1 - Seaweed thalli and cells

Summary

Introduction: the algae and their environments

Seaweeds

The term “seaweed” traditionally includes only macroscopic, multicellular marine red, green, and brown algae. However, each of these groups has microscopic, if not unicellular, representatives. All seaweeds at some stage in their life cycles are unicellular, as spores or gametes and zygotes, and may be temporarily planktonic (Amsler and Searles 1980; Maximova and Sazhin 2010). Some remain small, forming sparse but productive turfs on coral reefs (Hackney et al. 1989) while others, such as the “kelps” of temperate reefs, can form extensive underwater forests (Graham et al. 2007a). Siphonous algae such as Codium, Caulerpa and Bryopsis that form large thalli are, in fact, acellular. The prokaryotic Cyanobacteria have occasionally been acknowledged in “seaweed” floras (e.g. Setchell and Gardner 1919; Littler and Littler 2011a). They are widespread on temperate rocky and sandy shores (Whitton and Potts 1982) and are particularly important in the tropics, where large macroscopic tufts of Oscillatoriaceae and smaller but abundant nitrogen-fixing Nostocaceae are major components of the reef flora (Littler and Littler 2011a, b; Charpy et al. 2012). Benthic diatoms also form large and sometimes abundant tube-dwelling colonies that resemble seaweeds (Lobban 1989). An ancient lineage of (mostly) deep-water green algae, the Palmophyllales, that includes Verdigellas and Palmophyllum, have a palmelloid organization with complex thalli built from an amorphous matrix with a nearly uniform distribution of spherical cells (Womersley 1971; Zechman et al. 2010). On a smaller scale are the colonial filaments of some simple red algae, such as Stylonema (previously Goniotrichum). A “seaweed” is therefore problematic to precisely define: here “seaweed” refers to algae from the red, green, and brown lineages that, at some stage of their life cycle, form multicellular or siphonous macrothalli. In this book we shall consider macroscopic and microscopic marine benthic environments and how seaweeds respond to those environments.