16 Green Algae I

Introduction and Prasinophyceans

Part I—Introduction to the Green Algae

The green algae are named for their typical color, the bright grass-green characteristic of land plants. Green algae and land plants usually appear green because abundant chlorophylls a and b are not concealed by large amounts of differently colored accessory pigments. However, certain green algae that occupy very sunny habitats don’t appear green because they accumulate photoprotective orange or red pigments in amounts sufficient to obscure chlorophyll. Common examples include blankets of orange-red Trentepohlia on sea cliffs, purple-red films of Haematococcus in birdbaths, and red growths of Chlamydomonas nivalis in mountain snows. In addition to terrestrial habitats, green algae also commonly occupy marine shorelines and freshwater streams, ponds, and lakes. Some green algae are biogeochemically significant carbonate producers, and certain lipid-rich green algae are sources of petroleum deposits and modern renewable fuels. Green algae also participate in a wide variety of important biotic associations. Although some produce nuisance blooms under conditions of nutrient pollution, many green algae are sources of food for aquatic microbes and animals, and some are grown industrially to produce commercial food supplements.

Several green algae provide key laboratory research materials. In the past, Nobelist Melvin Calvin used laboratory cultures of the green alga Chlorella to elucidate the light-independent reactions of photosynthesis, now known as the Calvin cycle. Transplant experiments with the seaweed Acetabularia conducted in the 1930s allowed Hämmerling to postulate the existence of messenger RNA before it was chemically known. More recent examples include genetic studies conducted with collections of Chlamydomonas mutants, the complete genome sequencing of C. reinhardtii and other green species, and electrophysiological studies performed with the giant cells of Eremosphaera, Chara, and Nitella. Because green algae are more closely related than other protists to the ancestry of land plants, many green species are sources of information about the evolutionary origin of plant traits.

Green algae seem to be a monophyletic group that has diversified into a wide variety of body types: unicellular flagellates or colonies, nonflagellate unicells or colonies, unbranched and branched filaments, and multinucleate coenocytes. For many years green algae were classified according to these structural types. However, today we recognize that green algae have undergone extensive parallel evolution of body form. As a result, species of the same body type are not necessarily closely related, while close relatives may have diverse body structures. This chapter begins with a description of green algal relationships inferred from ultrastructural analyses, comparative biochemistry, life-history studies, and molecular features. Such relationships reveal the extent to which parallel evolution of body types has occurred in the green algae and serve as the basis for a more natural classification system. The second part of this chapter focuses on the earliest-divergent green algae, those commonly known as the prasinophytes or prasinophyceans. A survey of prasinophyte features and diversity helps to explain the evolutionary origin of two major green lineages that are described in subsequent chapters.

Part II—Prasinophyceans

Prasinophytes, the earliest-diverging modern green algae, take their name from the Greek word prasinos, meaning “green.” In some literature, they have been termed micromonadophytes. This non-monophyletic group includes at least seven clades (Guillou et al. 2004). Prasinophytes display few, if any, unique and defining derived characteristics. Rather, they are characterized by a collection of traits that are considered plesiomorphic for green algae (Sym and Pienaar 1993). Fossils that are structurally similar to cyst stages of modern prasinophytes are known from Lower Cambrian sediments, and biochemical evidence suggests that early green algae were present in the Neoproterozoic (Knoll et al. 2007).

Most prasinophyceans occur primarily as flagellate or nonflagellate unicells (Courties et al. 1994) (Figure 16.19). However, certain species occur as sessile (attached) dendroid (treelike) colonies or mucilaginous aggregations of nonflagellate cells, known as palmella stages. Although all known members of the prasinophyceans possess at least one green plastid, Pyramimonas gelidicola (Bell and Laybourn-Parry 2003) is known to also ingest particulate food and is thus mixotrophic. Most occur in marine habitats, where they can be important components of plankton communities (Worden et al. 2004). For example, Not and associates (2007) used molecular probes (see Chapter 5) to determine that Micromonas pusilla dominates the eukaryotic picoplankton (cell size fraction 0.2–3 µm) of the western English Channel year round. Pyramimonas australis forms blooms in sea ice holes (Moro et al. 2002). Complete genome sequences exist for some minute planktonic prasinophyceans, such as Ostreococcus tauri (Derelle et al. 2006) (see Chapter 4).