Photosynthesis in the “dark”: Near-infrared light drives substantial oxygen production in natural biofilms

As the prerequisite for life on Earth, plants, algae and cyanobacteria harvest solar light energy and convert it to drive the fixation of carbon dioxide into new biomass, while generating oxygen gas as a byproduct, in a process known as oxygenic photosynthesis. Most oxygenic phototrophs employ chlorophyll a and use visible light (400-700 nm) but certain cyanobacteria can produce chlorophyll d and f that can harvest light energy in the near infrared part of sunlight (>700 nm).

-  "This can in principle expand the ecological niches and habitats, where oxygenic photosynthesis can take place; e.g. in shaded places below dense plant or algal canopies where visible light is strongly limited. Cyanobacteria with chlorophyll d and f have been isolated from many different habitats, and such isolates have been studied in the lab. In contrast, almost nothing is known about their ecological importance and activity in natural habitats”, states the corresponding author Michael Kühl, Department of Biology, University of Copenhagen. 

In a study, recently published in eLife, a group of researchers from Denmark (University of Copenhagen and University of Aarhus) and Australia ( University of Technology Sydney) investigated, whether cyanobacteria with chlorophyll d and f can perform substantial oxygenic photosynthesis based on near infrared light in an intertidal beachrock habitat composed of cemented coral sand coated with a dense microbial biofilm.

- “ In order to answer this question we had to establish a way to visualize the presence and photosynthetic activity of cyanobacteria with chlorophyll d and f underneath other oxygenic phototrophs in the beachrock using visible light. We developed an oxygen sensitive paint and with a novel combination of nanoparticle-based O₂ imaging, hyperspectral imaging and confocal laser scanning microscopy, we found high rates of near infrared light-driven photosynthesis comparable to rates reported from microalgal biofilms using visible light”, Michael Kühl explains.

This indicates that cyanobacteria with chlorophyll d and f can play a hitherto overlooked role in the primary production of biomass in shaded habitats and emphasizes the importance of investigating to which degree cyanobacteria with chlorophyll dand f, using near infrared light in the “understory” of other oxygenic phototrophs, can enhance photosynthetic efficiency and productivity at the community/system level. Such knowledge could also inspire the design of more efficient photobioreactors, where layered biofilms with different photopigments simulating the beachrock habitat could lead to a higher productivity.