Phytochrome Superfamily Photoreceptor Antibodies

Cyanobacteria are an ancient group of photosynthetic prokaryotes that exist in diverse ecological niches ranging from desert soil to the deep ocean. Each habitat presents unique challenges, including how to respond to light conditions that fluctuate and are often suboptimal for photosynthesis. Cyanobacteria have evolved several photoreceptor-controlled mechanisms-particularly in their photosynthetic activity-that enable them to sense and respond to environmental light conditions. Given the global contribution of cyanobacteria to oxygen production, nitrogen fixation and carbon sequestration and their potential applications in industrial processes, there are widespread implications for understanding how cyanobacteria respond to light in order to improve photosynthetic efficiency and overall fitness.

Fig. 1 Common physiological responses controlled by cyanobacterial phytochrome superfamily photoreceptors. (Wiltbank LB, et al., 2019)

Phototaxis of cyanobacteria

Photosynthetic organisms such as cyanobacteria must carefully balance harvesting sufficient photons to maximally drive photosynthesis while avoiding the damaging effects of excess energy capture. Therefore, it is important for these cells to monitor the amount of light being received (irradiance level) in addition to the ambient light colour spectrum. It is also essential that these two signals are integrated to create the optimal conditions for photosynthetic activity. Cells can control irradiance levels if they have the capacity to move towards or away from light-a process known as phototaxis. Positive phototaxis is cell movement towards light, and negative phototaxis is movement away from light. Phototaxis and its regulation are complex90, as some colours of light promote positive phototaxis, whereas others promote negative phototaxis, and different irradiances of the same light colour can cause opposite phototactic responses.

Cell aggregation

Cell aggregation has a role in transitioning from a planktonic to a sessile lifestyle and appears to provide the shading required for protection of photosynthetic machinery from damage by excessive light. This must be tightly regulated so that when light conditions change, single cells can be released to optimize light capture for photosynthesis. Regulation of cyanobacterial cell aggregation involves extracellular glycoprotein biosynthesis in some species. The thermophilic species Thermosynechococcus vulcanus RKN has recently been developed as a model system to study light colourdependent regulation of cell aggregation. In this strain, aggregation is cellulose dependent. Whereas aggregation may occur in response to many environmental cues, in T. vulcanus, cellulose-dependent aggregation occurs when cells are exposed to relatively low temperatures (31°C versus an optimum temperature of 45°C). This is also dependent upon light conditions that are sensed by CBCRs: cells aggregate when grown in violet to blue light but not in green light. It is likely that selfshading during cell aggregation protects T. vulcanus cells from photodamage that would otherwise occur when photosynthesis occurs suboptimally at a lower temperature, while relatively high-energy, damaging violet and blue light continues to be absorbed.