Introduction to the Freshwater Algae

Robert G. Sheath1; John D. Wehr2    1 Department of Biological Sciences, California State University San Marcos, San Marcos, California, USA
2 Louis Calder Center—Biological Station, Fordham University, Armonk, New York, USA

Chapter Contents

I Introduction

Freshwater algae are globally ubiquitous and highly diverse, with tens or perhaps hundreds of thousands of species, in a myriad of forms and sizes (Andersen, 1992; Norton et al.; 2004; Mann and Vanormelingen, 2013; Guiry and Guiry, 2014). The algae represent between eight and 12 major evolutionary lineages (Graham et al., 2008; Cock et al., 2010), and all have representatives in inland waters. Current classifications consider most algae to be protists with chloroplasts, but there are also photosynthetic prokaryotes (the cyanobacteria) and a subset of the land plants, the Charales, which have been considered to be algae in previous texts (Patterson, 2014). With new molecular tools being applied to understanding algal taxonomy, systematics, and evolution, our understanding of this diversity is rapidly changing and expanding. Efforts to characterize this biological diversity (such as the Tree of Life Project: Maddison et al., 2007) have contributed to a better understanding of many groups of freshwater algae (e.g., Lane and Archibald, 2008; Entwisle et al., 2009; Hall et al., 2010; Ashworth et al., 2013; Stancheva et al., 2013a; Fučíková et al., 2014).

For simplicity, algae are treated in this book in the same sense as they are in recent phycology texts (e.g., Graham et al., 2008; Lee, 2008); that is, they are considered to be a loose (polyphyletic) group of organisms that have all or most of the following characteristics: aquatic, photosynthetic (possessing chlorophyll a), simple vegetative structures without a vascular system, and reproductive bodies that lack a sterile layer of protecting cells. There are important exceptions, however, particularly with regard to their habitat (Chapter 2), colorless relatives (lacking photosynthetic pigments; Chapters 10, 12, and 18), and species that switch between photosynthesis and predation on bacteria (mixotrophy; Chapters 10, 12, and 17). Both prokaryotic (those with cells lacking membrane-bound organelles) and eukaryotic taxa (cells with organelles) are considered in this book. Within the algae, there is an enormous range of vegetative morphologies, including the following:

1. Unicells: Species that occur as solitary cells that may be non-motile or motile, the latter of which move by using one or more flagella or gliding along surfaces via mucilage or other means. A wide variety of forms exist among unicells, including those contained within a gelatinous sheath (Fig. 1A); cells with intricate shapes, walls, and markings (Figs. 1B and C); cells having flexible cell shapes (Fig. 1D); and cells with two flagella of equal length (Fig. 1E) or flagella with unequal length (Fig. 1F). Cells can be drawn out into hornlike projections (Fig. 1G) or are contained in a hardened case or lorica (Fig. 1H).

2. Colonies: An aggregation of cells that are held together either in a loose (Fig. 1I and J) or tight well-organized pattern (Figs. 1K-L, 2A-C). Depending on the algal taxon, colonies may contain a variable number of cells, or they may be constant throughout their development (coenobium). Colonies may be composed of flagellated or non-flagellated cells. The basis for cellular connection varies among colonies, including a surrounding gelatinous matrix (Fig. 1I and J), gelatinous stalks (Fig. 1K), or a common parental wall (Fig. 1L). Cells may be arranged in a highly organized pattern (Fig. 2A), directly attached at their cellular edges (Fig. 2B), or at the middle portion of each cell (Fig. 2C). Alternatively, cells may be connected to others by their loricas (Fig. 2E).