498:
462:
450:
474:
510:
486:
40:
604:
618:
632:
31:
141:
of heterogeneity. Collecting a high number of samples can produce high spatial resolution, but at the cost of low temporal resolution since samples only reflect a singular a snapshot in time. In situ monitoring can provide high temporal resolution by collecting continuous real-time measurements, but low spatial resolution since the electrode is in a fixed location.
60:. The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. These redox gradients form both spatially and temporally as a result of differences in microbial processes, chemical composition of the environment, and oxidative potential. Common environments where redox gradients exist are
497:
546:
Redox gradients form along contaminant plumes, in both aquatic and terrestrial settings, as a function of the contaminant concentration and the impacts it has on relevant chemical processes and microbial communities. The highest rates of organic pollutant degradation along a redox gradient are found
272:
is also largely a function of hydrological conditions. In the event of a flood, saturated soils can shift from oxic to anoxic, creating a reducing environment as anaerobic microbial processes dominate. Moreover, small anoxic hotspots may develop within soil pore spaces, creating reducing conditions.
140:
can be measured by collecting samples in the field and performing analyses in the lab, or by inserting an electrode into the environment to collect in situ measurements. Typical environments to measure redox potential are in bodies of water, soils, and sediments, all of which can exhibit high levels
43:
Relative favorability of redox reactions in marine sediments based on energy. Start points of arrows indicate energy associated with half-cell reaction. Lengths of arrows indicate an estimate of Gibb's free energy (ΔG) for the reaction where a higher ΔG is more energetically favorable (Adapted from
256:
sediments exhibit redox gradients produced by variations in mineral composition, organic matter availability, structure, and sorption dynamics. Limited transport of dissolved electrons through subsurface sediments, combined with varying pore sizes of sediments creates significant heterogeneity in
156:
Redox gradients are commonly found in the environment as functions of both space and time, particularly in soils and aquatic environments. Gradients are caused by varying physiochemical properties including availability of oxygen, soil hydrology, chemical species present, and microbial processes.
543:, sulfate reduction, etc.) based on the conditions around them and further amplify redox gradients present in the environment. However, distribution of microorganisms cannot solely be determined from thermodynamics (redox ladder), but is also influenced by ecological and physiological factors.
79:
has a global redox gradient with an oxidizing environment at the surface and increasingly reducing conditions below the surface. Redox gradients are generally understood at the macro level, but characterization of redox reactions in heterogeneous environments at the micro-scale require further
461:
551:, where the water table meets soil and fills empty pores. Because this transition zone is both oxic and anoxic, electron acceptors and donors are in high abundance and there is a high level of microbial activity, leading to the highest rates of contaminant biodegradation.
132:
indicates a reducing environment (electrons will be donated). In a redox gradient, the most energetically favorable chemical reaction occurs at the “top” of the redox ladder and the least energetically favorable reaction occurs at the “bottom” of the ladder.
557:
sediments are heterogeneous in nature and subsequently exhibit redox gradients. Due to this heterogeneity, gradients of reducing and oxidizing chemical species do not always overlap enough to support electron transport needs of niche microbial communities.
473:
257:
benthic sediments. Oxygen availability in sediments determines which microbial respiration pathways can occur, resulting in a vertical stratification of redox processes as oxygen availability decreases with depth.
562:
have been characterized as sulfide-oxidizing bacteria that assist in connecting these areas of undersupplied and excess electrons to complete the electron transport for otherwise unavailable redox reactions.
34:
Depiction of common redox reactions in the environment. Adapted from figures by Zhang and Gorny. Redox pairs are listed with the oxidizer (electron acceptor) in red and the reducer (electron donator) in
509:
449:
176:
The following is a list of common reactions that occur in the environment in order from oxidizing to reducing (organisms performing the reaction in parentheses):
1058:
Peiffer, S.; Kappler, A.; Haderlein, S. B.; Schmidt, C.; Byrne, J. M.; Kleindienst, S.; Vogt, C.; Richnow, H. H.; Obst, M.; Angenent, L. T.; Bryce, C. (2021).
