Elemental Stoichiometry and Photophysiology Regulation of Synechococcus sp. PCC7002 Under Increasing Severity of Chronic Iron Limitation

Abstract

Iron (Fe) is an essential co-factor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp. PCC7002 we investigated the physiological responses to long-term acclimation to four levels of Fe availability representative of the contemporary ocean (36.7, 3.83, 0.47 and 0.047 pM Fe'). With increasing severity of Fe limitation, Synechococcus sp. cells gradually decreased their volume and growth while increasing their energy allocation into organic carbon and nitrogen cellular pools. Furthermore, total cellular content of pigments decreased. Additionally, with increasing severity of Fe limitation, intertwined responses of photosystem II functional cross-section (σPSII), re-oxidation time of the plastoquinone primary acceptor QA (τ), and non-photochemical quenching revealed a shift in the photophysiological response between mild to strong Fe limitation compared to severe limitation. Under mild and strong Fe limitation there was a decrease in linear electron transport accompanied by progressive loss of state transitions. Under severe Fe limitation, state transitions seemed to be largely supplanted by alternative electron pathways. In addition, mechanisms to dissipate energy excess and minimize oxidative stress associated with high irradiances increased with increasing severity of Fe limitation. Overall, our results establish the sequence of physiological strategies adopted by the cells under increasing severity of chronic Fe limitation, within a range of Fe concentrations relevant to modern ocean biogeochemistry.

Limitation by Fe, Zn, Co, and B12 Results in Similar Physiological Responses in Two Antarctic Phytoplankton Species

“…As previously observed for P. antarctica (Koch et al, 2019;Trimborn et al, 2019a) and other phytoplankton species (Allen et al, 2008;Botebol et al, 2017;Blanco-Ameijeiras et al, 2018), Fe limitation significantly affects cellular physiology. Typically Fe-limited cells grow more slowly, are often smaller, alter their photophysiology and have reduced particulate organic carbon (POC) quotas.…”

Photosynthetic energy conversion efficiency in the West Antarctic Peninsula

“…Similar results were seen in laboratory experiments with the diatom Thalassiosira oceanica , the haptophyte Chrysochromulina polylepis (Schuback et al 2015), and the cyanobacterium Synechococcus sp . (Blanco‐Ameijeiras et al 2018). The iron‐limited increase in is assumed to be an additional effect of the iron economizing physiology.…”

Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation

“…On the downside for Synechococcus , both the phycobilisome‐based light‐harvesting system and siderophore transport systems are costly cell structures (Lis et al., 2015; Ting Rocap, King, & Chisholm, 2002), which may explain the exceptionally high nitrogen demand and relatively low growth rates of coastal Synechococcus (Figure 5). For a given carbon content, the Synechococcus PCC7002 strain contains 64% more nitrogen than M. pusilla (Blanco‐Ameijeiras et al., 2018; Maat, Crawfurd, Timmermans, & Brusaard, 2014). In line with this, the protein investment for the phycobilisome‐based light‐harvesting system of Synechococcus is >3 times higher than for the chlorophyll a / b ‐based antennae of green algae (Ting, Rocap, King, & Chisholm, 2002).…”