Marine Macrophytic Algae

Professor Ralph E. Taggart

Department of Plant Biology and Department of Geological Sciences

Michigan State University

The evolution of protistan predators, such as the protozoan ciliates, in the Late Precambrian created significant selective pressure in the direction of increased size, leading to the evolution of multicellular eukaryotes. Photosynthetic multicellular forms are included in the Kingdom Plantae. The early evolution of multicellular plants in the oceans is difficult to trace, since many of the larger (macrophytic) marine algae have a poor fossil record. The plants generally lack resistant body parts and decay rather rapidly, so that most "seaweed" fossils are poorly preserved and difficult to study.

Division Phaeophyta

The Phaeophyta or "brown algae" are represented by approximately 1500 species, the vast majority of which are marine. Most of these species are benthic, which means that they grow attached to the bottom or some other substrate. These plants are the dominant seaweeds of relatively shallow-water habitats.

Like the kelp shown above, they have the largest and most complex body plans of any "algae", the stem-like stipe of kelps reaching 50 meters or more in length. The axis of the stipe appears quite stem-like and each side is lined with large- leaf-like structures, as well as gas-filled bladders that cause the upper end of the stipe to float on the ocean surface. The size and complexity of these plants might make them appear to be good candidates for the ancestors of land plants, but there are several persuasive lines of evidence that argue against this:

Their apparent complexity is largely illusion, as the stem-like and leaf-like structures are really composed of rather simple, undifferentiated cells with nothing in common with the anatomy of complex land plants.
The photosynthetic pathways are profoundly different from "higher" land plants, including different kinds and rations of chlorophylls and other accessory pigments and completely different food storage products.
Although the sexual life cycles have some crude parallels with those of land plants, they are really quite different in detail.

The brown algae would appear to represent a unique lineage that has diversified in the shallow-water marine environment and which has played no part in the evolution of land plants. The fossil record of the group is very poor and only a handful of fossils are known which can unambiguously be assigned to the Phaeophyta.

Division Rhodophyta

The 4,000 or so species of Rhodophyta or "red" algae, are beautiful but problematic plants. Almost all species are marine. Although there is considerable variation in growth form, in most cases the thallus or plant body is built up of complex branched filaments. The individual plants are not as large as those of the brown algae. The red algae are benthic, but their "mix" of photosynthetic pigments (which is similar in many respects to the Cyanobacteria) enable them to function in deeper water. The color of the thallus ranges from red through purple and black, depending upon the taxon-specific combination of chlorophylls and accessory pigments present. They are unique among the "higher" algae in that flagellated cells are not present in the sexual life cycle.

Some members of the group secrete calcium carbonate and contribute much of the bulk structure of so-called "coral reefs". The fossil record of the calcareous forms is quite good, but we know very little about the evolutionary history of the group as a whole. It is assumed that they are a unique lineage with close ties to the cyanobacteria and with no links to the various kinds of land plants.

Division Chlorophyta

The Chlorophyta or "green" algae are represented by approximately 7,000 species. Although there are many marine forms, most of these species are found in fresh water. The form of the thallus ranges from single-celled (the freshwater Chlamydomonas), through a wide rangle of simple colonial forms, to moderately complex body plans. The green algae are almost certainly the ancestors of all more complex land plants:

The photosynthetic pathways of the green algae, including the form of the various chlorophylls and accessory pigments (as well as their ratios) and the formation of starch as the primary food-storage end-product, are identical to those of land plants.
The basic nature of their sexual life cycles, including the alternation of haploid (gametophyte) and diploid (sporophyte) stages, is reflected in the sexual life histories of the various land plant lineages.

There is general agreement that the various land plant groups probably evolved from freshwater green algae, although the fossil record of the group is very poor, with the notable exception of calcareous charophytes.

Illustrations on this page are from Starr and Taggart, Biology: the Unity and Diversity of Life (Fourth Ed.), Wadsworth, 1987.

Ralph E. Taggart (taggart@pilot.msu.edu)