1218:"Redox potential (Eh) and pH as drivers of soil/plant/microorganism systems: a transdisciplinary overview pointing to integrative opportunities for agronomy"
210:
825:
Gorny, J.; Billon, G.; Lesven, L.; Dumoulin, D.; Madé, B.; Noiriel, C. (2015). "Arsenic behavior in river sediments under redox gradient: a review".
531:
Redox gradients form based on resource availability and physiochemical conditions (pH, salinity, temperature) and support stratified communities of
916:
Borch, Thomas; Kretzschmar, Ruben; Kappler, Andreas; Cappellen, Philippe Van; Ginder-Vogel, Matthew; Voegelin, Andreas; Campbell, Kate (2009).
280:
of a soil can be restored as water drains and the soil dries out. Soils with redox gradients formed by ascending groundwater are classified as
877:
1404:
419:< +700 mV. 300 mV is the boundary value that separates aerobic from anaerobic conditions in wetland soils. Redox potential (E
503:
Sediment cores like this one collected from estuaries, rivers, lakes, and bays often have redox gradients with depth down into the core.
467:
In productive ocean regions and enclosed basins, oxygen minimum zones and hypoxic zones may experience redox gradients in deep waters.
427:, and both have significant influence on the function of soil-plant-microorganism systems. The main source of electrons in soil is
1384:
695:
589:
1389:
1374:
582:
581:, and at the bottoms of aquatic environments, also exhibit redox gradients. The community of microbes—often metal- or
117:
485:
1379:
148:
imaging, however, further research is needed to fully understand contributions of redox species to polarization.
1280:"Biochemical processes in soil and groundwater contaminated by leachates from municipal landfills (Mini review)"
1369:
1364:
690:
536:
220:
1399:
665:
1424:
1409:
645:
194:
585:—produces redox gradients on the micrometer scale as a function of spatial physiochemical variability.
479:
In wetlands, organic-rich soils accumulate over time, and these soils often experience redox gradients.
763:
1332:
1129:
1071:
775:
518:
or gley soils like this one in the
Southern Black Forest in Germany often experience redox gradients.
145:
431:. Organic matter consumes oxygen as it decomposes, resulting in reducing soil conditions and lower E
246:) within the water column alter redox chemistry and which redox reactions can occur. Development of
242:
Redox gradients form in water columns and their sediments. Varying levels of oxygen (oxic, suboxic,
1060:"A biogeochemical–hydrological framework for the role of redox-active compounds in aquatic systems"
660:
428:
247:
243:
180:
57:
24:
1008:
Lau, Maximilian Peter; Niederdorfer, Robert; Sepulveda-Jauregui, Armando; Hupfer, Michael (2018).
144:
Redox properties can also be tracked with high spatial and temporal resolution through the use of
39:
1419:
1394:
1320:
1247:
1095:
953:
917:
850:
807:
609:
547:
at the oxic-anoxic interface. In groundwater, this oxic-anoxic environment is referred to as the
1301:
1239:
1163:
1145:
1087:
945:
937:
883:
873:
842:
799:
791:
655:
578:
184:
101:
1340:
1291:
1229:
1153:
1137:
1079:
1021:
929:
834:
783:
685:
623:
603:
548:
680:
670:
190:
109:
1336:
1133:
1075:
779:
1414:
1321:"The response of trace element redox couples to suboxic conditions in the water column"
1158:
1117:
559:
540:
226:
97:
1344:
1358:
1099:
957:
811:
637:
532:
1251:
1059:
854:
838:
787:
631:
554:
253:
1319:
Rue, Eden L.; Smith, Geoffrey J.; Cutter, Gregory A.; Bruland, Kenneth W. (1997).
157:
Specific environments that are commonly characterized by redox gradients include
128:
indicates an oxidizing environment (electrons will be accepted), and a negative E
1296:
1279:
1026:
1009:
911:
909:
907:
905:
903:
901:
899:
897:
230:
170:
158:
1141:
1083:
302:
generally ranges from −300 to +900 mV. The table below summarizes typical
1234:
1217:
650:
599:
30:
1305:
1243:
1149:
1091:
941:
887:
795:
285:
200:
1167:
949:
846:
803:
918:"Biogeochemical Redox Processes and their Impact on Contaminant Dynamics"
289:
162:
93:
284:, while soils with gradients formed by stagnant water are classified as
574:
570:
566:
515:
281:
216:
166:
933:
675:
76:
61:
53:
38:
29:
20:
764:"Soil redox dynamics under dynamic hydrologic regimes - A review"
88:
Redox conditions are measured according to the redox potential (E
206:
69:
65:
1116:
Zakem, Emily J.; Polz, Martin F.; Follows, Michael J. (2020).
