[B-CG07] [EE] Earth and Planetary Science Frontiers for Life and Global Environment

[BCG07-11] Multi-element, compound-specific isotope analysis of chloropigments: Insights into the biogeochemical cycle of the microbial mat in the shallow hypersaline environment

*Yuta Isaji¹, Hodaka Kawahata¹, Junichiro Kuroda¹, Toshihiro Yoshimura¹, Nanako O. Ogawa², Daisuke Araoka³, Akiko Makabe², Atsushi Suzuki³, Takazo Shibuya², Francisco J. Jiménez-Espejo², Yoshinori Takano², Stefano Lugli⁴, Andrea Santulli⁵, Vinicio Manzi⁶, Marco Roveri⁶, Naohiko Ohkouchi² (1.Atmosphere and Oceanic Research Institute, University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technolog, 3.National Institute of Advanced Industrial Science and Technology, 4.Università degli Studi di Modena e Reggio Emilia, 5.Consorzio Universitario della Provincia di Trapani, 6.University of Parma)

Keywords:chloropigments, carbon isotope, nitrogen isotope, magnesium isotope, microbial mat, hypersaline environment

In the last several decades, compound-specific isotope analysis of source-specific organic compounds (biomarkers) enabled scientists to access essential information of modern and past environments. Among numerous biomarkers, chloropigments are particularly powerful because they are synthesized only by photoautotrophs, and their tetrapyrrole nucleus is composed of carbon and nitrogen, thus providing constraints on the physiology of the photoautotrophs and the biogeochemical cycles of the photic zone from both carbon and nitrogen aspects. Another unique characteristic of the chloropigments is that they are metallo-organic complexes with Mg²⁺ as a central metal ion, potentially providing the opportunity to investigate into the biogeochemical processes that involve Mg. In this study, we investigated carbon, nitrogen, and magnesium isotopic compositions (δ¹³C, δ¹⁵N, δ²⁶Mg) of the chloropigments isolated from the hypersaline benthic microbial mat and gypsum crusts formed in the solar salterns of Trapani, Italy. The mats and the gypsums exhibit stratification of yellow, green, and pink layers, which are inhabited mainly by photosynthetic bacteria such as cyanobacteria and purple sulfur bacteria, together with diverse chemotrophic and heterotrophic microorganisms. We measured isotopic compositions of chlorophyll a (Chl a) originating from cyanobacteria living in the upper layers, and bacteriochlorophyll a (BChl a) from purple sulfur bacteria living directly below. The relationship between Chl a and BChl a were similar for both δ¹³C and δ¹⁵N in all examined samples. Lower δ¹³C of BChl a (ave. –22.9‰) compared to Chl a (ave. –17.2‰) suggests that purple sulfur bacteria are assimilating ¹³C-depleted carbon source supplied by mineralization of the organic matter in the deeper parts of the mats and gypsums. Striking difference of δ¹⁵N between Chl a (ave. 13.6‰) and BChl a (ave. –2.5‰) indicate that different substrate is utilized by cyanobacteria and purple sulfur bacteria. We infer that cyanobacteria are assimilating ¹⁵N-enriched ammonium because δ¹⁵N of nitrate ranged between –0.3 to 3.8‰, indicating that nitrate is not the main nitrogen source. Unlike δ¹³C and δ¹⁵N, δ²⁶Mg of Chl a and BChl a did not show any clear trend; δ²⁶Mg of Chl a varied between –1.77‰ and –0.39‰ and that of BChl a –2.13‰ and –0.11‰, on DSM3 scale. Moreover, δ²⁶Mg of Mg²⁺ in the brine ranged between –0.88 to –1.12‰, indicating that there are apparently both positive and negative isotopic fractionation between the source Mg and the chloropigments. Although more fundamental studies are necessary to understand the mechanisms determining Mg isotopic signature of the chloropigments, our results imply that δ²⁶Mg of chloropigments may vary substantially in response to the changing physiological states and environmental conditions of the photoautotrophs.