Effect of cell size on growth rates of two Skeletonema species and their competitve interactions

We investigated shifts in growth rates and cell size reduction-restoration processes in two species of diatoms, Skeletonema japonicum and Skeletonema dohrnii. Growth rates greatly fluctuated from 1.0 to 2.1 per day, even under the same conditions. Rates increased from 1.1 to 2.1 per day with decreasing valve diameter; however, rates quickly decreased to 1.0 per day when cell size reached the threshold for initiating auxosporulation. We also conducted co-culture experiments using different size combinations of the two species. The experiment was started using the same cell densities (75 cells · mL^-1 ) of both species, and when batch cultures reached late exponential phase, the cultures were reinoculated twice into new medium. When large (valve diameter of 17 μm) S. japonicum cells and small (6 μm) S. dohrnii cells were co-cultured, the S. dohrnii contributed 99% of the total cell density on day 16 (S. dohrnii: 263,900 cells · mL^-1 ; S. japonicum: 2,000 cells · mL^-1 ). In contrast, when small (9 μm) S. japonicum cells and large (15 μm) S. dohrnii cells were co-cultured, small S. japonicum cells accounted for 97% of the total cell density after only 13 d (S. dohrnii: 1,900 cells · mL^-1 ; S. japonicum: 62,500 cells · mL^-1 ). This study demonstrated that diatom growth rates covary with cell size, and this phenomenon potentially determines the outcome of competition between species.