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Microbial respiration and extracellular enzyme activity in sediments from the Gulf of Mexico hypoxic zone
Hill, B., C. Elonen, L. Anderson, AND J. Lehrter. Microbial respiration and extracellular enzyme activity in sediments from the Gulf of Mexico hypoxic zone. AQUATIC MICROBIAL ECOLOGY. Inter-Research, Luhe, Germany, 72:105-116, (2014).
Summertime hypoxic conditions in the northern Gulf of Mexico (GOM) occur as a result of vertical gradients in salinity, driven by freshwater inputs from the Mississippi River that causes stratification of coastal waters and limits mixing of the more aerated surface layer with the less aerated (hypoxic) lower layers. Organic matter derived primarily from phytoplankton production enhanced by Mississippi River nutrients, fuels respiration beneath the pycnocline leading to hypoxic or anoxic waters and sediments. In shallow, continental shelf regions of the ocean, such as the northern GOM, 80-90% of the water column particulate organic matter sinks to the sea bed before being decomposed. Once deposited on the sea bed, organic matter is processed by both aerobic and anaerobic processes, with the relative balance between oxic and anoxic respiration depending on the amount of organic matter loading and the influx of oxygenated waters. This study demonstrates the utility of microbial enzymes related to respiration (DHA) and nutrient acquisition (EEA) for assessing ecosystem functions in benthic marine ecosystems. The proposed structural equation model quantified the linkages between sediment chemistry, EEA, and DHA. The spatial pattern of sediment microbial activity in the northern GOM is consistent with physico-chemical differences reported for the different zones of similarity, and provides some insight into relative C and N-limitation of microbial activity. The change in microbial DHA and EEA with increasing sediment depth supports the concepts of nutrient recycling within the benthic environment, and the tight coupling of DHA and EEA with sediment chemistry suggests that microbial enzymes might provide insight into the importance of legacy effects on recovery from hypoxic conditions.
This study explores the relationship between sediment chemistry (TC, TN, TP) and microbial respiration (DHA) and extracellular enzyme activity (EEA) across the Gulf of Mexico (GOM) hypoxic zone. TC, TN, and TP were all positively correlated with each other (r=0.19-0.68). DHA was positively correlated with -glucosidase (BG, r=0.33), [-N-acetylglucosaminidase +L-leucine amino peptidase] ([NACE+LAP, 0.66), acid phosphatase (AP, 0.18), and aryl sulfatase (SULF, 0.52). These EEA were all positively correlated each other (0.30-0.79). DHA (0.23-0.34) and [NACE+LAP] (0.38-0.57) were positively correlated with TC, TN, and TP, while BG (0.61-0.67), AP (0.55-0.63), and SULF (0.43-0.55) were correlated only with TC and TN. While DHA and EEA were positively correlated with TC and TN, they were negatively correlated with TC:TN, reflecting the steeper gradient in TC across the GOM zones of similarity compared to that of TN. Structural equation modeling (SEM) revealed a significant causal relationship between sediment chemistry (especially TN), EEA, and DHA, explaining 87% of the variance in DHA. Sediment C, N, and P did not vary with increasing depth, yet there were significant decreases in DHA and EEA. DHA and EEA also reflected the differences in C and N supply associated with the four zones of similarity. EEA represents the interface between microbial demands for, and environmental supplies of, nutrients, effectively linking the ecological stoichiometric theory (EST) with the concept of threshold elemental ratios (TER). As such, the relative activities of the functional classes of extracellular enzymes are both a measure of nutrient availability and ecosystem metabolism that may be used to assess large-scale phenomena such as regional impacts of anthropogenic disturbances. The present study demonstrates the utility of microbial enzymes related to respiration (DHA) and nutrient acquisition (EEA) for assessing ecosystem functions in benthic marine ecosystems.