A model for internal self‐shading in planktonic organisms and its implications for the usefulness of ultraviolet sunscreens

I present a bio‐optical model that estimates the internal self‐shading caused by absorbing matter in planktonic organisms as well as the specific benefit afforded by the accumulation of sunscreen compounds. The model is applied to the examination of case studies of sunscreen‐bearing microorganisms and to the prediction of the effects of organismal size on both the inherent sensitivity to ultraviolet radiation and the possible amelioration of this condition by means of sunscreen accumulation. The model predicts that sunscreens cannot be used as a photoprotective mechanism of any relevance by picoplankters (cell radii, < 1 µ m), which are also the most inherently sensitive size‐class. Conversely, the microplankters (cell radii, 10−> 100 µ m) can use sunscreens with efficiencies comparable to well‐studied damage‐repair mechanisms. Among nanoplankters (cell radii, 1−< 10 µ m), sunscreens can afford considerable benefits but only at the expense of relatively heavy investments and with restricted efficiencies.