Sergio A. Sañudo-Wilhelmy

Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters, stimulating productivity of phytoplankton and so influencing the global carbon cycle. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium, which supplies more than half of the new nitrogen used for primary production. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability—for example, in the subtropical North Pacific Ocean—our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.

Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N2 fixation far from the mouth and provides important pathways for sequestration of atmospheric CO2 in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr−1 of the new production supported by NO3 delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2.5 Tmol of C to the atmosphere [Mikaloff-Fletcher SE, et al. (2007) Inverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport. Global Biogeochem Cycles 21, doi:10.1029/2006GB002751]. The enhancement of N2 fixation and consequent C sequestration by tropical rivers appears to be a global phenomenon that is likely to be influenced by anthropogenic activity and climate change.

Dissolved trace element cycles in the San Francisco Bay estuary

Dissolved trace element (copper, nickel, cadmium, zinc, cobalt, and iron) concentrations were measured in surface water samples collected from 27 stations in the San Francisco Bay and Sacramento—San Joaquin Delta during April, August and December of 1989. The trace element distributions were relatively similar for all three sampling periods, and evidenced two distinct biogeochemical regimes within the estuarine system. The two regimes were comprised of relatively typical trace element gradients in the northern reach and anthropogenically perturbed gradients in the southern reach of the estuary. These dichotomous trace element distributions were consistent with previous reports on the distributions of nutrients and some other constituents within the estuary. In the northern reach, trace element and dissolved phosphate concentrations were non-conservative. Simple estuarine mixing models indicated substantial internal sources of dissolved copper (46–150%), nickel (250–500%) and cadmium (630–780%) relative to riverine inputs in April and August, and sizable internal sinks for dissolved cobalt (> 99%) and iron (> 70%) during the same periods. Dissolved zinc fluxes varied temporally, with a relatively large (135%) internal source in April and a relatively small (29%) internal sink in August. Concentrations of many trace elements (copper, nickel, cadmium, zinc, and cobalt) in the southern reach were anomalously high relative to concentrations at comparable salinities in the northern reach. Mass balance calculations indicated that those excesses were primarily due to anthropogenic inputs (waste-water discharges and urban runoff) and diagenetic remobilization from benthic sediments. The magnitude of these excesses was amplified by the long hydraulic residence time of dissolved constituents within the South Bay. The influence of other factors was evident throughout the system. Notably, upwelling appeared to elevate substantially dissolved cadmium concentrations at the mouth of the estuary and authigenic flocculation appeared to dominate the cycling of dissolved iron in both the northern and southern reaches of the system. Biological scavenging, geochemical scavenging and diagenic remobilization were also found to be important in different parts of the estuary. Additional complementary information is required to quantify accurately these processes.

The impact of surface-adsorbed phosphorus on phytoplankton Redfield stoichiometry

The Redfield ratio of 106 carbon:16 nitrogen:1 phosphorus in marine phytoplankton is one of the foundations of ocean biogeochemistry, with applications in algal physiology, palaeoclimatology and global climate change. However, this ratio varies substantially in response to changes in algal nutrient status and taxonomic affiliation. Here we report that Redfield ratios are also strongly affected by partitioning into surface-adsorbed and intracellular phosphorus pools. The C:N:surface-adsorbed P (80–105 C:15–18 N:1 P) and total (71–80 C:13–14 N:1 P) ratios in natural populations and cultures of Trichodesmium were close to Redfield values and not significantly different from each other. In contrast, intracellular ratios consistently exceeded the Redfield ratio (316–434 C:59–83 N:1 intracellular P). These high intracellular ratios were associated with reduced N2 fixation rates, suggestive of phosphorus deficiency. Other algal species also have substantial surface-adsorbed phosphorus pools, suggesting that our Trichodesmium results are generally applicable to all phytoplankton. Measurements of the distinct phytoplankton phosphorus pools may be required to assess nutrient limitation accurately from elemental composition. Deviations from Redfield stoichiometry may be attributable to surface adsorption of phosphorus rather than to biological processes, and this scavenging could affect the interpretation of marine nutrient inventories and ecosystem models.

A trace metal clean reagent to remove surface-bound iron from marine phytoplankton

Many recent studies have investigated Fe biogeochemical cycling, chemical speciation, and limitation of phytoplankton growth in the ocean. In current models of marine iron biogeochemistry, however, two critical parameters remain uncertain. These are the partitioning of particulate iron into scavenged and interior pools, and the iron quotas (Fe/C ratios) of natural plankton communities. These values have not been measured in natural samples, because the only reagent that is available to remove surface adsorbed Fe from cells and other particles (Ti(III) citrate/EDTA [Limnol Oceanogr 34 (1989) 1113]) contains substantial levels of contaminating Fe, and is therefore useful only for Fe radiotracer experiments. We developed a new reagent that differentiates between intra- and surface adsorbed Fe pools in marine phytoplankton as effectively as the Ti wash, but that is also trace metal clean, chemically stable, and harmless to cells. This reagent uses oxalate as a reductant to remove surface adsorbed Fe from phytoplankton cells and other particles. A simple cleaning protocol reduces Fe concentrations in the oxalate solution to levels suitable for trace metal clean field measurements. The oxalate reagent was used to measure scavenged and interior Fe pools in suspended particles collected at four stations in the Southern Ocean. Sixteen percent to eighty-six percent of the total Fe associated with these samples was found to be surface-adsorbed. The oxalate reagent provides a new tool to accurately measure the physical partitioning of Fe in marine particles and could be used along with appropriate corrections for lithogenic Fe to estimate the intracellular Fe quotas of natural plankton communities.