424:
1010:"Synthesizing redox biogeochemistry at aquatic interfaces"
412:
limits that are tolerable by plants are +300 mV <
16:
Variation of the redox potential with distance (or depth)
80:
research and more sophisticated measurement techniques.
1325:
Deep Sea
Research Part I: Oceanographic Research Papers
1118:"Redox-informed models of global biogeochemical cycles"
250:also contributes to formation of redox gradients.
455:Wetland soils often experience redox gradients.
108:can be calculated using half reactions and the
592:for coverage of microbial processes in SMTZs.
440:Examples of redox gradients in the environment
92:) in volts, which represents the tendency for
928:(1). American Chemical Society (ACS): 15–23.
872:. Amsterdam Boston: Elsevier/Academic Press.
8:
116:of zero represents the redox couple of the
1295:
1233:
1157:
1025:
311:
19:For broader coverage of this topic, see
707:
445:
922:Environmental Science & Technology
870:Introduction to marine biogeochemistry
491:Some soils experience redox gradients.
1211:
1209:
1207:
1205:
1203:
1201:
1199:
1197:
1003:
1001:
999:
997:
995:
993:
991:
989:
987:
757:
755:
753:
751:
749:
747:
745:
743:
741:
739:
737:
735:
733:
731:
7:
1273:
1271:
1269:
1267:
1265:
1263:
1261:
1195:
1193:
1191:
1189:
1187:
1185:
1183:
1181:
1179:
1177:
1111:
1109:
1053:
1051:
1049:
1047:
1045:
1043:
1041:
1039:
1037:
985:
983:
981:
979:
977:
975:
973:
971:
969:
967:
827:The Science of the Total Environment
762:Zhang, Zengyu; Furman, Alex (2021).
729:
727:
725:
723:
721:
719:
717:
715:
713:
711:
309:values for various soil conditions:
52:is a series of reduction-oxidation (
14:
165:, contaminant plumes, and marine
768:Science of the Total Environment
630:
616:
602:
508:
496:
484:
472:
460:
448:
56:) reactions sorted according to
839:10.1016/j.scitotenv.2014.10.011
788:10.1016/j.scitotenv.2020.143026
696:Sulfate-methane transition zone
590:sulfate-methane transition zone
535:. Microbes carry out differing
203:reduction (Manganese reducers)
1:
1345:10.1016/S0967-0637(96)00088-X
344:Aerated – moderately reduced
219:reduction (sulfate reducers:
1278:Vodyanitskii, Yu N. (2016).
1405:Oceanographical terminology
1297:10.1016/j.aasci.2016.07.009
1027:10.1016/j.limno.2017.08.001
118:standard hydrogen electrode
1441:
1284:Annals of Agrarian Science
1142:10.1038/s41467-020-19454-w
1084:10.1038/s41561-021-00742-z
423:) is also closely tied to
209:reduction (iron reducers:
84:Measuring redox conditions
68:, contaminant plumes, and
18:
1235:10.1007/s11104-012-1429-7
583:sulfate-reducing bacteria
374:Aerated – highly reduced
1216:Husson, Olivier (2013).
691:Sediment-water interface
273:With time, the starting
261:Terrestrial environments
221:Sulfur-reducing bacteria
152:Environmental conditions
1385:Environmental chemistry
666:Hypoxia (environmental)
527:Role of microorganisms
211:iron-reducing bacteria
45:
36:
1390:Environmental science
1375:Chemical oceanography
1122:Nature Communications
868:Libes, Susan (2009).
646:Anaerobic respiration
195:denitrifying bacteria
42:
33:
248:oxygen minimum zones
238:Aquatic environments
146:induced-polarization
96:to transfer from an
1337:1997DSRI...44..113R
1134:2020NatCo..11.5680Z
1076:2021NatGe..14..264P
780:2021ScTEn.76343026Z
661:Dead zone (ecology)
405:Generally accepted
181:Aerobic respiration
25:Reduction potential
610:Environment portal
579:hydrothermal vents
359:Aerated – reduced
46:
37:
1064:Nature Geoscience
934:10.1021/es9026248
879:978-0-08-091664-4
656:Gibbs free energy
403:
402:
185:aerobic organisms
159:waterlogged soils
102:electron acceptor
1432:
1380:Electrochemistry
1349:
1348:
1316:
1310:
1309:
1299:
1275:
1256:
1255:
1237:
1228:(1–2): 389–417.
1213:
1172:
1171:
1161:
1113:
1104:
1103:
1055:
1032:
1031:
1029:
1005:
962:
961:
913:
892:
891:
865:
859:
858:
822:
816:
815:
759:
686:Remineralization
640:
635:
634:
626:
624:Chemistry portal
621:
620:
619:
612:
607:
606:
549:capillary fringe
512:
500:
488:
476:
464:
452:
315:Soil conditions
312:
1440:
1439:
1435:
1434:
1433:
1431:
1430:
1429:
1370:Biogeochemistry
1365:Aquatic ecology
1355:
1354:
1353:
1352:
1318:
1317:
1313:
1277:
1276:
1259:
1215:
1214:
1175:
1115:
1114:
1107:
1057:
1056:
1035:
1007:
1006:
965:
915:
914:
895:
880:
867:
866:
862:
824:
823:
819:
761:
760:
709:
704:
681:Redox potential
671:Marine sediment
636:
629:
622:
617:
615:
608:
601:
598:
529:
524:
523:
522:
519:
513:
504:
501:
492:
489:
480:
477:
468:
465:
456:
453:
442:
441:
434:
422:
418:
411:
398:
383:
368:
353:
338:
324:
308:
301:
279:
271:
263:
240:
193:(denitrifiers:
191:Denitrification
154:
139:
131:
127:
123:
115:
110:Nernst equation
107:
91:
86:
62:coastal marshes
58:redox potential
28:
17:
12:
11:
5:
1438:
1436:
1428:
1427:
1422:
1417:
1412:
1407:
1402:
1400:Marine geology
1397:
1392:
1387:
1382:
1377:
1372:
1367:
1357:
1356:
1351:
1350:
1331:(1): 113–134.
1311:
1290:(3): 249–256.
1257:
1222:Plant and Soil
1173:
1105:
1070:(5): 264–272.
1033:
963:
893:
878:
860:
817:
706:
705:
703:
700:
699:
698:
693:
688:
683:
678:
673:
668:
663:
658:
653:
648:
642:
641:
627:
613:
597:
594:
560:Cable bacteria
541:methanogenesis
528:
525:
521:
520:
514:
507:
505:
502:
495:
493:
490:
483:
481:
478:
471:
469:
466:
459:
457:
454:
447:
444:
443:
439:
438:
437:
432:
429:organic matter
420:
416:
409:
401:
400:
396:
390:
386:
385:
381:
375:
371:
370:
366:
360:
356:
355:
351:
345:
341:
340:
336:
331:
327:
326:
322:
316:
306:
299:
277:
269:
262:
259:
239:
236:
235:
234:
227:Methanogenesis
224:
214:
204:
198:
188:
153:
150:
137:
129:
125:
121:
113:
105:
98:electron donor
89:
85:
82:
50:redox gradient
15:
13:
10:
9:
6:
4:
3:
2:
1437:
1426:
1423:
1421:
1418:
1416:
1413:
1411:
1408:
1406:
1403:
1401:
1398:
1396:
1393:
1391:
1388:
1386:
1383:
1381:
1378:
1376:
1373:
1371:
1368:
1366:
1363:
1362:
1360:
1346:
1342:
1338:
1334:
1330:
1326:
1322:
1315:
1312:
1307:
1303:
1298:
1293:
1289:
1285:
1281:
1274:
1272:
1270:
1268:
1266:
1264:
1262:
1258:
1253:
1249:
1245:
1241:
1236:
1231:
1227:
1223:
1219:
1212:
1210:
1208:
1206:
