Genetic heterogeneity and physiological variation among seasonally separated clones of Skeletonema marinoi (Bacillariophyceae) in the Gullmar Fjord, Sweden

Abstract

Water samples and plankton net hauls were collected 24 times from Gullmar Fjord on the Swedish west coast from February 2004 to March 2005. The abundance of Skeletonema marinoi was estimated and individual clones isolated. Abundance was highest during the spring blooms in February to March. Subsequently, S. marinoi was detected in all samples but at lower abundances. At the end of September a second peak was recorded. All clones were pre-adapted to the same culturing conditions for more than 1.5 years. Large subunit (LSU) rDNA (D1–D3) was sequenced from 23 clones isolated from three different seasons, February, June and September. Six microsatellite loci were genotyped for 19 clones to estimate within-season genetic diversity. Three clones from each season were selected for physiological experiments at different salinity and temperature combinations and monitored for average number of divisions per day, maximum cell densities, biovolume, and total RNA concentration per cell. Differentiation of physiological response among the clones was partly attributed to the month of isolation. The February isolates had a significantly higher division rate and larger biovolumes, while the September clones attained higher cell densities. The June clones were isolated during the time of the year when the natural abundance is lowest, and exhibited the smallest genetic and physiological variation, which suggests that they were under strong selection pressure. The differential physiological responses and degree of genetic heterogeneity among seasonally separated clones could indicate that different populations succeed each other in the fjord.