Effects of temperature, salinity, irradiance and diurnal periodicity on growth and photosynthesis in the diatom Nitzschia americana: light-limited growth

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Abstract
The effects of variations in temperature (10, 15, 20, 25, 30°C) and salinity (8, 15, 20, 26, 32 p.p.t.) on cell size and rates of photosynthesis and population growth were evaluated in axenic, light-limited (30 μE m −2 s −1 ) cultures of an estuarine clone of the diatom Nitzschia americana . Experimental conditions were chosen to reflect the range of natural conditions which occur in the clone's native environment, the Cape Fear River Estuary, Nqrth Carolina. Rates of light-limited gross photosynthesis; or photosynthetic efficiency (PSE), were determined from short-term (1 h) 14 C incubations. Diurnal variation in PSE was analyzed using 14 C samples taken during times of estimated maximum and minimum rates of diurnal photosynthesis. The salinity-dependent temperature response of PSE is characterized by a gradual increase in rates up to a temperature optimum at −25°C, beyond which rates rapidly decline to zero at an upper lethal limit (30–40°C). A similar pattern was observed in population growth rates as a function of salinity and temperature. Independent of temperature, optimum salinity for growth was 26 p.p.t. A maximum growth rate of 2.4 div d −1 was measured at 25°C and 26 p.p.t. The effect of non-optimum salinity is a reduction in growth rates relative to a predicted temperature-dependent maximum. Salinity-dependent patterns of variation in cell volume, in general, mirrored the response of population growth such that cultures with relatively high growth rates were dominated by small cells. Significant diurnal variation was observed in PSE; maximum diurnal rates were generally 1.5–3.5 times greater than minimum diurnal rates.

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