A wide range of logic gates (including: AND, NOR, OR, NAND, XOR) have been envisaged using Marimo with light as ‘input signal’, ‘output signal’ and ‘actuation power’. In other words, depending on the desired functionality, light is interchangeable between all three functions [input signal (data), output signal (data), actuation power (energy)]. This provides potentially zero electric grid consumption when operating on sunlight. Further, processing based on variable density units can be both modular and scalable. A key benefit to the devices presented is, therefore, the efficiency inherent in utilising photosynthetic (i.e. carbon- capturing), self-growing, solar-powered devices whose running costs are essentially null, regardless of whether their application lies in creating unconventional computing devices (logic gates, oscillators) or engineering (motor, e.g. applied to electricity generation).

In order to extend the systems’ capabilities into logic gates, a refined set up has been designed. The layouts for these gates can be seen in Fig. 7. The main alteration is that the float has transparent and opaque sections, as required. Inputs are represented by the presence or absence of a light beam entering the system, and outputs by a light beam exiting the system. Such a system could also be implemented using other vertically-controlled units [17–19].

Fig. 7 Logic gates based on Marimo: (a) NOR (b) AND (c) OR (d) NAND (e) XOR. The Marimo are represented with green balls. The floats are represented by orange and/or black circles. The black section of a float is opaque, while the orange section is transparent. The input signals are shown as A, B, C, or D. The output signal is observed by sensor F1 or F2. An ‘always-on’ light sources is portrayed by a black box Full size image

During the development of the Marimo logic gates it was observed that Marimo can hold a stable position between the ‘top’ and ‘bottom’ of the containment vessel for prolonged periods (upwards of 30 min, video available in the Additional file 4), with a suitable lighting configuration. This enables the potential advancement of logic gates to include multi-state operations. Further, by graduating the light level, an analogue system can be conceived.

Additional file 4: Video of the Marimo whilst vertically stable. (MP4 12,592 kb)

It was observed that Marimo often take a period (several hours) before forming bubbles and movement in a new/refreshed environment. This induction period may be a combination of several factors including: rising concentration of dissolved gases in local water before bubble formation, re-acclimatisation of the organisms to the rapid change in environment, and the state of the Marimos’ photosynthetic systems prior to beginning the experiment [21].

The 1D rotational movement (of Marimo motor) can be extended to 2D movement. One can envisage such a system whereby multiple Marimo are located inside individual transparent chambers (spheres), which are themselves located near the inside surface of a larger partly transparent sphere. Marimo exposed to illumination will generate gas (via photosynthesis) creating positive buoyancy. The net change in buoyancy will rotate the larger sphere (rover) away from the light source. Each of the smaller spheres would contain a gas vent (located at the largest distance from centre of larger sphere) such that the gas can escape when the smaller sphere moves to the ‘top’ of the larger sphere. It is worth noting that because the surface area of a sphere increases to the square of its radius, the potential power of a Marimo rover would increase quadratically with its size. Potentially, such a light-powered Marimo rover could autonomously travel on land or in water (if outer sphere is suitably configured), and navigate topology. The topology of the land could be inferred by remotely tracking the movement of the rover (e.g. add RF reflector and track via satellite). A prototype rover is under construction and will be reported on in the future.

Potential applications of actuators and individual logical gates based on Marimo can be found in situations where speed of operation is not essential, but device longevity is. For example, a logical device such as the one presented here could be used as a light-sensitive controller for a microbial fuel cell system, such as the one presented in Ref. [22]. The rationale behind this approach is that such a controller — which could be effective for the decades-long lifespan of the Marimo used and would not require an electrical power source — would switch between power usage or storage based on the user’s diurnal energy demands. Furthermore, any low speed of operation could be mitigated in the various applications of Marimo motors, such as the aforementioned electricity generation. If, for example, Marimo motors were used to generate electricity via induction from only sunlight input, device efficiency could be improved through scaling and design optimisation. Whilst electricity generation could not be expected to be enormous through these means, we consider the concept worthy of further investigation as a current opinion on alternative fuel sources is that a variety of ‘green’ methods should be utilised in combination to achieve better climate outcomes. As such, certain perceived ‘failings’ of Marimo-based devices may be mitigated by factors such as their ability to capture carbon and not requiring heavily-refined or enriched materials in their production, as is the case for solar panels.

The potential speed of operation is an important consideration in biomimetic designs, as it is comparable to data processing and actuation time. While most plants move relatively slowly (compared to animals), some have evolved fast responses. For example, the Venus flytrap (Dionaea muscipula) can capture a fly by closing its trap (a modified leaf) in less than one tenth of a second when triggered (physical contact with plant’s hairs) [23]. Therefore, the speed of operation cannot be considered to be limited solely due to its biological nature. Whilst the current investigation did not examine variation in Marimo diameter, this would be an important aspect to focus on in future studies as there could be a relationship between the size of a Marimo and its movement.

Although the applications outlined above are proposed specifically in applications where slow speed of operation is not an impediment, the principles of operation outlined in this paper can be applied to a smaller scale which could be used, in turn, to enhance the operation speed. As photosynthesis is a characteristic of multiple species of plants, algae, bacteria and protozoa, the limitation on the size of a living oxygen-generating buoyancy system is therefore bounded by the physical dimensions of a single photosynthetic cell plus a surrounding medium for capturing any oxygen generated, although smaller systems may be considered as viable through the use of decellularised chloroplasts. We propose that, if immobilised in a suitable porous, water-stable, light-permeable substance such as sodium alginate, small colonies of microalgae (e.g. Micromonas pusilla, approximately 3 μm in length [24]) or cyanobacteria (approximately 1 μm in length) could be utilised in much the same manner as the previously described Marimo devices. The use of microfluidic technology has been demonstrated as a reliable method for generating alginate beads with embedded substances in the range of 10 μm [25], hence we propose this to be a realistic estimate of the size of a single photosynthetic, variable-density buoyancy system, for use in miniaturised versions of the Marimo devices previously described, which could be engineered to have much faster operation times through exploiting lower-mass components and shorter travelling distances for the photosynthetic elements.