Growth of

A strain of Cyclotella meneghiniana was isolated from the River Danube. The optimum irradiance and temperature for growth were determined in batch cultures. The optimum temperature for growth was 25 °C. The estimated saturated irradiance was near 125 µE m-2 s-1. The calculated Ik for growth and photosynthesis were 30 ± 3 and 44 ± 6 µE m-2 s-1 respectively. A continuous culture technique was used to test the growth and cell composition at the transition state between batch, nutrient sufficiency, and a low rate of nutrient supply. Algal response was characterized by high intracellular N and C with negative growth rate. At a low dilution rate (0.06 d-1), the continuous culture theory was not followed. This critical rate is characterized by low chlorophyll-a content per cell and may depend on the mode of nutrition

Isotope dilution analysis of combined nitrogen in natural waters: I. Ammonium

Dissolved free amino acids (DFAA) are an overlooked and potentially very important substrate for primary production. In the open ocean, an understanding of the flux of regenerated nitrogen is fundamental to our studies of carbon and nitrogen turnover. A major aim of these studies is to provide more accurate estimates of the drawdown of atmospheric carbon dioxide. We have estimated the concentration and regeneration rate of DFAA in seawater using a novel stable isotope dilution technique. Seawater is first spiked to an estimated 10% of the DFAA pool with a known quantity of 15N-labelled algal protein hydrolysate. One portion is filtered immediately and the DFAA concentration analysed by isotope dilution. A second portion is incubated in situ. After a set period this is filtered and DFAA regeneration is estimated by isotope dilution analysis of the filtrate. Combination with data from analysis of particulate nitrogen provides an assay of DFAA uptake by phytoplankton. DFAA are extracted from 50 mL seawater by ligand exchange chromatography. Amino acids are eluted, concentrated and derivatized prior to GC/MS anslysis. The new procedure is sensitive, precise, accurate and, as it is based on mass spectrometric detection, it has greater selectivity than methods based on high-performance liquid chromatography/optical detection. Furthermore, the method can provide concentration and flux data on all protein amino acids and is applicable in a number of fields

Growth of

The growth and cell composition of C. meneghiniana in P-, N- or Si-limited continuous cultures under different dilutions are discussed. Carbon, nitrogen and phosphorus content per cell or per unit cell volume increased with growth rate in all chemostats. Maximum growth rate depends on the type of limiting nutrient. The maximum growth rates are. 1.65 ± 0.04, 0.73 ± 0.07 and 0.95 ± 0.08 d-1 for P-, N- and Si-limited cultures respectively. These values are close to those calculated by Droop, Caperon, Goldman and Monod models. The concentrations of N and P in the river Danube are much higher than the requirements for growth and uptake of C. meneghiniana. This indicates that neither P nor N are limiting to the growth of C. meneghiniana in natural habitat