Mass sexual reproduction in the toxigenic diatoms Pseudo-nitzschia australis and P. pungens (Bacillariophyceae) on the Washington Coast, USA

Sexual reproduction is documented for the first time in field populations of the pennate diatoms Pseudo-nitzschia australis Freng. and P. pungens (Grunow ex Cleve) Hasle (var. cingulata Villac and hybrids between var. cingulata and var. pungens). A bloom dominated by these species began on June 26, 2006, along Kalaloch Beach, Washington, USA, coincident with a drop in the Si(OH)(4):NO3 ratio to below two. Multimodal size distributions were detected for both species, and synchronous auxosporulation occurred within the smallest size class during a 3-week window. Auxospores and initial cells created a new class of large cells, and cells in the intermediate size classes increased in abundance during auxosporulation. Mating cells of both species were attached to colonies of surf-zone diatoms. Paired gametangia, gametes, zygotes, auxospores, and large initial cells were found. Auxosporulation began first for P. pungens (June 30), apparently once a critical, high cell concentration was reached, followed by P. australis (July 5), when the total Pseudo-nitzschia cell concentration reached 929,000 cells . L-1. Low frequencies of auxosporulation occurred throughout the bloom but increased 4-fold for P. australis and 3-fold for P. pungens when macronutrients were reduced to low levels on July 11. A 2-year life cycle was estimated for P. australis and 3 years for P. pungens, both with annual auxosporulation. Domoic acid (DA) in razor clams reached a maximum of 38 mu g DA . g(-1) on July 18. A significant relationship existed between the percent of cells within the new size range and DA concentrations in razor clams on the same beach.