Composition characteristics of dissolved organic matter derived from microalgae

Introduction.Algal-derived dissolved organic matter (ADOM) released by microalgae during growth and metabolism is a key autogenous component of DOM in water, which can be divided into Intracellular Organic Matter (IOM) and Extracellular Organic Matter (EOM). Their compositions, aromaticity and humification degree differ significantly, while the differences in DOM content and structure of EOM and IOM among various microalgae remain unclear. This study focused on the source of microalgal DOM and analyzed DOC content and optical structural characteristics of EOM and IOM based on physiological and metabolic processes of microalgae.

Methods.Five microalgae, Platymonas subcordiformis, Phaeodactylum tricornutum, Chaetoceros muelleri, Isochrysis zhanjiangensis and Nitzschia Closterium, were selected in this study. The EOM and IOM were extracted by F/2 medium. Three-dimensional fluorescence spectroscopy and ultraviolet-visible absorption spectroscopy were used for analysis.

Results.There were significant differences in the DOC content of different microalgae sources, and the concentration levels of EOC and IOC (230.45 ± 24.49 and 68.95 ± 9.11 mg/L) of P. subcordiformis were the highest, nitzschia Closterium had the lowest contents of EOC and IOC (65.87 ± 1.14 mg/l, 26.18 ± 0.61 mg/L) . EOM is mainly composed of humic-like and tryptophan-like components with high humification, high DOC content and difficult degradation. IOM is mainly composed of tyrosine-like components with strong autogenicity, high aromaticity and easy degradation, which are directly related to the endogenesis and metabolism of microalgae.

Conclusion.The release of microalgae-derived DOM from intracellular to extracellular is an evolutionary process from fresh protein fraction to humic fraction. ADOM is mainly composed of protein-like and humic-like components, in which IOM is rich in protein-like substances, and EOM is relatively high in humic-like components. This study can provide a theoretical basis for the scientific selection of feed microalgae and water quality control in aquaculture.