FIG. 2.

Cross sections of B. frigida stained with FUN-1 and documented by CLSM. The micrographs depict mostly vital thallus areas in 2a–2b and 2d–2f and mostly metabolically inactive thallus areas in 2c. The stratification of the thallus is presented by the epicortical gelatinous sheath (e); the cortex (c), which is encrusted with melanin and thus dark in fluorescence microscopy; the layer of clustered algal cells (a); and the medulla (m, 2a–2b). Areas of intense production of extracellular gelatinous substances (g) are found occasionally (2b, 2d). Some areas of the investigated thallus sections show exposure-induced impairment of photobiont vitality and are predominantly metabolically inactive (2c), as demonstrated by the green-colored hyphal cells of the mycobiont (im) and of the photobiont (ip), while degenerating photobionts (dp) are intensely stained by passively accumulating dye (2c, 2e). Metabolically active photobiont cells are characterized by the accumulation and pH-induced change of color in small spherical vacuoles at the margin of the algal cells (ap, 2e). Within the medulla of several samples (2f), yet-unidentified metabolically active cell clusters survive the exposure conditions (encircled areas), neither resembling fungal hyphae nor algal cells. The samples are (2a) SVT, rock, non-irradiated; (2b) EVT-2, P-MRS, transportation control; (2c) EVT-2, P-MRS, 1.4×10³ kJ m⁻²; (2d) EVT-2, S-MRS, 1.4×10⁵ kJ m⁻²; (2e) EVT-2, P-MRS, 1.4×10⁴ kJ m⁻²; (2f) SVT, rock, non-irradiated. (Color images available at www.liebertonline.com/ast)