Daniel J. Repeta

The relationship between δ13C of organic matter and [CO2(aq)] in ocean surface water: Data from a JGOFS site in the northeast Atlantic Ocean and a model

The delta 13C of suspended particulate organic matter (SPOM) in surface waters increased from -22.9 to -18.1% during April 25-May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47 degrees N, 20 degrees W). During the same period, nearly parallel increases in sinking POM delta 13C were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton delta 13C and [CO2 (aq)] are inversely related, the increases in both sinking and suspended POM delta 13C were highly negatively correlated with mixed-layer [CO2(aq)] that generally decreased from 13.2-10.1 micromoles/kg during the five weeks. The change in SPOM delta 13C per change in [CO2(aq)], however, appears to be somewhat greater than that expected from previous, though less direct, ocean and laboratory evidence. By adapting a model of plant delta 13C by FARQUHAR et al. (1982), it is shown that under a constant phytoplankton demand for CO2 an inverse, nonlinear SPOM delta 13C response to ambient [CO2(aq)] is expected. Such trends are unlike the negative linear relationships indicated by data from the NABE Site and or from Southern Hemisphere waters. Such differences between predicted and observed SPOM delta 13C vs. [CO2(aq)] trends and among observed relationships can be reconciled, however, if biological CO2 demand is allowed to vary. This has significant implications for the use of the delta 13C of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO2 concentrations and biological productivity.

Microbial community transcriptomes reveal microbes and metabolic pathways associated with dissolved organic matter turnover in the sea

Marine dissolved organic matter (DOM) contains as much carbon as the Earths atmosphere, and represents a critical component of the global carbon cycle. To better define microbial processes and activities associated with marine DOM cycling, we analyzed genomic and transcriptional responses of microbial communities to high-molecular-weight DOM (HMWDOM) addition. The cell density in the unamended control remained constant, with very few transcript categories exhibiting significant differences over time. In contrast, the DOM-amended microcosm doubled in cell numbers over 27 h, and a variety of HMWDOM-stimulated transcripts from different taxa were observed at all time points measured relative to the control. Transcripts significantly enriched in the HMWDOM treatment included those associated with two-component sensor systems, phosphate and nitrogen assimilation, chemotaxis, and motility. Transcripts from Idiomarina and Alteromonas spp., the most highly represented taxa at the early time points, included those encoding TonB-associated transporters, nitrogen assimilation genes, fatty acid catabolism genes, and TCA cycle enzymes. At the final time point, Methylophaga rRNA and non-rRNA transcripts dominated the HMWDOM-amended microcosm, and included gene transcripts associated with both assimilatory and dissimilatory single-carbon compound utilization. The data indicated specific resource partitioning of DOM by different bacterial species, which results in a temporal succession of taxa, metabolic pathways, and chemical transformations associated with HMWDOM turnover. These findings suggest that coordinated, cooperative activities of a variety of bacterial “specialists” may be critical in the cycling of marine DOM, emphasizing the importance of microbial community dynamics in the global carbon cycle.

Chemical characterization of high molecular weight dissolved organic matter in fresh and marine waters

Abstract The high molecular weight fraction of dissolved organic matter in a suite of lakes, rivers, seawater, and marine sediment interstitial water samples was collected by ultrafiltration and characterized by molecular level and spectroscopic techniques. Proton nuclear magnetic resonance spectra of all samples show a high degree of similarity, with major contributions from carbohydrates, bound acetate, and lipids. Molecular level analyses of neutral sugars show seven monosaccharides, rhamnose, fucose, arabinose, xylose, mannose, glucose, and galactose, to be abundant, and to occur in comparable relative amounts in each sample. Previous studies have emphasized the distinctive composition of dissolved humic substances in fresh and marine waters, and have attributed these differences to sources and transformations of organic matter unique to each environment. In contrast we find a large fraction of freshwater high molecular weight dissolved organic matter (HMWDOM; > 1kD) to be indistinguishable from marine HMWDOM in bulk and molecular-level chemical properties. Aquatic HMWDOM is similar in chemical composition to biologically derived acylated heteropolysaccharides isolated from marine algal cultures, suggesting a biological source for some fraction of persistent HMWDOM. High molecular weight DOC contributes 51 ± 26% of the total DOC, and monosaccharides 18 ± 8% of the total HMWDOC in our freshwater samples. These contributions are on average higher and more variable, but not significantly different than for surface seawater (30% and 16% respectively). Biogeochemical processes that produce, accumulate, and recycle DOM may therefore share important similarities and be broadly comparable across a range of environmental settings.

Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column

We usedthe nitrogen isotope ratio of algae, suspend edparticles andnitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in 15 Ni n the eastern basin (� 0.370.5%) than in the western basin (+2.471.4%), suggesting that nitrogen suppliedby biological N 2 fixation may be an important source of new nitrogen in the eastern basin, where preformednitrate from the Atlantic Ocean could have been depleted during its transit eastward. The d 15 N of nitrate in the deep Mediterranean (B3% in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5%), also suggesting an important role for N2 fixation. The eastwardd ecrease in the d 15 No f surface PON is greater than the eastwarddecrease in the d 15 N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the d 15 N of phytoplankton biomass from the d 15 N of the nitrate upwelledfrom below. Variations in prod uctivity associatedwith frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the d 15 N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) andnitrate concentration in the Alboran Sea. 15 N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.470.7%. The d 15 N in sinking particles caught at 100 m depth (4.9–5.6%) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Dd 15 N[PON-chlorin]=+6.471.4%) in the surface, similar to the offset reportedearlier in cultures for cellular N andchl- a. This indicates that d 15 N of phytoplankton biomass was retainedin surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity. r 2002 Elsevier Science Ltd. All rights reserved.

The organic geochemistry of Peru margin surface sediments: I. A comparison of the C37 alkenone and historical El Niño records

Abstract The alkenone-U k 37 “paleothermometer” has potential as a sedimentary marker for El Nino/ Southern Oscillation (ENSO) events in the Peru upwelling regime. We assessed this potential by comparing the historical ENSO record with detailed U k 37 profiles for 210 Pb-dated cores from the Peru margin oxygen minimum zone (OMZ). Sediments with the greatest potential for holding a sedimentary U k 37 record of El Nino events are sediments from the center of the OMZ sectioned at intervals ≤ the yearly sedimentation rate. The U k 37 signals of individual El Nino events were substantially attenuated in the sediments we examined, and periods of frequent ENSO activity (e.g., 1870–1891) were more readily identified than isolated ENSO events in periods of less frequent ENSO activity. Detailed depth profiles of the C 37 alkenones in a core from ≈253 m (O 2 k 37 values for the five 2 mm sections from 0–1 cm suggests that the U k 37 may be unaffected by the alkenone loss. Correlation between the C 37 alkenone concentration profiles in two cores from ≈15°S collected nine years apart are consistent with the use of these compounds for “molecular stratigraphy.”

A high resolution historical record of Holocene anoxygenic primary production in the Black Sea

I reconstruct the historical record of anoxygenic primary production in the Black Sea over the last 8200 years by an analysis of Chlorobium carotenoids in sediments. Anoxygenic photosynthesis in the Black Sea occurs only when hydrogen sulfide-containing deep water penetrates the euphotic zone. The data show that the Black Sea has experienced large fluctuations in the extent of anoxygenic primary production and therefore the depth of the H2S chemocline throughout much of the Holocene. Changes in the depth of the H2S chemocline also shift the dominant phytoplankton populations in the water column to noncarbonate-bearing species and result in the formation of intense dark bands in sediments.

Nitrogen and carbon isotopic ratios of chlorophyll from marine phytoplankton

The relationship between the nitrogen and carbon isotopic ratios of chlorophyll a and total biomass was explored in cultured marine phytoplankton to assess the utility of chlorophyll as an isotopic proxy for photoautotrophs. A near constant nitrogen isotopic depletion of 5.06 6 1.13‰ (95% confidence interval) in chlorophyll a relative to total nitrogen was observed in 8 species. This value was similar to isotopic differences between chlorophyll a and marine particles (5.27 6 1.48‰ (1 s); n 5 6) and sediments (5.39 6 0.67‰ (1s); n 5 4) in a variety of settings. These findings suggest that a 5.1‰ isotopic depletion of chlorophyll a relative to total algal nitrogen is a robust relationship that justifies the use of chlorophyll as a nitrogen isotopic surrogate for photoautotrophs. Although interspecies differences in Dd 15 Ncell-Chla exist, and growth rate has a small effect on this parameter, the field data suggest these factors are probably minimized in the ocean where multiple species and growth rates occur. The nitrogen isotopic depletion of chlorophyll a probably occurs during the transamination of glutamic acid in d-aminolevulinic acid biosynthesis, the first committed precursor to chlorophyll. The carbon isotopic composition of chlorophyll from 12 batch cultures and 7 species of marine phyto- plankton was elevated by 0.32 6 1.61‰ (95% confidence interval) relative to total cellular carbon. No significant interspecies variance was observed that was not attributable to intraspecies variance. There was a moderate inverse correlation between growth rate and Dd 13 Ccell-Chla, and it is hypothesized that this parameter is largely responsible for the large range of intraspecies Dd 13 Ccell-Chla values observed in batch cultures. Copyright

