Phototactic responses in the gametes of the brown alga, Ectocarpus siliculosus.

The action spectrum of phototaxis was determined and the photoreceptive mechanism was studied in Ectocarpus gametes (Ectocarpales, Phaeophyceae) using a computerized cell-tracking system. The fine structures of the stigma and the flagellar swelling were analyzed, and the reflective function of the stigma was demonstrated for the first time. Under monochromatic light stimulation, Ectocarpus gametes show mainly positive phototaxis between 370 nm and 520 nm. The action spectrum has a minor peak near 380 nm, and two major peaks at 430 nm and 450 nm or 460 nm and a shoulder at 470 nm adjoining a remarkable depression near 440 nm. Under unilateral stroboscopic illumination with more than four pulses per second, the gametes show clear phototaxis. However, the response is disturbed at lower frequencies. Addition of methyl cellulose, which increases the viscosity of the medium and slows down gamete rotation, decreases the threshold frequency. These results indicate that rotation of the gamete plays an essential role in the photoreceptive mechanism. Under equal intensities of bilateral illumination at an angle of 90°, most of the gametes swim on the resultant between the two light beams. This response is disturbed when the angle of the two light beams is as large as 120°. Observations by transmission electron microscopy show that the flagellar swelling fits precisely into a concave depression of the chloroplast at the central region of the stigma. Electron-dense material is present in that sector of the flagellar swelling which faces away from the stigma. Epifluorescence microscopy without a barrier filter and epipolarization microscopy reveal that stigmata reflect blue light. A hypothesis is formulated which discusses the possibility that the reflected light is focused onto the flagellar swelling.

Comparative receptor study in gamete chemotaxis of the seaweeds Ectocarpus siliculosus and Cutleria multifida. An approach to interspecific communication of algal gametes.

Gamete fusion in phaeophytes is initiated by olefinic hydrocarbons as messenger substances. They are secreted by the female gametes and act on male gametes or gametangia. The sensitivity thresholds of the androgametes of Ectocarpus siliculosus and Cutleria multifida, two brown algae of different evolutionary stages, for ectocarpene, which is produced by either species, were found to be 0.89 nmol/l and 8.9 nmol/l sea water, respectively. The molecular recognition mechanism is shown to be identical for these two species and probably also for most other phaeophytes. The pheromones involved are structurally related and sometimes resemble each other in their overall pi-electron distribution (e.g. ectocarpene and multifidene). Knowing the specific messenger, the typical response behaviour of each receptor involved, the pheromone productivity of the considered species as well as other biological parameters (phytogeographic distribution, the natural benthic habitat and times of fertilization), a method is presented to uncover possible interspecific cross-reactions of such algal communication systems as a 'chemical warfare' device in their fight for living space.