Background The coral-Symbiodiniaceae symbiosis is fundamental for the coral reef ecosystem. Corals provide various inorganic nutrients to their algal symbionts in exchange for the photosynthates to meet their metabolic demands. When becoming symbionts, Symbiodiniaceae cells show a reduced proliferation rate and a different life history. While it is generally believed that the animal hosts play critical roles in regulating these processes, far less is known about the molecular underpinnings that allow the corals to induce the changes in their symbionts. Results We tested symbiont cell proliferation and life stage changes in vitro in response to different nutrient-limiting conditions to determine the key nutrients and to compare the respective symbiont transcriptomic profiles to cells in hospite. We then examined the effects of nutrient repletion on symbiont proliferation in coral hosts and quantified life stage transitions in vitro using time-lapse confocal imaging. Here, we show that symbionts in hospite share gene expression and pathway activation profiles with free-living cells under nitrogen-limited conditions, strongly suggesting that symbiont proliferation in symbiosis is limited by nitrogen availability. Conclusions We demonstrate that nitrogen limitation not only suppresses cell proliferation but also life stage transition to maintain symbionts in the immobile coccoid stage. Nutrient repletion experiments in corals further confirmed that nitrogen availability is the major factor limiting symbiont density in hospite. Our study emphasizes the importance of nitrogen in coral-algae interactions and, more importantly, sheds light on the crucial role of nitrogen in symbiont life history regulation.

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We investigated variations in cell growth and ATP Sulfurylase (ATPS) activity when two cyanobacterial strains— Synechocystis sp. PCC6803 and Synechococcus sp. WH7803—were grown in conventional media, and media with low ammonium, low sulfate and a high CO 2 /low O 2 atmosphere. In both organisms, a transition and adaptation to the reconstructed environmental media resulted in a decrease in ATPS activity. This variation appears to be decoupled from growth rate, suggesting the enzyme is not rate-limiting in S assimilation and raising questions about the role of ATPS redox regulation in cell physiology and throughout Earth history.

Here we investigated variations in cell growth and ATP sulfurylase activity when two cyanobacterial strains – Synechocystis sp. PCC6803 and Synechococcus sp. WH7803 – were grown comparatively between conventional media and media with low ammonium, low sulfate and a controlled high CO2/low O2 atmosphere, which might resemble some Precambrian environments. In both organisms, a transition and adaptation to the reconstructed environmental media resulted in a decrease in ATPS specific activity. This decrease in activity appears to be decoupled from growth rate, suggesting the enzyme is not rate-limiting in S assimilation and raising questions about the role of ATPS redox regulation in cell physiology and thorughout history.