Bioelectrochemical biosensors for in-situ monitoring microbial activity in outdoor soil mesocosms from postharvest rice field

The decomposition of buried straw in rice fields during post-harvest generates volatile fatty acids (VFA), thus activating methanogenesis (1). In this study, bioelectrochemical biosensors were used to measure the in-situ electrical current produced by electroactive microorganisms, related to the biodegradation of buired straw,  in outdoor mesocosms containing rice paddy soil from the Ebro Delta (Spain) .

Three  biosensors (BS1-BS3), based on bioelectrochemical cells buried in the water saturated soil (at a -10 cm), were used in 3 rice paddy soil mesocosms, with a poised working electrode (graphite) potential at +0.2V vs Ag/AgCl, by using a potentiostat. During 5 months (November 2022-March 2023), the production of electrical current (I) in the soil mesocosms was monitored using chronoamperometry. The presence of electroactive microbial biofilms on the electrodes was assessed by cyclic voltammetry (CV). Simultaneously, soil chemical parameters were monitored (total and soluble COD, VFA and CH4 emission), and microbial diversity (bacteria and archaea) in  the soil and the electrodes biofilms was assessed by 16S rRNA-metabarcoding.

Chronoamperometry data in BS1-BS3 showed a marked current production curve from the day 3 to 5 after straw addition, with an I max of 110-180µA (4.26-6.91 µA cm^-2) at day 10, remaining higher than the baseline for 30-45 days, and concomitant with VFA accumulation  (69-28 mg-eq Acetic kg^-1 soil , 7-40 days) and a high emission rate of CH4 (198.1±101.0 mg C-CH4· m^-2 soil · h^-1 7 days after straw addition. The CV revealed electroactive profiles in the 3 biosensors, similar in BS1-BS2 (oxidation peak -0.16/-0.22 V vs Ag/AgCl, similar to Geobacter), and different in BS3 (oxidation peak +0.26 V vs Ag/AgCl), revealing different electroactive microbial communities. 16S-based metataxonomy revealed an enrichment of well known electroactive bacteria on the three anode biofilm but with different relative predominances, encompassing mainly Desulfobulbus in BS1-BS3,  Geobacter mainly in BS1, but in less predominance in BS2 and BS3, Proteiniclasticum solely in BS3, and Clostridium in BS2 and BS3.  Methanogenic archaea such as Methanosarcina and Methanobacterium were also depicted on the anode, but at lower relative abundance than observed in the soil, where ammonium oxidizing archaea (Nitrososphaera and candidatus Nitrosocaldus) were also predominant.

The results showed the capacity of the bioelectrochemical-based biosensors for real time detection of microbial in-situ degradation processes of buried edible organic carbon (straw) in the soil of rice  paddy fields, also linked to methane emissions.

Aknowledgements
This research was funded by Agencia Estatal de Investigación (PID2019-111572RB-I00/AEI/10.13039/501100011033 ) from Spain.

References
1. Martínez-Eixarch, M., Alcaraz, C., Viñas, M., Noguerol, J., Aranda, X., Prenafeta-Boldú, F. X., Saldaña-De la Vega, J.A., Català, M.M. & Ibáñez, C. (2018). Neglecting the fallow season can significantly underestimate annual methane emissions in Mediterranean rice fields. PLoS One, 13(5), e0198081. DOI: 10.1371/journal.pone.0202159