Towards a carbon budget of the diazotrophic cyanobacterium Crocosphaera: effect of irradiance

Abstract

We examined changes in carbon fluxes in the marine unicellular diazotrophic cyanobacterium (UCYN) Crocosphaera watsonii in response to irradiance. We estimated changes in the total organic carbon and nitrogen contents, incorporation of carbon into reserves, and exudation processes of extracellular exopolymeric substances (EPS), as well as in the formation of transparent exopolymeric particles (TEP). Variability of cellular carbon pools and extracellular EPS and TEP under conditions of different irradiance and the significance of carbon release in the environment is discussed. Our results show a relationship between growth rate, metabolic processes of carbon incorporation in cells, exudation and irradiance. When growth is light-limited, both carbon production and exudation increase with light. No inhibition occurs at growth-saturating irradiances and half of the total particulate organic carbon is then found as intracellular carbohydrates. While the overall growth becomes light saturated, carbon fixation is further stimulated by higher irradiances, leading to an increased EPS exudation. We found that EPS exudation in C. watsonii is not only a way for cells to release excess carbon, but also occurs when growth is light-limited, even at the lowest irradiance tested. The formation of TEP increases with irradiance, with TEPcarbon amounting to up to 30% of the total particulate organic carbon. Our data will help to quantify the role of unicellular diazotrophs in the export of both carbon and nitrogen through and below the euphotic zone in the marine environment.

Diazotrophic community and associated dinitrogen fixation within the temperate coral Oculina patagonica

“…The high relative abundance of potentially autotrophic as compared with heterotrophic diazotrophs in O. patagonica suggests that these bacteria may provide, besides DDN, also fixed C to the coral host. In the open ocean, unicellular, diazotrophic cyanobacteria are known to exude large amounts of C into the oligotrophic environment (Rabouille et al ., ) and symbiotic autotrophic cyanobacteria within sponges supply a significant amount of fixed C to their host (Erwin and Thacker, ). Similarly, coral‐associated cyanobacteria may constitute an alternative C source for the coral host that could be beneficial when the supply from Symbiodinium is disrupted.…”

Impact of increasing carbon dioxide on dinitrogen and carbon fixation rates under oligotrophic conditions and simulated upwelling

“…Unicellular diazotrophic cyanobacterium clade UCYN‐A and its symbiotic algae transfer nitrogen and C to each other (Thompson et al 2012), thus high CO 2 may indirectly benefit them. Unicellular diazotrophic cyanobacterium clade UCYN‐B fix C themselves, so they could potentially benefit directly from elevated CO 2 (Mohr et al 2010b; Rabouille et al 2017). This study was aimed to test the hypothesis that N 2 fixation will increase with increasing p CO 2 levels in the natural environment.…”

Independence of a Marine Unicellular Diazotroph to the Presence of NO3−

“…All culture experiments were conducted on monocultures of Crocosphaera watsonii WH8501 [27,28], in acid-cleaned boro-silicate culture vessels, maintained under tightly controlled conditions of light and temperature in a Sanyo MLR 351 incubator as described in [29] and using the same experimental setup. Irradiance was provided by fluorescent tubes (Sanyo FL40SS W/37, Osaka, Japan).…”

“…The total carbon and nitrogen contents per cell are needed to deduce the total nitrogen and carbon biomass formed in this experiment #1. We pooled data of carbon and nitrogen content in C. watsonii from the present study as well as our past studies [29,31,35] to derive an average content in our strain. The most robust reference appears to be the volumetric content as it allows comparison of N contents in cells with different sizes.…”