Participation of diatoms in the amino acid cycle of coastal waters; uptake and excretion in cultures

Micro-algae are thought to be involved in the turnover of pools of dissolved free amino acids (DFAA) in seawater either by their excretion or by the uptake of these compounds. Our study showed that, in vitro, 3 benthic diatom species were able to deplete small additions of individual L-amino acids down to concentrations below 10 to 40 nmol l⁻¹, as determined by HPLC. When nitrogen was limited, all test species scavenged their media for 10 or 11 amino acids added but Amphiprora cf. paludosa assimilated 3 amino acids also in the presence of surplus nitrate. Navicula salinarum took up all 10 amino acids supplied, whereas Nitzschia closterium assimilated none of them, when nitrate was present. Excretion of DFAA could not be detected in the medium of growing nitrate-sufficient cultures; N. closterium, N. salinarum and A. cf. paludosa excreted respectively less than 0.03, 0.05 and 0.1 % of the cellular nitrogen as amino acids. Only silicate deprivation induced A. cf. paludosa to excrete 1 % cellular nitrogen as DFAA. The results show that benthic diatom populations may act as net consumers of DFAA, even when the concentrations are as low as 100 to 1,000 nmol l⁻¹, the range reported for nearshore seawater and the porewater of mudflats.