Reflective properties of different eyespot types in dinoflagellates

The reflective properties of different types of dinoflagellate eyespots wer e investigated using confocal laser scanning microscopy in the epireflectio n contrast mode. Although the eyespots studied differed with respect to loc alization (cytosol or plastid) and organization of the globule layer(s), al l types effectively absorbed and reflected blue-green laser light (principa l lines of 488/514 nm). The relative orientation of the eyespot surface tow ards the light source strongly influenced the reflective properties. Maxima l reflection occurred when the eyespot surface was approximately perpendicu lar to the light source and rapidly decreased at increasing angles of light incidence. Horizontal and vertical optical sectioning of live and fixed ce lls resolved differences in the reflection patterns. Focusing of reflected light on the basal portion of the longitudinal flagellum was observed for t he cytosolic eyespot of Glenodinium sp. and the triple membrane-bounded eye spot of Peridinium foliaceum, presumably a vestige of a host plastid. This flagellum is thought to be mainly involved in mediating orientational movem ent responses. In contrast, the reflection patterns obtained from the eyesp ot of Woloszynskia pascheri, which represents the third and most commonly o bserved dinoflagellate eyespot type within a plastid, point to only minor f ocusing. Reflection signals could be followed a considerable distance into the sulcus in all cases, indicating that in dinoflagellate eyespots, irresp ective of the presumed receptor location (plasma membrane overlying the eye spot and/or the basal part of the longitudinal flagellum), back reflection of non-absorbed light can enhance the excitation probability of the photore ceptor(s). Such a combined reflection/absorption screen allows maximal cont rast modulation and will, in conjunction with the specialized geometry of t he dinoflagellate eyespots, increase the directionality of these eyespot ap paratuses considerably.