Static allometry of unicellular green algae: scaling of cellular surface area and volume in the genus Micrasterias (Desmidiales)

The surface area-to-volume ratio of cells is one of the key factors affecting fundamental biological processes and, thus, fitness of unicellular organisms. One of the general models for allometric increase in surface-to-volume scaling involves fractal-like elaboration of cellular surfaces. However, specific data illustrating this pattern in natural populations of the unicellular organisms have not previously been available. This study shows that unicellular green algae of the genus Micrasterias (Desmidiales) have positive allometric surface-to-volume scaling caused by changes in morphology of individual species, especially in the degree of cell lobulation. This allometric pattern was also detected within most of the cultured and natural populations analysed. Values of the allometric S:V scaling within individual populations were closely correlated to the phylogenetic structure of the clade. In addition, they were related to species-specific cellular morphology. Individual populations differed in their allometric patterns, and their position in the allometric space was strongly correlated with the degree of allometric S:V scaling. This result illustrates that allometric shape patterns are an important correlate of the capacity of individual populations to compensate for increases in their cell volumes by increasing the surface area. However, variation in allometric patterns was not associated with phylogenetic structure. This indicates that the position of the populations in the allometric space was not evolutionarily conserved and might be influenced by environmental factors.

细胞的表面积体积比（surface area-to-volume ratio）是影响基础生物学过程、进而决定单细胞生物适合度的关键因素之一。用于阐释表面积体积缩放异速增长的通用模型之一，涉及细胞表面的类分形化构建。然而此前尚未有可用于展示单细胞生物自然种群中该规律的特异性数据。本研究表明，鼓藻目（Desmidiales）鼓藻属（Micrasterias）的单细胞绿藻，会因单个物种的形态变化——尤其是细胞裂片化程度的改变——呈现出正向的表面积体积异速缩放关系。该异速模式在绝大多数被分析的培养种群与自然种群中均被检测到。单个种群内的表面积体积异速缩放（allometric S:V scaling）数值，与该进化枝的系统发育结构密切相关；此外，其还与物种特异性的细胞形态存在关联。不同种群的异速模式存在差异，且它们在异速生长空间中的位置与表面积体积异速缩放程度强相关。这一结果说明，异速形态模式是单个种群通过增加细胞表面积以补偿体积增长的能力的重要关联因素。不过，异速模式的变异与系统发育结构并无关联，这表明种群在异速生长空间中的位置并非进化保守的，可能受环境因素影响。