No abstract

Several recent lines of literature point toward strong photoreactivity of phytoplanktonic detritus. We examined effects of irradiation of algal membrane fragments in various stages of decay, with emphasis on transfer of materials from solid to dissolved phase (photodissolution). After simulated solar irradiation for 24 h, up to several tens of percent of particulate organic matter converted to photodissolved organic matter (PDOM). Prior microbial decay enhanced PDOM production. PDOM had initially high C : N ratios, which decreased with irradiation time. Dissolved organic nitrogen dominated nitrogen photodissolution, followed by minor photoammonification and negligible nitrite plus nitrate production. Chromophoric particulate organic matter bleached at visible wavelengths and underwent dissolution, creation, and bleaching at ultraviolet (UV) wavelengths, resulting in net loss of color in particulates and net gain of largely UV-absorbing PDOM that also exhibited humic-type fluorescence. Solid phase proteinaceous material became less accessible to proteases after microbial decay but regained this accessibility upon irradiation. Irradiation under anoxic conditions roughly halved production of PDOM, including chromophores and humic fluorophores. Oxygen enhancement of these reactions, along with production of peroxides, implies a strong role for photosensitization. Pigments, unsaturated lipids, and tryptophan emerged as likely sources of reactive oxygen species. Lipid peroxides appeared as a reactive intermediate product. If these reactions in the ocean scale to pigment loss as found in our experiments, at least 5-15% of particulate organic matter could undergo photodissolution before settling in some planktonic environments. This photodissolution would enhance remineralization by photic zone microbial communities and thus upper ocean elemental recycling.

Carboxydotrophic activity in forest soils was enriched by incubation in a flowthrough system with elevated concentrations of headspace CO (40 to 400 ppm). CO uptake increased substantially over time, while the apparent K m ( app K m ) for uptake remained similar to that of unenriched soils (<10 to 20 ppm). Carboxydotrophic activity was transferred to and further enriched in sterile sand and forest soil. The app K m s for secondary and tertiary enrichments remained similar to values for unenriched soils. CO uptake by enriched soil and freshly collected forest soil was inhibited at headspace CO concentrations greater than about 1%. A novel isolate, COX1, obtained from the enrichments was inhibited similarly. However, in contrast to extant carboxydotrophs, COX1 consumed CO with an app K m of about 15 ppm, a value comparable to that of fresh soils. Phylogenetic analysis based on approximately 1,200 bp of its 16S rRNA gene sequence suggested that the isolate is an ␣-proteobacterium most closely related to the genera Pseudaminobacter, Aminobacter, and Chelatobacter (98.1 to 98.3% sequence identity).Carbon monoxide (CO) regulates concentrations of hydroxyl radical (the primary oxidizing agent in the troposphere [33,39,40]) and several greenhouse gases (e.g., methane and ozone [16,27]). Due to its direct and indirect effects on atmospheric chemistry, Daniel and Solomon (19) have suggested that short-term cumulative radiative forcing due to anthropogenic CO emission may be greater than that due to nitrous oxide (see also reference 25).Although chemical oxidation in the troposphere consumes 75 to 85% of annual CO emissions (7,8,16), biological oxidation contributes significantly to CO regulation (10, 36). In particular, soils consume 7 to 25% of net global annual emissions (10,36,40,45,50). A number of studies have addressed various aspects of soil CO consumption (e.g., 2-4, 6, 12-15, 20, 28, 45), but much remains to be learned about the microorganisms involved.Certain fungi (31), algae (9), actinomycetes and streptomycetes (4, 26), ammonium oxidizers (5, 32), and methanotrophs (5, 23, 30) oxidize CO. However, apparent half-saturation constants ( app K m ) for many of these organisms (e.g., 465 to 1,110 ppm [10,11,15]) substantially exceed values measured for soils (5 to 51 ppm [e.g., references 12 and 35]), with results for a thermophilic streptomycete (88 ppm) being exceptional (26). Accordingly, Conrad et al. (15) have concluded that known CO-oxidizing bacteria (carboxydotrophs) cannot account for observed soil CO consumption. In addition, King (35) has suggested that neither methanotrophs nor ammonia oxidizers are important CO oxidizers in a Maine forest soil.Since enrichments for carboxydotrophs typically contain headspace CO concentrations of Ն10%, the populations isolated from them may represent taxa that are not representative of those that dominate activity in situ. Thus, in this study forest soil was incubated with a flow of air containing CO at 40 to 400 ppm. Sterile sand and soil inoculated with previously e...

In the past decade, technological advances in optical sensors have facilitated an increased understanding of the relationship between optical characteristics and biogeochemistry of our oceans. In particular, long-pathlength liquid core waveguide cells (LCWs) are being used to "map" chromophoric dissolved organic matter (CDOM), as a biogeochemical tracer, in various coastal and open ocean regions. At present, two LCW cell types are used in the study of marine CDOM, and concerns about discrepancies in data collected with the different cell types and problems with baseline offsets have arisen. We conducted a direct comparison of absorption coefficient spectra of a dissolved spectrophotometric standard, molecular weight standard (MWS), and dilution series of natural seawater obtained in type I and type II LCWs to assess data agreement and potential colloidmediated biases. Although no statistical difference was observed for the dissolved standard, we found the type I to have a slight bias toward higher absorption coefficient values for MWS 14-150 kDa (within 95% confidence interval) and natural seawater. Seawater CDOM spectral slopes differed significantly between type I and type II LCWs, with a maximum difference in slope of 0.0006 nm -1 . Fastidious elimination of microbubbles from the capillary cells greatly reduced baseline offsets and markedly improved CDOM spectral slope precision.