HR: 12:05h
AN: H32C-08    [Abstracts]
TI: Kinetic Modeling of Arsenic Cycling by a Freshwater Cyanobacterium as Influenced by N:P Ratios: A Potential Biologic Control in an Iron-Limited Drainage Basin
AU: * Markley, C T
EM: cmarkley@geo.tamu.edu
AF: Texas A&M University, Halbouty Bldg MS 3115, College Station, TX 77843 United States
AU: Herbert, B E
EM: herbert@geo.tamu.edu
AF: Texas A&M University, Halbouty Bldg MS 3115, College Station, TX 77843 United States
AB: Elevated As levels are common in South Texas surface waters, where As is derived from the natural weathering of geogenic sources and a byproduct of historical uranium mining. The impacted surface waters of the Nueces River drainage basin supply Lake Corpus Christi (LCC), a major drinking water reservoir for the Corpus Christi area. The soils and sediments of the Nueces River drainage basin generally have low levels of reactive iron (average concentration of 2780 mg/kg), limiting the control of iron oxyhydroxides on As geochemistry and bioavailability. Given these conditions, biologic cycling of As may have a large influence on As fate and transport in LCC. Sediment cores from LCC show evidence for cyanobacterial blooms after reservoir formation based upon stable isotopes, total organic matter and specific elemental correlations. While algae have been shown to accumulate and reduce inorganic As(V), few studies have reported biologic cycling of As by cyanobacteria. Therefore, As(V) uptake, accumulation, reduction, and excretion in a 1.0 $\mu$M As(V) solution by the freshwater cyanobacterium, {\it Anabaena sp.} Strain PCC 7120, was measured over time as a function of low, middle and high N:P ratios (1.2, 12, 120) to determine nutrient effects on As cycling by the cyanobacterium. Total As(V) reduction was observed in all three conditions upon completion of the ten-day experiment. Maximum As(V) reduction rates ranged from (0.013 mmol g C$^{-1}$ day$^{-1}$) in the low N:P solution to (0.398 mmol g C$^{-1}$ day$^{-1}$) in the high N:P solution. Increased cell biomass in the low N:P ratio solution compensated for the low maximum reduction rate to allow total As(V) reduction. Kinetic equations commonly used to model algal-nutrient interactions were utilized in modeling the current data. The Michaelis-Menten enzyme saturation equation modified with a competitive inhibition term adequately modeled As(III) excretion in the high and middle N:P ratio test conditions. The low N:P test condition further required a growth term to adequately model As(III) excretion by the cyanobacterium. The impact of N:P ratios on As reduction rates implies that N:P cycling can be coupled to As biogeochemistry in surface waters through the action of phytoplankton.
DE: 1871 Surface water quality
DE: 1065 Trace elements (3670)
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting