Shewanella sp. O23S: Respiration to biominerals

Shewanella sp. O23S (related to Shewanella baltica; and isolated from a former gold mine in SW Poland) is a versatile metal and metalloid reducer [1-2] and a potential candidate for the bioremediation of polymetallic industrial streams [3]. In a recent study [1], the strain was shown to reduce selenate (SeO42-) and selenite (SeO32-) to extracellular red Se(-S)0, and arsenate (AsO43-) to arsenite (AsO33-). When cysteine was metabolized, yielding H2S, arsenite produced extracellular As-S biominerals. After seven days of bacterial incubation with only either Se or As oxyanions, kinetic analysis indicated the following reduction yields: SeO32- (90%), AsO43- (60%), and SeO42- (less than 10%). The mix of SeO32- with AsO43- led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO42- incubated with AsO43- boosted Se removal (71%). These results indicate a complex metabolic relation between
As and Se oxyanions leading to either inhibition or stimulation outcomes in Shewanella sp. O23S. All biominerals formed were extracellular, amorphous and presented a negative surface charge in the -24 to -38 mV range. The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process, wherein the bacterial cells harvest energy by chemolitotrophy and dispose of the solid by-products outside the cell [4]. While Se(-S)0 biominerals were mainly present with granular morphology, As-S displayed both nanorod (AsS, realgar) and granular (As2S3, orpiment) morphologies.
This study provides evidence about the mixed microbial metabolism and the biomineralization of arsenic and selenium, and opens future research directions on the enzymatic systems and genetic determinants [5] involved in this process with potential bioremediation and resource recovery applications. A particularly interesting research direction pertains to the enzymatic system(s) involved in these transformations: multi-substrate respiratory enzyme or multiple enzymes capable of binding As and Se oxyanions?

References
[1] Staicu et al. (2022) Environ Pollut 306:119451 https://doi.org/10.1016/j.envpol.2022.119451
[2] Drewniak et al. (2015) Int J Mol Sci 16(7):14409-27 https://doi.org/10.3390/ijms160714409
[3] Staicu et al. (2021) Lett Appl Microbiol https://doi.org/10.1111/lam.13578
[4] Staicu & Barton (2021) J Inorg Biochem 222:111509 https://doi.org/10.1016/j.jinorgbio.2021.111509
[5] Uhrynowski et al. (2019) Int J Mol Sci 20(5):1018 https://doi.org/10.3390/ijms20051018