Rhodophyta Red Algae

Rhodophyta, or red algae, are distinguished from other algal groups by the presence of chlorophylls a and d in combination with certain accessory pigments (phycobiliproteins), non-aggregated photosynthetic lamellae in the chloroplasts, specialized food reserves, unique sexual reproduction, and the absence of flagellation in all phases of the life cycle. The 4000-6000 extant species are primarily marine, mostly inhabiting warm tropical waters (Graham and Wilcox, 2000; Sounders and Hommersand, 2004). Phylogenies based on molecular data suggest the group is monophyletic (Le Gall and Saunders, 2007). The red algae are almost all multicel-lular and structurally more complex than other algae, with specialized pit connections between cells and a complicated mode of reproduction. Various red algae are commonly preserved as fossils, because they possess calcified skeletons that form as a result of calcium carbonate precipitation within the cell walls. In this feature, they differ from other lime-precipitating algae that deposit calcium carbonate only on the thallus. In the reds, the calcite is typically deposited in a grid-like pattern.

Red algae are widespread in the fossil record, extending back to the late Mesoproterozoic. To date, the oldest red alga is a member of the Bangiales, Bangiomorpha pubescens (FIGS. 4.43, 4.44), from the Hunting Formation (~1.2Ga) of Somerset Island, arctic Canada (Butterfield et al., 1990; see Chapter 2). Not only is B. pubescens the oldest taxonom-ically resolved eukaryote on record, but it also exhibits the oldest example of eukaryotic sex and complex multicellular-ity (Butterfield, 2000, 2001).

Rhodophyta Fossils
FIGURE 4.43 Bangiomorpha pubescens (Mesoproterozoic). Bar = 50 pm. (CourtesyN. J. Butterfield.)

Most Paleozoic calcareous red algae grew in open-marine carbonate shelf environments, although, as a group, they tolerated a variety of environments. These wide environmental variations suggest that individual taxa may provide important clues relative to ancient environments. Far more difficult, however, is the problem of relating Paleozoic red algae to living groups, because the taxonomy of the fossil forms often remains uncertain. This is due in part to the fact that reproductive structures have rarely been described. Their absence has suggested to some that the reproductive organs were externally produced and not calcified. Others suggest, however, that spores were produced within cells and that

Bangiomorpha Pubescens
FIGURE 4.44 Bangiomorpha pubescens (Mesoproterozoic). Bar = 50pm. (CourtesyN. J. Butterfield.)

these reproductive propagules are impossible to distinguish from vegetative cells.

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