Competition and facilitation between unicellular nitrogen‐fixing cyanobacteria and non—nitrogen‐fixing phytoplankton species

Abstract

Recent discoveries show that small unicellular nitrogen-fixing cyanobacteria are more widespread than previously thought and can make major contributions to the nitrogen budget of the oceans. We combined theory and experiments to investigate competition for nitrogen and light between these small unicellular diazotrophs and other phytoplankton species. We developed a competition model that incorporates several physiological processes, including the light dependence of nitrogen fixation, the switch between nitrate assimilation and nitrogen fixation, and the release of fixed nitrogen. Model predictions were tested in nitrogen-limited and lightlimited chemostat experiments using the unicellular nitrogen-fixing cyanobacterium Cyanothece sp. Miami BG 043511, the picocyanobacterium Synechococcus bacillaris CCMP 1333, and the small green alga Chlorella_cf sp. CCMP 1227. Parameter values of the species were estimated by calibration of the model in monoculture experiments. The model predictions were subsequently tested in a series of competition experiments at different nitrate levels. The model predictions were generally in good agreement with observed population dynamics. As predicted, in experiments with high nitrate input concentrations, the species with lowest critical light intensity (S. bacillaris) competitively excluded the other species. At low nitrate input concentration, nitrogen release by Cyanothece enabled stable coexistence of Cyanothece and S. bacillaris. More specifically, model simulations predicted that fixed nitrogen release by Cyanothece enabled S. bacillaris to become four times more abundant in the species mixture than it would have been in monoculture. This intricate interplay between competition and facilitation is likely to be a major determinant of the relative abundances of unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing phytoplankton species in the oligotrophic ocean.

Distribution of nano-sized Cyanobacteria in the western and central Pacific Ocean

“…Jiao et al 2005) that they were numerically abundant in the water with high availability of nutrients. These results suggest that nanocyanobacteria probably prosper where Synechococcus are limited by low availability of nutrients, especially nitrate, as observed in competition experiments using laboratory cultures (Agawin et al 2007). …”

“…This suggests that for nanocyanobacteria, which probably include nitrogen fixers, the cost of light limitation is larger than the benefit of enhanced nutrient supply in winter. Simultaneously, the deepening of the mixed layer would provide nitrate from the deep layer, which would decrease the ecological advantage of nitrogenfixing nanocyanobacteria (Agawin et al 2007). During the KH-04-5 and KH-05-2 cruises, the position of the front largely coincided with the border of the area depleted with surface soluble reactive phosphorus and the area with detectable soluble reactive phosphorus (Fig.…”

The evolutionary path to terminal differentiation and division of labor in cyanobacteria

“…Although there is a growing literature on modelling the ecology and population dynamics of nitrogen fixing cyanobacteria [88,69,65,3], the factors that can affect the evolution of multicellularity and differentiation in these organisms has not been examined. In this work we try to approach several fundamental questions.…”

Assessing the role of dust deposition on phytoplankton ecophysiology and succession in a low-nutrient low-chlorophyll ecosystem: a mesocosm experiment in the Mediterranean Sea

“…Induced variations in taxonomic, cell size and functional diversity of phytoplankton communities result in competition or coexistence of multiple species (Hutchinson, 1961;Tilman et al, 1982). Functional changes in phytoplankton cell size and functional groups diversified by peculiar biogeochemical properties (Iglesias- , for instance associated with nitrogen-fixation (Le Quéré et al, 2005;Agawin et al, 2007), diatom/cyanobacterial association (Carpenter et al, 1999), or even mixotrophy ability (Raven, 1997), cause the alternation of successful species whose eco-physiological characteristics will eventually determine the quality (elemental and biochemical composition) and quantity of primary production transferred up to the marine food web (Finkel et al, 2010 and references therein).…”