In this paper, Schulze-Makuch and Irwin, considering the fact that in some environments the magnetic field could theoretically be of sufficient magnitude as to act as an energy source for living systems, proceed to describe three models of such organisms that could feed onto these energy sources—in other words, three models of magnetotrophic organisms.

No organism that we know of on Earth is able to use magnetism as an energy source or to store information. That being said, some living systems can still be sensitive to magnetic fields. If magnetic fields were to be much stronger than on Earth, for instance on magnetars, living systems could use this energy to survive. In what follows, the authors introduce three models of magnetotrophic organisms that can obtain energy via the Lorentz force.¹

The fact that no organism on Earth use magnetic fields as an energy source is likely because of the general weakness of Earth’s magnetic field when compared against other energy sources, for example chemical energy or light. The energy from Earth’s magnetic field is about 11 orders of magnitude lower than what can be obtained from a chemoautotrophic reaction or a photon on Earth. However certain stellar bodies—such as magnetars or neutron stars—can have magnetic fields of \(10^{15}\) Gauss, which is roughly 16 orders of magnitude stronger than on Earth. Magnetic fields of this strength would likely be favoured over chemical or light energy.

Nevertheless, on Earth several living systems have learned to interact with magnetic fields. For instance, in animals, we know that birds and lobsters use magnetic positional information. Plants also alter their gene expression and phenotype in response to magnetic fields. Finally, microbes such as magnetotactic bacteria, produce what we call magnetosomes—iron-rich magnetic particles within a lipid bilayer membrane—that enable orientation and migration along field lines. To this effect, chains of magnetite crystals that could represent magnetosomes have been found in the Martian meteorite ALH84001, and could indicate the presence of bacteria in the past.

Free energy can be extracted from magnetic fields via charge separation—either via Lorentz force directly or via induction. We write the Lorentz force as follow:

$$ F_\text{L} = q(E + v \times B) $$

where \( E \) is the electric field, \( v \) the velocity of the charge in the magnetic field and \( B \) the magnetic field strength. We note that the Lorentz force applies to one single charge, whereas a living cell would obviously involve the activity of millions of charges acting in parallel.

In general, induction—generating energy through a periodically changing magnetic field—would be considered mostly for very specific environments. Moreover, the energy that can rather be obtained by the Lorentz force directly is often many orders of magnitude larger than by induction. We will thus focus on the latter in what follows.

Schulze-Makuch and Irwin investigated three different types of models for magnetotrophic organisms. The first one is shown on Figure 1. We first assume an organism that is divided into two compartments by a membrane that is permeable to cations and anions through ion-specific channels. Let’s also assume that it is in a free-floating environment (like water) that causes the organism to tumble through cycles of rotation, where it is at times in perpendicular, then parallel orientation relative to the magnetic field.

The authors further describe how this organism can exploit the magnetic force in four steps, during which the orientation of the organism will change:

In the second model proposed by Schulze-Makuch and Irwin, we assume a substrate of molecular sulfur, iron compounds and water—in which lives an oval-shaped organism (see Figure 2). In this setting, the sulfur, ferric ions and water are engulfed, ingested or otherwise absorbed into the organism. An exothermic reaction occurs, in which sulfur is oxidized and ferric ions are reduced, which yields free energy that can be used by the organism. Additionally, negatively charged bisulfate ions are drawn through selective anionic channels and ferrous ions are pushed through selective cationic channels by the magnetic force. This engages molecular machinery that adds a high-energy phosphate bond which can create an energy-bearing molecule.

In the final model considered by the authors, consider the following morphology: the organism this time is shaped as a long cellular microbe, lined with rows of cilia-like structures (Figure 3). As in the first model, this organism tumbles in cycles of parallel and perpendicular alignment relative to the magnetic field. When this happens, the cilia on one side contains positive charges while the other side carries negative ones, and a band of elastic molecules connects with molecules on either side of the cell.

When the microbe reorients perpendicular to the magnetic field, the magnetic force bends one row of cilia forward and the other row backwards. Gear-like machinery in the membranes then induce conformational changes, causing the bands to stretch. As soon as the organism resumes a parallel orientation to the magnetic field, the elastic band recoils to its unstretched conformation, and releases energy that can be used to form high-energy bonds.

In this paper, Schulze-Makuch and Irwin have tried to demonstrate the possibility for a theoretical magnetotrophic organism to take advantage of strong magnetic fields on stellar objects such as magnetars.

Such an organism would be a case of “life as we do not know it” since no organism on Earth is known to use its (much weaker) magnetic field as a metabolic energy source. Moreover, the use of magnetic fields for reproduction is even more speculative, and only a handful investigations have been carried out in using a magnetic rather than a chemical code for this purpose.

The authors finally conclude their discussion on the theoretical possibility of magnetotrophic organisms by summarizing their hypothesized properties with those of “life as we know it” (Table 1).