The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico

H. L. Buss, Maryann Victoria Bruns, M. J. Schultz, J. Moore, C. F. Mathur, Susan Louise Brantley

Research output: Contribution to journalArticle

55 Citations (Scopus)

Abstract

Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2-2 m thick) is overlain by saprolite (2-8 m) topped by soil (0.5-1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro-organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron-oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g-1 regolith, were higher than 108 g -1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl-extractable iron. Biochemical tests for aerobic iron-oxidizers were also positive at 0.15-0.6 m, at 2.1-2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite-bedrock interface is dominated by Fe(II) and where autotrophic iron-oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.

Original languageEnglish (US)
Pages (from-to)247-260
Number of pages14
JournalGeobiology
Volume3
Issue number4
DOIs
StatePublished - Oct 1 2005

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saprolite
Puerto Rico
weathering
mountains
heterotrophs
iron
minerals
bedrock
mountain
mineral
regolith
cells
DNA
substrate
bacterium
bacteria
quartz
diorite
oxidants
growth models

All Science Journal Classification (ASJC) codes

  • Ecology, Evolution, Behavior and Systematics
  • Environmental Science(all)
  • Earth and Planetary Sciences(all)

Cite this

@article{3bc523786b7d4e8b90414afd6afc3dcc,
title = "The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico",
abstract = "Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2-2 m thick) is overlain by saprolite (2-8 m) topped by soil (0.5-1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro-organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron-oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g-1 regolith, were higher than 108 g -1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004{\%} to 0.02{\%} of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl-extractable iron. Biochemical tests for aerobic iron-oxidizers were also positive at 0.15-0.6 m, at 2.1-2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite-bedrock interface is dominated by Fe(II) and where autotrophic iron-oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.",
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The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico. / Buss, H. L.; Bruns, Maryann Victoria; Schultz, M. J.; Moore, J.; Mathur, C. F.; Brantley, Susan Louise.

In: Geobiology, Vol. 3, No. 4, 01.10.2005, p. 247-260.

Research output: Contribution to journalArticle

TY - JOUR

T1 - The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico

AU - Buss, H. L.

AU - Bruns, Maryann Victoria

AU - Schultz, M. J.

AU - Moore, J.

AU - Mathur, C. F.

AU - Brantley, Susan Louise

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Y1 - 2005/10/1

N2 - Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2-2 m thick) is overlain by saprolite (2-8 m) topped by soil (0.5-1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro-organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron-oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g-1 regolith, were higher than 108 g -1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl-extractable iron. Biochemical tests for aerobic iron-oxidizers were also positive at 0.15-0.6 m, at 2.1-2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite-bedrock interface is dominated by Fe(II) and where autotrophic iron-oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.

AB - Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to saprolite. The rindlet zone (0.2-2 m thick) is overlain by saprolite (2-8 m) topped by soil (0.5-1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro-organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron-oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g-1 regolith, were higher than 108 g -1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl-extractable iron. Biochemical tests for aerobic iron-oxidizers were also positive at 0.15-0.6 m, at 2.1-2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the saprolite-bedrock interface is dominated by Fe(II) and where autotrophic iron-oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and saprolite formation.

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