Abstract
Cyanobacteria form a very large and diverse phylum of prokaryotes that perform oxygenic photosynthesis. Many species of cyanobacteria live colonially in long trichomes of hundreds to thousands of cells. Of the filamentous species, many are also motile, gliding along their long axis, and display photomovement, by which a trichome modulates its gliding according to the incident light. The latter has been found to play an important role in guiding the trichomes to optimal lighting conditions, which can either inhibit the cells if the incident light is too weak, or damage the cells if too strong. We have developed a computational model for gliding filamentous photophobic cyanobacteria that allows us to perform simulations on the scale of a Petri dish using over 105 individual trichomes. Using the model, we quantify the effectiveness of one commonly observed photomovement strategy—photophobic responses—in distributing large populations of trichomes optimally over a light field. The model predicts that the typical observed length and gliding speeds of filamentous cyanobacteria are optimal for the photophobic strategy. Therefore, our results suggest that not just photomovement but also the trichome shape itself improves the ability of the cyanobacteria to optimize their light exposure.
Highlights
Cyanobacteria are a very large and diverse phylum of photoautotrophic prokaryotes [1]
Previous models of the photomovement of filamentous cyanobacteria by Burkart [26] and Hader [27] using partial differential equations demonstrated how a combination of phototaxis and step-down photophobic responses can lead to the accumulation of trichomes in light traps
Using a cell-based model, we have shown how gliding speed and trichome length complement photophobia and improve the exposure optimization process
Summary
Cyanobacteria are a very large and diverse phylum of photoautotrophic prokaryotes [1]. The opposite reaction, called a step-down reaction, occurs when an organism enters a dark area from a bright area and reverses direction, remaining in the light Gliding filamentous cyanobacteria, such as Phormidium uncinatum, display step-down photophobic reactions when their ‘heads’ (i.e. the leading 10–20% of the trichome) are shaded, causing the trichome to reverse direction. Previous models of the photomovement of filamentous cyanobacteria by Burkart [26] and Hader [27] using partial differential equations demonstrated how a combination of phototaxis and step-down photophobic responses can lead to the accumulation of trichomes in light traps. Myxobacteria glide using a slime extrusion (A-motility) and by extension and retraction of type IV pili that can attach to other individuals and pull them closer (S-motility) Like the cyanobacteria, they reverse their direction of movement periodically. The advantage of our approach is that we can scale our simulations to the full domain size (i.e. a 10 cm Petri dish) by using massively parallel graphics processing units (GPUs)
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