they have never seen before for overwintering. Even cows generally align north and south. One thing that unites these disparate creatures from different phyla and families: the ability to sense and align with the earth’s geomagnetic field. Where did they get this ability? How does it work? Hearing has ears; seeing has eyes, but this “sense without a receptor” has been a mystery for half a century. Now, some scientists think bacteria may have shared their technology with animals, but testing that possibility is a work in progress.
Robert Wells has the news from the University of South Florida with a photo of a sea turtle laying eggs on a beach. “Animals’ Magnetic ‘Sixth’ Sense May Come from Bacteria, New Paper Suggests.” It’s a question “that has been unresolved despite 50 years of research.”
“The search for a mechanism has been proposed as one of the last major frontiers in sensory biology and described as if we are ‘searching for a needle in a needle stack,’” says Robert Fitak, an assistant professor in UCF’s Department of Biology, part of UCF’s College of Sciences.
Fitak and researchers in the United Kingdom and Israel recently authored an article in Philosophical Transactions of the Royal Society B that proposes a hypothesis that the magnetic sense comes from a symbiotic relationship with magnetotactic bacteria. [Emphasis added.]
A look at the paper shows that this hypothesis is little more than a hunch, requiring much more research. Nonetheless, the idea could be a breakthrough, uniting the disparate groups of navigating animals around the common theme of bacterial symbiosis.
Magnetotactic Bacteria Lug Rocks Around
The research paper published by the Royal Society (open access), of which Fitak is a co-author with three other scientists, introduces the tiny protagonists of the hypothesis: magnetotactic (magnetism-sensing) bacteria (MTB):
MTB are a diverse group of prokaryotes which are characterized by having their direction of motility influenced with a magnetic field. Unlike animals, the mechanism behind their magnetotaxis is well understood, using specialized intracellular organelles called magnetosome[s] (magnetic mineral crystals, each enveloped by a phospholipid bilayer membrane).
The incorporation of inorganic mineral particles inside organisms not unique to bacteria. We humans have rocks in our heads, too: tiny particles of calcium carbonate, called otoconia, in the ear’s balance organs. In fish they are called otoliths (“ear rocks”) but they have the same basic function (see 2015 paper in Developmental Dynamics for description). These minerals, surrounded by proteins and neurons in the utricle and saccule of the inner ear, respond to gravity and acceleration and transmit that information to the brain. That’s how we know which way is up when we bend over or stand, even with eyes closed. Without these minerals, vertigo becomes a serious problem.
A similar response to a physical force takes place in MT bacteria, except with magnetism instead of gravity. The magnetosomes, usually arranged in single file, are attracted to magnetic fields, including the earth’s magnetic field. They align with the field and feel the pull. The whole bacterium responds by aligning with the resultant magnetic moment and travels in that direction. The mere presence of magnetite inside a bacterium, however, does not guarantee a response. As with every other functional entity in life, bacteria have genes and proteins that construct these biominerals and arrange them in magnetosomes with tight controls. It’s not a simple process. The authors believe that unrelated MTB shared these genes, located on “Magnetosome Islands” (MAI), by horizontal gene transfer. Even so, the coupling of magnetic-sensitive organelles to cellular motion in the bacteria is not well understood.
Score Points for the MTB Theory
Research into the puzzle of animal magnetic navigation has led to two leading theories. One theory, the “radical pair” theory, invokes quantum mechanical effects in specific proteins called cryptochromes. Excitation of unpaired electrons in response to UV light might act as a sensor of the earth’s magnetic field. The other theory involves magnetic materials in the membranes of cells. Both theories have produced contradictory results. Now, the USF team is suggesting that navigating animals have developed symbiotic relationships with magnetotactic bacteria.
The symbiotic magnetic-sensing hypothesis is particularly plausible given the growing recognition of the varied roles symbiotic bacteria play in their hosts’ physiology (e.g., as evidenced by this special issue). However, three major criticisms of the hypothesis have thus far emerged: (i) the lack of empirical evidence for the existence of such a symbiosis, (ii) the likelihood of having symbiotic MTB, given that their prevalence in potential host species remains rarely reported, and (iii) the absence of a known mechanism that would allow eukaryotes to sense or communicate with symbiotic MTB. Here, we review and discuss recent evidence of the hypothesis, demonstrate that such a symbiosis is probable, show that MTB DNA is routinely found within animal tissue, and discuss possible host-MTB sensing mechanisms, show that MTB DNA is routinely found within animal tissue, and discuss possible host-MTB sensing mechanisms.
