Electron & Biomass Dynamics of Cyanothece Under Interacting Nitrogen & Carbon Limitations

Abstract

Marine diazotrophs are a diverse group with key roles in biogeochemical fluxes linked to primary productivity. The unicellular, diazotrophic cyanobacterium Cyanothece is widely found in coastal, subtropical oceans. We analyze the consequences of diazotrophy on growth efficiency, compared to NO3–-supported growth in Cyanothece, to understand how cells cope with N2-fixation when they also have to face carbon limitation, which may transiently affect populations in coastal environments or during blooms of phytoplankton communities. When grown in obligate diazotrophy, cells face the double burden of a more ATP-demanding N-acquisition mode and additional metabolic losses imposed by the transient storage of reducing potential as carbohydrate, compared to a hypothetical N2 assimilation directly driven by photosynthetic electron transport. Further, this energetic burden imposed by N2-fixation could not be alleviated, despite the high irradiance level within the cultures, because photosynthesis was limited by the availability of dissolved inorganic carbon (DIC), and possibly by a constrained capacity for carbon storage. DIC limitation exacerbates the costs on growth imposed by nitrogen fixation. Therefore, the competitive efficiency of diazotrophs could be hindered in areas with insufficient renewal of dissolved gases and/or with intense phytoplankton biomass that both decrease available light energy and draw the DIC level down.

The balance between photosynthesis and respiration explains the niche differentiation between Crocosphaera and Cyanothece

“…It increases when either cell density increases and/or increases in sizes increase, or cells accumulates intracellular storage products such as granules. The non-linear relationships between OD 735 and cell abundance have been shown previously, both in Crocosphaera and Cyanothece [26] , [34] . The daily changes in cell size as the cells undergo division are also small (<10 %) in both species.…”

“…The daily changes in cell size as the cells undergo division are also small (<10 %) in both species. Therefore, the diurnal increase in OD 735 reflects mostly biomass production, specifically the increase in cellular C content as a result of photosynthesis, whereas the nocturnal decrease in OD 735 is caused by the consumption of cellular C as a consequence of respiration [26] , [34] . Since ODs can be influenced by various factors, absolute OD 735 of two different species may not reflect the same C content.…”

“…The photobioreactor monitored and averaged the OD 735 data over every 1 or 5 min intervals and we normalized them to the OD 735 values recorded at 1 h after the onset of the light phase (1L). Based on our earlier studies [20] , [26] , [34] , we considered the increase in OD 735 during the light phase as a proxy for C accumulation by photosynthesis. In contrast, the decrease in OD 735 at the end of the light phase and also during the dark phase as a proxy for C consumption by respiration (see Discussion).…”

Ecological Interactions of Cyanobacteria and Heterotrophs Enhances the Robustness of Cyanobacterial Consortium for Carbon Sequestration

“…P. alkaliphilum mainly performs protein synthesis during day time, as previously observed (Singer and Doolittle, 1975;Hood et al, 2016). Additionally, the differences between the rate of 13 C/ 12 C increase during day 1 and day 2 suggest that C incorporation into cyanobacterial proteins is a two-step process: the newly fixed carbon first accumulates in an intermediate glycogen pool before being assimilated into proteins (Polerecky et al, 2021;Rabouille et al, 2021). Such an intermediate glycogen pool would initially consist mainly of 12 C, as it was synthesized before the addition of 13 C-bicarbonate.…”