Light absorption by cyanobacteria: Implications of the colonial growth form

We assessed differences inthe light absorption characteristics ofnine species of cyanobacteria, colonial and single-celled, and four species of eucaryotic phytoplankton species. The results indicated that the positive scaling of light absorption by both single cells and cyanobacteria colonies to their size and Chl a content is similar to that of eucaryotic algae, but the large differences in the extent of pigment packaging within single-celled cyanobacteria (i.e. cl) were unrelated to their size. Cyanobacteria colonies, in contrast, could avoid increased self-shading with increasing size by reducing their internal pigment concentration as colony size increased, as do eucaryotic cells. The results indicated that cyanobacteria colonies act as distinct optical units, rather than as the sum of their component cells. The ability of many cyanobacteria species to develop colonies may help explain their ability to develop nuisance blooms. Cyanobacteria are important components of both marine and freshwater phytoplankton communities that differ, both structurally and biochemically, from eucaryotic algae. Some differences, such as the lack of chloroplasts in their cells, differences in pigment complement, and lower cell Chl a content relative to eucaryotic algae (Bricaud et al. 19 8 8; Agusti 199 1 a) should affect the absorption of light by cyanobacteria. Agusti (199 la) demonstrated that small cyanobacteria exhibit lower light absorption per unit of biomass than similarly sized eucaryotic algae do. In addition, many cyanobacteria develop large planktonic olonies, which, by increasing the pigment packaging within the colony, may also influence their absorption of light. The development of colonies, which modify the functional size range of cyanobacteria, may increase population growth Acknowledgments We thank D. E. Canfield, Jr., for support during this study and C. M. Duarte for comments on the manuscript. rates by reducing loss factors (i.e. sinking rates, e.g. Walsby and Reynolds 1980, and grazing losses, cf. Paerl 1988), thereby allowing colonial cyanobacteria to develop large planktonic blooms (e.g. Reynolds and Walsby 1975; Paerl and Ustach 1982). However, the colonial growth form may also involve a cost, as increased pigment packaging within the colonies may reduce the efficiency of light absorption by cyanobacteria pigments compared to that of cells grown in isolation. Yet, the tendency of blooming cyanobacteria to develop large colonies (Duarte et al. 1990) suggests that the colonial growth form may involve some adaptive advantages for growth within the highly shaded environment associated with dense algal blooms (cf. Agusti 1991 b). Previous studies of light absorption by cyanobacteria have been limited to picoplankton-size species of the genera Synechococcus or Synechocystis (e.g. Osborne and Geider 1989; Bricaud et al. 1988; Stramsky and Morel 1990). Thus, the light absorption properties of cyanobacteria as a group have not been adequately characterized, and the implications for light absorption of the colonial vs. isolated growth abits remain untested. Our goals here are twofold: to assess whether the light absorption properties of cyanobacteria differ from those of eucaryotic algae and to examine the implications for light absorption of colonial vs. isolated growth. To address these questions we grew nine cyanobacterial species in batch cultures (250 ml). These species included unicellular (Microcystis sp. clone 007, Microcystis p. clone 008, Microcystis aeruginosa UTEX 2B 2385, Raphidiopsis curvata, Oscillatoria sp., and Synechococcus sp.) and colonial forms growing either as isolated filaments (Lyngbya sp., Anabaena circinalis) or as aggregates of cells or filaments (Anabaenaflos-aquae, Microcystis sp. clone 007).