MTB Are Everywhere
One of the team’s findings was that magnetotactic bacteria are not rare. They are far more widespread than thought:
Thus, it is important to emphasize that MTB are not an anecdote of nature, but in fact, MTB are ubiquitous across aquatic and anaerobic environments and have a global distribution, occupying aquatic and sedimentary environments, deep-sea sediments and hemi-pelagic environments: inhabiting environments of both neutral pH values to inhabiting extreme environments such as saline-alkaline lakes and even thermals.
They ran over 55,000 metagenomic surveys, looking for MTB of known species in various animal products and ecological environments. While tentative, the results strongly suggested that MTB genes are associated with animal material in about 53 percent of cases. This answered criticism (ii).
Do MTB Relate?
A deeper question is whether MTB form symbiotic relationships with other organisms. They related a discovery of a deep-sea microbe covered with MTB that was able to move in the direction of the magnetosomes; this was the “first empirical evidence supporting a symbiotic interaction between MTB and a eukaryote.” Then, they reported a finding that reed warblers given antibiotics lost their magnetic sense. This suggested that the birds relied on symbiotic MTB to navigate. This answered criticism (i).
The Coupling Question
In the final section of the paper, the four scientists responded to criticism (iii). They gave tentative mechanisms by which MTBs, if present in the bodies of navigating animals, might help the animals orient. A tool is of no use if a being doesn’t know how to use it. How does a magnetically aligned microbe inside of a bird, for instance, cause the bird to fly in line with the geomagnetic field?
In the case of one-celled eukaryotes, they suggest that multiple MTB might cluster along one axis, prompting movement in that direction. For multicellular cases, MTB might bunch together at one end of cells, aiding cell-to-cell communication if the host has receptors to interpret MTB secretions. For animals with sensory cilia in their cells, MTB within the cilium might trigger alignment of the cilium with the magnetic moment, “facilitating a neural response.” And for complex organisms like birds, movement of MTB across the eye might allow the bird to perceive the magnetic orientation in real time and make flight adjustments accordingly. How the bird would perceive that as useful information instead of noise was not explained.
These suggestions for how animals could use MT bacteria for navigation are crude at best. The authors admit the study is in its infancy. It will take much more observation and experimentation to even locate the MT bacteria in these animals, let alone see whether they establish symbiotic relationships with them, and couple the bacteria’s magnetic moments with actual responses in the animals. Fifty years of study and scientists are still at the level of hypothesis! “All in all,” they say in conclusion, “the symbiotic magnetic-sensing hypothesis is a hypothesis worth considering.”
In Illustra’s documentary Living Waters, the exquisite navigation equipment of sea turtles was showcased. The turtles can determine both the strength and angle of a magnetic field line and move precisely at the angle needed to reach their feeding grounds without visual cues. When in the vicinity of a target, they can incorporate other cues, like smell and vision, to get there. Years or decades later they can remember how to get back, often arriving within meters of the beach where they were hatched. The feat implies the existence of far more than symbiosis with bacterial partners aligned with compass points. They must have the ability to build a magnetic map in their brains and store it in memory, then run the route forwards and backwards.
Other magnetic navigators, like migratory birds, trout, butterflies, eels and even fruit flies, possess similarly precise navigation equipment. Last year, The Scientist noted that while humans began to use compasses in the 12th century,
Other animals have been magnetic navigators for much, much longer. Many different species — ranging from newts and insects to sea turtles, fish, and birds — are able to orient themselves relative to the Earth’s magnetic field. Among mammals, naked mole rats, deer, and even dogs also seem to have this gift. Researchers have recently shown that the brainwaves of human beings respond to changes in magnetic fields, though it’s far from clear whether or not we can make any navigational use of this effect.
Not Darwinian Evolution
If these authors are correct, tiny bacteria had magnetic navigation first. If they passed it along to other bacteria by horizontal gene transfer, that is not Darwinian evolution; it is sharing of existing technology. And if higher animals incorporated the technology, they still had to establish permanent symbiotic relationships with the bacteria, and then couple it with capabilities like mapping, memory, and integration with other senses. Was that a result of chance? Nowhere do these scientists explain how bacteria invented magnetotaxis and encoded it into their genes. They also don’t explain how higher animals took this technology and built precision systems onto it. Evolution has failed to explain this exquisite sense. Design science deserves a turn at bat. (Incidentally, some bats can also navigate with magnetism.)
If MTB are found to be our symbiotic partners, too, wouldn’t it be cool to see if we could restore the ability to navigate by training a sense that we may have lost through devolution? Instead of carrying a compass in our pockets, we could use one that’s built in.