Environmental and physiological controls on daily trace element incorporation in Tridacna crocea from combined laboratory culturing and ultra-high resolution LA-ICP-MS analysis

Recent advances in spatially-resolved in-situ geochemical analysis enabled recognizing daily variability in B, Mg, Sr and Ba within bivalve shells of the genus Tridacna (e.g. Sano et al., 2012; Warter and Müller, 2017). However, the causes for such daily chemical variations are not fully understood. We carried out laboratory culture experiments to investigate the effect of varying temperatures and light levels on organism physiology and shell geochemistry of the giant clam Tridacna crocea at daily time-resolution. We find that both temperature and light strongly influence shell growth rates. Besides demonstrating light enhanced calcification, we also observe growth inhibition during culture in continuous darkness. ‘Ultra-high resolution’ LA-ICP-MS analysis (Warter and Müller, 2017) in tandem with 135Ba-labelling allows both to identify newly secreted, cultured aragonite and investigate its trace elemental composition at (sub)daily resolution (B/Ca, Mg/Ca, Sr/Ca, Ba/Ca). Overall, average Element/Calcium (El/Ca) ratios reveal an inverse relation with temperature and light. Strikingly, an increasing trend in all El/Ca ratios is observed in the direction of shell growth, which is interpreted to be metabolically-controlled, possibly related to organism ‘stress’. Daily, cyclic trace element variability can be clearly resolved at ~5–15μm in relatively fast growing shells, which experienced a diurnal light cycle but constant temperatures. We conclude that light (but not temperature) is the primary controlling factor responsible for daily trace element variability. We further demonstrate that high El/Ca ratios correspond to growth during night time, i.e. low light intensities, corroborating field observations by Sano et al. (2012). Yet, because light levels were kept constant during night and day, respectively, light intensity cannot account for the entire range of the El/Ca variability. We hypothesize that Tridacna physiology and biochemistry is controlled by a circadian rhythm, initiated by the external factor light, but endogenously mediated by the giant clam and/or its photosynthetic symbionts.