Bacteriohopanetetrol- x : constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system

[摘要] Interpreting lipid biomarkers in the sediment archiverequires a good understanding of their application and limitations in modernsystems. Recently it was discovered that marine bacteria performinganaerobic ammonium oxidation (anammox), belonging to the genus Ca.  Scalindua,uniquely synthesize a stereoisomer of bacteriohopanetetrol (“BHT- x ”). Theratio of BHT- x over total bacteriohopanetetrol (BHT, ubiquitously synthesizedby diverse bacteria) has been suggested as a proxy for water column anoxia.As BHT has been found in sediments over 50 Myr old, BHT- x has the potentialto complement and extend the sedimentary biomarker record of marine anammox,conventionally constructed using ladderane lipids. Yet, little is knownabout the distribution of BHT- x in relation to the distribution of ladderanesand to the genetic evidence of Ca.  Scalindua in modern marine systems. Here, weinvestigate the distribution of BHT- x and the application of the BHT- x ratio inrelation to distributions of ladderane intact polar lipids (IPLs), ladderanefatty acids (FAs) and Ca.  Scalindua 16S rRNA genes in suspended particulatematter (SPM) from the water column of the Benguela upwelling system (BUS),sampled across a large oxygen gradient. In BUS SPM, high BHT- x abundances wererestricted to the oxygen-deficient zone on the continental shelf (at[O 2 ]   45  µ mol L −1 , in all but one case). HighBHT- x abundances co-occurred with high abundances of the Ca.  Scalindua 16S rRNAgene (relative to the total number of bacterial 16S rRNA genes) andladderane IPLs. At shelf stations with [O 2 ]  >  50  µ mol L −1 , the BHT- x ratio was  0.04 (in all but one case). Inapparent contradiction, ladderane FAs and low abundances of BHT and BHT- x (resulting in BHT- x ratios >  0.04) were also detected in oxygenatedoffshore waters ([O 2 ] up to 180  µ mol L −1 ), whereasladderane IPLs were undetected. The index of ladderane lipids with fivecyclobutane rings (NL 5 ) correlates with in situ temperature. NL 5 -derived temperatures suggested that ladderane FAs in the offshore waterswere not synthesized in situ but were transported down-slope from warmer shelfwaters. Thus, in sedimentary archives of systems with known lateral organicmatter transport, such as the BUS, relative BHT and BHT- x abundances should becarefully considered. In such systems, a higher BHT- x ratio may act as a saferthreshold for deoxygenation and/or Ca.  Scalindua presence: our results andprevious studies indicate that a BHT- x ratio of ≥  0.2 is a robustthreshold for oxygen-depleted waters ([O 2 ]   50  µ mol kg −1 ). In our data, ratios of ≥  0.2 coincided with Ca.  Scalindua 16SrRNA genes in all samples ( n =62 ), except one. Lastly, when investigatingin situ anammox, we highlight the importance of using ladderane IPLs over BHT- x and/orladderane FAs; these latter compounds are more recalcitrant and may derivefrom transported fossil anammox bacteria remnants.

[学科分类] 大气科学