Description: We investigated trace element stoichiometries of the nitrogen-fixing marine cyanobacterium Crocosphaera subtropica ATCC51142 under steady-state growth conditions. We utilized exponentially fed batch cultures and varied iron (Fe) concentrations to establish nutrient limitation in C. subtropica growing at a constant growth rate (0.11 d -1 ). No statistical difference in cell density, chlorophyll a , particulate organic carbon (C), nitrogen (N) and phosphorus (P) were observed between consecutive days after Day 14, and cultures were assumed to be at steady state with respect to growth for the remaining 11 d of the experiment. Cultures were limited by P in the highest Fe treatment (41 nmol l -1 ) and by Fe in the 2 lower-concentration Fe treatments (1 and 5 nmol l -1 ). Cell size and in vivo fluorescence changed throughout the experiment in the 1 nmol l -1 Fe treatment, suggesting ongoing acclimation of C. subtropica to our lowest Fe supply. Nevertheless, Fe:C ratios were not significantly different between the Fe treatments, and we calculated an average (±SD) Fe:C ratio of 32 ± 14 µmol mol -1 for growth at 0.11 d -1 . Steady-state P-limited cells had lower P quotas, whilst Fe-limited cells had higher manganese (Mn) and cobalt (Co) quotas. We attribute the increase in Mn and Co quotas at low Fe to a competitive effect resulting from changes in the supply ratio of trace elements. Such an effect has implications for variability in elemental stoichiometry in marine phytoplankton, and potential consequences for trace metal uptake and cycling in marine systems.

Description: Highlights • Low- and medium pressure TTGs are formed continuously. • High-pressure TTGs are mainly formed during episodic lithospheric recycling events. • The ratio of low- to medium-pressure TTGs corresponds to time-dependent crustal thickness. • Formation of high-pressure TTGs requires a delamination or subduction type process. The appearance of the earliest felsic crust can be estimated by dating zircons and rocks of tonalite-trondhjemite-granodiorite (TTG) composition. However, the necessary geodynamic processes that form the basis for metamorphism and differentiation as well as the role of emerging TTG crust on evolving crustal dynamics is still poorly understood. To investigate the formation of felsic crust with TTG composition, we conduct a detailed analysis of a series of previously published 3D high-resolution magmatic-thermomechanical models at elevated mantle temperature corresponding to Archean conditions. In these models we observed two distinct phases during coupled cyclic tectonomagmatic crust-mantle evolution: a long quiet growth phase followed by a short rapid overturn phase. Results of the detailed model analysis presented here suggest that (1) Low- and medium-pressure TTGs are formed at the base of the crust during both growth and overturn phase. The formation of low- and medium-pressure TTGs is linked with Moho depth. The ratio of low- to medium-pressure TTGs changes with crustal growth or thinning and gives an approximation for crustal thickness. (2) To form high-pressure TTGs an entirely different mechanism is required, as hydrated basaltic rocks need to be buried below the crust. Direct partial melting of cold eclogitic drips can be excluded as a valid mechanism due to their low temperatures and rapid sinking into the deep mantle. Rather we suggest delamination (peeling-off) or subduction as the main process for some high-pressure TTG production.