Investigating Algal Primary Productivity and In Situ Production of Aquatic Fluorescent Organic Matter in a Simulated Freshwater System 

Freshwater systems play a disproportionately important role in global carbon cycling, yet the contribution of algal primary productivity to the production and transformation of dissolved organic matter (DOM) remains poorly researched, particularly with respect to fluorescent dissolved organic matter (FDOM). While protein-like fluorescence is widely associated with autochthonous microbial activity, humic-like fluorescence has traditionally been attributed to terrestrial inputs, despite emerging evidence for its in-situ microbial production. This study investigates the role of freshwater algae in the production, composition and temporal dynamics of protein-like and humic-like FDOM under controlled laboratory conditions. A previously developed simulated freshwater (SFW) model, free from external dissolved organic carbon inputs was optimised for algal growth and monitored for algal-derived fluorescence signatures using bench top fluorescence spectroscopy. Monoculture and mixed algal communities were grown under defined nutrient regimes, spanning low to elevated concentrations representative of nutrient enrichment and bloom conditions. We quantify algal primary productivity using single-turnover active fluorometry (STAF), providing high-resolution insight into photosystem II efficiency and productivity dynamics. Excitation–emission matrix spectroscopy is applied throughout algal growth phases to characterise changes in FDOM intensity, composition and persistence, with particular focus on protein-like and humic-like components. Findings show a coupling of primary productivity measurements with algae derived fluorescence signatures. This research aims to investigate the dynamics between algal metabolism, nutrient availability and FDOM production. The findings will improve understanding of algal contributions to the cycling of DOM in freshwater systems and identify the role and usefulness of fluorescence-based monitoring tools, particularly in environments experiencing nutrient enrichment and increasing algal biomass.