Seasonality affects dinitrogen fixation associated with two common macroalgae from a coral reef in the northern Red Sea

Nitrogen (N) is often a limiting nutrient for primary production in coral reef eco systems. In this context, dinitrogen (N-2)-fixing prokaryotes (diazotrophs) associated with benthic primary producers can relieve N limitation. Macroalgae are key reef players that are generally able to rapidly uptake dissolved inorganic nutrients. They may thus particularly benefit from the activity of associated diazotrophs. With this rationale, this study investigated N-2 fixation activity and net primary production associated with 2 dominant coral reef macroalgae (the green algal genus Caulerpa and the brown algal genus Lobophora) during all 4 seasons in a fringing northern Red Sea reef using the acetylene using the acetylene reduction assay and oxygen production and consumption measurements. Both macroalgae exhibited associated N-2 fixation activity during all seasons with lowest activity in winter and significantly higher activity (1 and 2 orders of magnitude increase for Lobophora and Caulerpa, respectively) during the nutrient-depleted summer, while net primary production for both macroalgae remained relatively constant over all 4 seasons. Primary production rates of the macroalgae were comparable to corals from the same area on a yearly average. Conversely, average N2 fixation rates of both macroalgae were approximately 5-fold higher than rates reported for hard corals that were incubated in parallel experiments. These results indicate that macroalgae can capitalize on higher inputs of N from epibiotic diazotrophs, which in turn could prove an ecological advantage when competing for space with corals.