Composition analysis of organic matter released by cosmopolitan coral reef-associated green algae

Coral reef-associated benthic algae can control important metabolic processes in reef ecosystems via organic matter release. However, information about the chemical composition of macroalgae exudates is very limited. We therefore quantified total and dissolved organic carbon (TOC, DOC) and total and dissolved organic nitrogen (TON, DON) of exudates released by 2 cosmopolitan coral reef-associated green algae species, Halimeda opuntia and Caulerpa serrulata. Glycosyl composition and content analyses, along with protein, lipid and chlorophyll a (chl a) quantifications of algae exudates were conducted. Findings revealed that organic matter released from both algae predominately consisted of carbohydrates (59 +/- 5%) and proteins (32 +/- 7%), which mainly (89 to 93%) dissolved in surrounding waters. Traces of fatty acids (C16:0; C18:0) and chl a were also found in algae incubation waters, but in a quantitatively negligible amount. Carbohydrate analysis further showed that glucose was the dominant glycosyl released by algae, accounting for 77 +/- 8% of the carbohydrate fraction and 42 +/- 8% of TOC. Galactose (9 +/- 4% of carbohydrate fraction), mannose (6 +/- 3%), xylose (4 +/- 1%), rhamnose (3 +/- 1%) and fucose (2 +/- 1%) were also detected in all incubation water samples. High glucose and protein contents of algae exudates found in the present study confirm assumptions on the fast microbial degradability of these exudates, with ensuing potential negative effects on 02 availability and on interactions between corals and algae in coral reef ecosystems.