Dissolved Organic Nitrogen Hydrolysis Rates in Axenic Cultures of Aureococcus anophagefferens (Pelagophyceae): Comparison with Heterotrophic Bacteria

ABSTRACT The marine autotroph Aureococcus anophagefferens (Pelagophyceae) was rendered axenic in order to investigate hydrolysis rates of peptides, chitobiose, acetamide, and urea as indicators of the ability to support growth on dissolved organic nitrogen. Specific rates of hydrolysis varied between 8 and 700% of rates observed in associated heterotrophic marine bacteria.

Carotenoid diagenesis in recent marine sediments. II. Degradation of fucoxanthin to loliolide

Abstract The quantitative distributions of loliolide and the major phytoplankton carotenoids: fucoxanthin, diadinochrome, diatoxanthin, and β-carotene in two cores of anoxic marine sediment recovered from the Peru continental shelf are reported. Our results demonstrate that the rapid degradation of carotenoids in sediments is not a result of their high degree of unsaturation as has been previously suggested. Instead, carotenoids exhibit a wide range of degradation rates that are proportional to the ability of specific pigments to form unstable bicyclic furanoxides. Carotenoid furanoxides undergo subsequent fragmentation to loliolide, isololiolide, dihydroactinidiolide and other, as yet undetermined, low molecular weight products. This degradation pathway accounts for the relative rates of removal for specific carotenoids (fucoxanthin = fucoxanthinol > diadinoxanthin > diatoxanthin = carotene), the distribution of carotenoids reported by Wpatts and Maxwell (1977) and C ardoso et al. (1978) in ancient sediments, the occurrence of novel carotenoid transformation products in surface sediments reported by r pidout et al. (1984), and the distribution of loliolides in recent sediments recovered from the Namibian shelf reported by k plok et al. (1984a,b). We predict that loliolide and isololiolide will inherit a specific stereochemistry from their carotenoid precursors, but that dihydroactinidiolide will be racemic. For every μmole of fucoxanthin degraded in Peru sediments, 0.7–1.1 μmole of loliolide is produced. Summation of fucoxanthin and loliolide at each subsurface horizon yields an estimate of the total deposition of fucoxanthin at t = 0. Throughout the 0–20 cm depth of our samples, this parameter is remarkably constant to ±16%. Individual horizons exhibit excursions which may reflect changes in surface productivity. Extrapolation of our measurements to deeper sediments may therefore be of some value in deciphering questions on environmental conditions of deposition and paleoproductivity.

Rapid bacterial degradation of polysaccharides in anoxic marine systems

Extracellular hydrolysis of organic macromolecules is often assumed to be the slow step in remineralization of organic matter. We tested this assumption by comparing the degradation of four polysaccharides (pullulan, laminarin, and two polysaccharides isolated from the marine cyanobacterium Synechococcus WH7335) to determine whether size, linkage position, or anomeric linkage affected rates or mechanisms of carbohydrate degradation by mixed cultures of anaerobic bacteria enriched from marine sediments. Gel permeation chromatography and nuclear magnetic resonance spectroscopy (NMR) were used to follow the extracellular conversion of high molecular weight polysaccharides to lower molecular weight polysaccharides and oligosaccharides which were subsequently remineralized. In all cases, substrate degradation was rapid. NMR spectra showed that preferential hydrolysis occurred at specific chemical linkages, and extracellular enzymatic hydrolysis of polysaccharides occurred far more rapidly than bacterial uptake and remineralization of the lower molecular weight oligosaccharides produced through enzymatic hydrolysis. Substrate size was not a significant determinant of remineralization rate: high molecular weight does not always correlate with slow degradation rate. The hypothesis that extracellular enzymatic hydrolysis is a slow step in the degradation of macromolecular organic matter in marine systems needs to be critically re-examined.