We present the first in situ detection of molecular iodine emitted from the brown macroalga <i>Laminaria digitata</i> under natural stress conditions. We show that the release of I<sub>2</sub> occurs in short, strong bursts with a complex time signature. The new data indicate that algal control of I<sub>2</sub> 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 H<sub>2</sub>O<sub>2</sub>.