Shape dynamics of silica scales (Chrysophyceae, Stramenopiles) associated with pH.

Abstract

We investigated the shape dynamics of silica scales in relation to environmental pH in two freshwater algal flagellates, Mallomonas striata and Synura echinulata, using landmark-based methods of geometric morphometrics. pH has been implicated as a primary factor controlling the occurrence and distribution of these protists. Moreover, scales are preserved in sediments and provide important proxy data of past and recent environmental changes. We concluded that the pH of the cultivation medium significantly affected scale biogenesis and influenced the final shape of produced silica scales. In addition, we asked how size of the scales was related to their shape (allometric effect). Scales produced under various levels of pH treatment were significantly different in shape in both investigated species, even though we eliminated the effect of size. We also investigated phenotypic plasticity (defined as the extent of scale shape variation) within the investigated pH treatments. Increased phenotypic plasticity is generally accepted to be induced by environmental stress. The impact of pH on M. striata and S. echinulata population growth was used to evaluate optimal/suboptimal conditions. We revealed increased scale shape plasticity in either suboptimal conditions (both species) or in high level pH treatments (pH 8.2 and 8.7, in M. striata). The inability of cells to compose a perfectly fitting, functional scale case, when the shape of individual scales varied considerably, may cause absence of Synurales in highly alkaline natural environments.