ROLE OF ALUMINUM AND GROWTH RATE ON CHANGES IN CELL SIZE AND SILICA CONTENT OF SILICA‐LIMITED POPULATIONS OF ASTERIONELLA RALFSII VAR. AMERICANA(BACILLARIOPHYCEAE)1

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Abstract
Changes in cell size and silica content were examined in response to aluminum additions in cultures of the acidophilic diatom Asterionella ralfsii var. americana Körn at pH 6.0. The effects of Al were examined over a range of steady‐state growth rates using silica‐limited semicontinuous cultures (Si:P = 8.0). Additions of ≥ 2.8 μmol · L−1 total Al decreased mean cell length, total surface area, and biovolume up to 40–50%. The effects of Al were dependent on growth rate with the magnitude of size reduction increasing at higher growth rates. The proportion of small (approximately 15–20 μm) cell length classes increased relative to large (approximately 50 μm length) cell length classes when total Al exceeded 2.8 μmol · L−1, particularly at higher growth rates. The relationship between cell quota and steady‐state growth rate fit a Droop relationship at 0 and 2.8 μmol·L−1 total Al, but this fit was highly variable in the presence of Al. Cell quotas in the 6.22 μmol·L−1 total Al treatment were highest at low growth rates; therefore, a Droop relationship was an inappropriate descriptor of growth rate. Cells also became 30–40% more heavily silicified per unit surface area in the presence of Al and at growth rates ≥0.22 day−1. Although the mechanisms responsible for size reductions in response to Al additions are unclear, the relationship between metal concentration and frustule morphology may be useful as an indicator of Al loading to acidified lakes.