Effects of cell density, temperature, and light intensity on growth and stalk production in the biofouling diatom Achnanthes longipes (Bacillariophyceae)

Achnanthes longipes Ag. is a marine stalk-forming diatom that grows in dense biofilms. The effects of cell density, temperature, and light on growth and stalk production were examined in the laboratory to determine how they affected the ability of this diatom to form a biofilm. Stalk production abruptly increased when A. longipes was cultured at a density of 5.4 x 10(3) cells.mL(-1), with a lag before stalk production occurring in cultures initiated at lower densities. Growth occurred at all temperatures from 8 to 32degrees C, with maximum growth at 26degrees C. Growth rate was light saturated at 60 mumol photons.m-(2) .s(-1). Stalk production was determined as the proportion of cells producing stalks and stalk length in response to various temperatures and light intensities at high (5000 cells.mL(-1)) and low (500 cells.mL(-1)) densities. More cells formed stalks at high density, with no difference in stalk length. The proportion of cells producing stalks was maximal at 20degrees C, with little change at 17-32degrees C. Stalk length was at a maximum between 14 and 26degrees C. Stalk production showed little change in response to varying light intensity. The results of an earlier investigation on the effects of bromide concentration on stalk formation were expressed as the proportion of cells forming stalks and the lengths of the stalks. Both measures of stalk production varied with bromide concentration, with maximum values at 30 mM bromide. The increased stalk production at higher densities may be a means of elevating cells above the substrate to avoid competition in the dense biofilm.