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Thirteen species from the red algal flora of Helgoland (southern North Sea) were exposed to UVA ÷ UVB radiation for various periods in the laboratory, and dark-adapted variable fluorescence (F v : Fro) was measured immediately after the UV treatment and again after various recovery times in white light. With the exception of Porphyra ~mbilicalis, all species showed a decrease in F v : F m on exposure to UV radiation, followed by recovery towards the initial values during the next 24-48 h in white light. The rate of the initial decrease was greater, and the extent of recovery was less, in deep subtidal species (e.g. Delesseri~ s~guinea, Placamium carfflagineum) than in intertidal or shallow subtidal species, although there was no direct correlation between these indicators of sensitivity to UV radiation and the depth range of a species, and only slight differences were detected between populations of a single species collected from different depths. There was also little evidence of a seasonal change in sensitivity to UV radiation in Delesseria or Plocamium. The decrease in variable fluorescence in Delesseria and Plocamium was proportional to the logarithm of the exposure to UV radiation, and reciprocity between irradiance and the length of irradiation appeared to hold for up to ~6h. The exposure to UVA that reduced F~ : F m by 50% ('50% exposure') immediately after the treatment was about I0 kJ m -2 for both Delesseria and Plocamium, but Plocamium appeared to recover more rapidly than Delesseria because the 50% exposure after 24 h recovery was about 37 kJ m -2 for Delesseria compared with 70-120 kJ m -z for different populations of Plocamium. The removal of UVB from the UV radiation treatments had no detectable effect o~ the inhibition of F v : F~ in either Ddesseria or a deep-water population of Plocamium, but reduced the inhibition by 30-50% in a shallow-water population of Plocamium. When only half an intact blade of De]esseria was exposed to UV radiation, variable fluorescence was reduced in the irradiated half but was completely unaffected in the unirradiated half, and steep gradients of Fv : F m values were measured which persisted for over 48 h after the irradiation treatment.

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability, 1 while fragile and heavy bulk materials can affect their transport and deployment. Here, we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide coated polyethylene terephthalate achieve an activity of 4266 µmol H2 g -1 h -1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attain a high CO:H2 selectivity of 7.2 under a lower 0.1 sun irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices display unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm 2 standalone artificial leaves, which sustain a comparable performance and stability of ≈24 h to their 1.7 cm 2 counterparts. Bubbles formed under operation further enable the 30-100 mg cm -2 devices to float, while lightweight reactors facilitate gas collection during outdoor testing on a river. The leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open water applications, while avoiding competition with land use.