1204:
1202:
1200:
1198:
1196:
1194:
1192:
1190:
1188:
1186:
1184:
1182:
1180:
1178:
1174:
1169:
1165:
1160:
1155:
1151:
1147:
1143:
1139:
1135:
1131:
1127:
1123:
1119:
1112:
1110:
1106:
1101:
1097:
1093:
1089:
1085:
1081:
1077:
1073:
1069:
1065:
1061:
1054:
1052:
1050:
1048:
1046:
1044:
1042:
1040:
1038:
1034:
1028:
1023:
1019:
1015:
1011:
1004:
1002:
1000:
998:
996:
994:
992:
990:
988:
986:
984:
982:
980:
978:
976:
974:
972:
970:
968:
964:
959:
955:
951:
947:
943:
939:
935:
931:
927:
923:
919:
912:
910:
908:
906:
904:
902:
900:
898:
894:
889:
885:
881:
875:
871:
864:
861:
856:
852:
848:
844:
840:
836:
832:
828:
821:
818:
813:
809:
805:
801:
797:
793:
789:
785:
781:
777:
773:
769:
765:
758:
756:
754:
752:
750:
748:
746:
744:
742:
740:
738:
736:
734:
732:
730:
728:
726:
724:
722:
720:
718:
716:
714:
712:
708:
701:
697:
694:
692:
689:
687:
684:
682:
679:
677:
674:
672:
669:
667:
664:
662:
659:
657:
654:
652:
649:
647:
644:
643:
639:
638:Oceans portal
633:
628:
625:
614:
611:
605:
600:
595:
593:
591:
586:
584:
580:
576:
572:
568:
564:
561:
556:
552:
550:
544:
542:
538:
534:
526:
517:
511:
506:
499:
494:
487:
482:
475:
470:
463:
458:
451:
446:
436:
430:
426:
415:
408:
395:
391:
388:
387:
380:
376:
373:
372:
365:
361:
358:
357:
350:
346:
343:
342:
335:
332:
329:
328:
321:
317:
314:
313:
310:
305:
298:
293:
291:
287:
283:
276:
268:
260:
258:
255:
251:
249:
245:
237:
232:
228:
225:
222:
218:
215:
212:
208:
205:
202:
199:
196:
192:
189:
186:
182:
179:
178:
177:
174:
172:
168:
164:
160:
151:
149:
147:
142:
134:
119:
111:
103:
99:
95:
83:
81:
78:
73:
71:
67:
63:
59:
55:
51:
44:Libes, 2011).
41:
32:
26:
22:
1425:Soil science
1410:Oceanography
1328:
1324:
1314:
1287:
1283:
1225:
1221:
1125:
1121:
1067:
1063:
1017:
1013:
925:
921:
869:
863:
830:
826:
820:
771:
767:
587:
565:
553:
545:
539:processes (
530:
413:
406:
404:
393:
378:
363:
348:
333:
330:Waterlogged
319:
303:
296:
294:
274:
266:
264:
252:
241:
175:
155:
143:
135:
124:a positive E
87:
74:
49:
47:
1128:(1): 5680.
1014:Limnologica
833:: 423–434.
571:tidal flats
569:, found in
537:respiration
389:Cultivated
325:range (mV)
231:methanogens
173:sediments.
171:hemipelagic
1359:Categories
774:: 143026.
702:References
651:Chemocline
399:< +500
392:+300 <
384:< −100
377:−300 <
369:< +100
362:−100 <
354:< +400
347:+100 <
339:< +250
286:stagnosols
183:(aerobes:
1420:Sediments
1395:Limnology
1306:1512-1887
1244:0032-079X
1150:2041-1723
1100:233876038
1092:1752-0894
1020:: 59–70.
958:206997593
942:0013-936X
888:643573176
812:226249448
796:0048-9697
290:planosols
201:Manganese
94:electrons
1252:17059599
1168:33173062
950:20000681
855:24877798
847:25461044
804:33143917
596:See also
575:glaciers
567:Biofilms
533:microbes
516:Gleysols
318:Typical
282:gleysols
163:wetlands
1333:Bibcode
1159:7656242
1130:Bibcode
1072:Bibcode
776:Bibcode
555:Benthic
254:Benthic
244:hypoxic
217:Sulfate
167:pelagic
112:. An E
1304:
1250:
1242:
1166:
1156:
1148:
1098:
1090:
956:
948:
940:
886:
876:
853:
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