We present the first in situ detection of molecular iodine emitted from the brown macroalga Laminaria digitata under natural stress conditions. We show that the release of I-2 occurs in short, strong bursts with a complex time signature. The new data indicate that algal control of I-2 release in the form of an oscillatory time-dependence may be based on a nonlinear autocatalytic reaction scheme which is closely linked to the production of H2O2.We present the first in situ detection of molecular iodine emitted from the brown macroalga Laminaria digitata under natural stress conditions. We show that the release of I-2 occurs in short, strong bursts with a complex time signature. The new data indicate that algal control of I-2 release in the form of an oscillatory time-dependence may be based on a nonlinear autocatalytic reaction scheme which is closely linked to the production of H2O2.