When most Midwesterners think of algae, they probably envision microscopic green cells causing pondwater to be green, or slimy or filamentous growths on rocks. Indeed, such descriptions are appropriate for most of the algae likely to be encountered in freshwater habitats. Similar algae are also abundant In the ocean, but there are many additional sorts of larger, macroscopic (easily visible to the naked eye) marine algae that are unlike anything found in freshwater. Some such as the giant kelps routinely grow taller than a two-story building, forming lush underwater forests. Many others are smaller in stature, yet very plant-like in their growth habits.

These plant-like marine algae are commonly referred to as seaweeds, or sometimes as macroalgae. Though they often have growth forms resembling the vascular plants that dominate the land and freshwater, these marine macroalgae lack the xylem and phloem tissues that vascular plants use for internal transport of materials (it should be noted though that some large macroalgae such as the giant kelps have structures that are analogous to the internal transport systems of vascular plants).

These macroscopic algae are classified into three different phyla, each of which has tremendous species diversity:

Though the green, brown, and red macroalgae are all photosynthetic, plant-like organisms, they differ greatly from one another in a number of fundamental respects. For example, there are differences in photosynthetic pigments, differences in energy storage compounds, and different modes of reproduction. Indeed, the red algae in particular are so strange and different from the rest of life on earth (often with unique and complex life cycles and modes of reproduction) that some biologists have argued they should be classified as a completely separate kingdom of life.

Though there are marine vascular plants (seagrasses and mangroves) that are abundant in some coastal areas, the diverse forms of algae dominate most marine habitats. In many areas the green, brown, and red macroalgae are the dominant plant-like organisms, filling niches that in freshwater are typically dominated by aquatic vascular plants. Some marine algae live lifestyles with no counterparts in freshwater. For example, coraline algae are specialized marine red algae that accumulate calcium carbonate (limestone) in their tissues. These coraline algae actually create rock as they grow, and play important roles in reef fromation by cementing coral skeletons together.

Many of the bushy or leafy red algae are extremely beautiful, as can be seen in this month's photos. This red alga was given to me by a member of the Twin Cities Marine Aquarium Society several years ago. Other than that it probably comes from tropical or subtropical waters, I know nothing of its place of origin. Since it could have originated in places as distant as the Caribbean or the Indo-Pacific, I have no confidence in my identification of this species, but it somewhat resembles the Halymenia species shown in Littler, et. al. 1989.

The remarkable thing about this species is that it has been growing reliably in our reef aquaria for several years. In contrast, most of the other large, bushy or leafy red macroalgae I have tried to grow have done well for only short periods of time, only to die off for unknown reasons. This is a great shame, since many of the marine red macroalgae are not only beautiful, but also major components of marine ecosystems that I would like to have represented in our aquaria.

Great strides have been made by aquarists in recent decades in their ability to grow a great many marine organisms. For example, it is now commonplace to grow stony corals in aquaria, though just a few decades ago this was thought to be impossible. Some (but not all) of the green macroalgae also grow well in aquaria, as do some species of coraline red algae. However, reliable closed-system aquarium culture techniques for many of the green, brown, and red macroalgae still elude us. Mastery of the aquarium growth requirements of these wonderfully diverse marine "plants" is one of the great challenges still facing aquarists.

Above: Red macroalgae growing alongside several green macroalgae (Caulerpa sertularioides on right, C. mexicana in background, and a small growth of dark green Halimeda sp. immediately to the right of the main clump of red algae). Other organisms visible include fluorescent green, bumpy mushroom anemones (bottom left, bottom center, and bottom right), striped mushroom anemones (barely visible on right side), and a greenish-grey chicken liver sponge (climbing up one of the Caulerpa sertularioides "fronds").

Reference

Littler, D.S., Littler,M.M, Bucher, K.E., and Norris, J.N. 1989. Marine Plants of the Caribbean: A Field Guide From Florida to Brazil. Smithosonian Institution Press, Washington, D.C.