A REVISED ESTIMATE OF THE IRON USE EFFICIENCY OF NITROGEN FIXATION, WITH SPECIAL REFERENCE TO THE MARINE CYANOBACTERIUM TRICHODESMIUM SPP. (CYANOPHYTA)1

Estimates of the iron use efficiency (IUE) for diazotrophic plant growth have been used to suggest iron limitation of marine N2 fixation. However, in the course of these inferences, neither the physiological complexity of these estimates nor the specific physiological parameters of marine diazotrophs were evaluated. Here, a semiempirical prediction of the IUE of diazotrophic growth for Trichodesmium was computed from considerations of the Fe content and reaction rates of the nitrogenase complex and PSI:PSII ratios, as well as field measurements of Mehler activity, cellular Fe‐superoxide dismutase activity, and diel variability in C and N2 fixation. With a PSI:PSII ratio of 1 and 48% Mehler activity, the instantaneous IUE (0.33 mol C fixed·mol cellular Fe− 1 ·s− 1 ) was only 4‐fold lower than that calculated for a phytoplankter growing on reduced N. We computed a range of daily integrated IUE values from 2900 to 7700 mol C·mol Fe− 1 ·d− 1 , accounting for the diel variability in C and N2 fixation as well as the uncertainties in cyanobacterial nitrogenase biochemistry and PSI:II ratios of field‐collected Trichodesmium. The lowest observed Fe‐superoxide dismutase:C quota of 2.9 (μmol:mol) suggests a maintenance requirement for this enzyme. The maintenance Fe:C requirement of 13.5 μmol:mol (derived from cultures of Trichodesmium IMS 101) and values of the IUE yielded an Fe requirement ranging from 27 to 48 Fe:C (μmol:mol) to achieve a diazotrophic growth rate of 0.1 d− 1 . Based on these predicted requirements, the Fe:C contents of Caribbean Sea and most North Atlantic Ocean populations sampled thus far exceed that required to support the observed rates of N2 fixation.

Iron and marine nitrogen fixation: progress and future directions

A synthesis of the current understanding of potential iron limitation of pelagic nitrogen fixation is given, considering biochemical bases of Fe requirements and empirical observations of growth and Fe quotas of cultures and field populations of Trichodesmium. The potential for iron limitation of heterotrophic diazotrophy in the marine environment is also evaluated.

Trace metal distributions off the Antarctic Peninsula in the Weddell Sea

Dissolved trace metals (Ag, Al, Cd, Co, Cu, Fe, Ni, Pb, and Zn), inorganic nutrients (PO4, H4SiO4), and chlorophyll a were measured at 19 stations along a surface water transect from the Antarctic Peninsula into the Weddell Sea. The range of concentrations of metals and nutrients measured along the western rim of the Weddell Sea were consistent with previous results from the Southern Ocean. Metal levels measured in the Weddell Sea showed two distinct patterns: (1) metals (Al, Co, and Pb) that generally had lower levels in the Weddell Sea; and (2) dissolved constituents (Ag, Cd, Cu, Fe, Ni, Zn, PO4, H4SiO4) that showed an enrichment in the Weddell Sea, as compared to other ocean basins. While this dichotomy suggested that high metal concentrations may result from natural processes, the impact of anthropogenic processes on metal levels in Antarctic waters is also evident. A comparison of the stable lead isotopic composition reported for surface waters of the Weddell Sea and our Southern Hemisphere aerosol samples suggested that the cycling of Pb in those waters has been influenced by industrial Pb from South America. The importance of biological activity on the cycling of bioactive metals in the Weddell Sea was suggested by the inverse relationship between chl a concentrations and trace metal residence time. A strong linear relationship between Cd and PO4 was observed, as in other oceans. The Cd/PO4 ratio along the western rim of the Weddell Sea was consistent with previous ratios reported for the northern part of the Weddell Sea and the Antarctic Circumpolar Current. However, those ratios were significantly higher than the previously reported Cd/PO4 ratio for the southern part of the Weddell Sea, suggesting that Cd/PO4 ratios within the same oceanographic basin are susceptible to temporal and spatial variability. We also showed that the Ag/Cu ratio could potentially be used as a geochemical tracer of different water masses in the world ocean. The Ag/Cu ratio in the Weddell Sea was essentially the same as the ratio reported for the Pacific Ocean, suggesting that Weddell Sea surface waters may influence the composition of trace metals in subsurface waters of the Pacific.

Multiple B-vitamin depletion in large areas of the coastal ocean

B vitamins are some of the most commonly required biochemical cofactors in living systems. Therefore, cellular metabolism of marine vitamin-requiring (auxotrophic) phytoplankton and bacteria would likely be significantly compromised if B vitamins (thiamin B1, riboflavin B2, pyridoxine B6, biotin B7, and cobalamin B12) were unavailable. However, the factors controlling the synthesis, ambient concentrations, and uptake of these key organic compounds in the marine environment are still not well understood. Here, we report vertical distributions of five B vitamins (and the amino acid methionine) measured simultaneously along a latitudinal gradient through the contrasting oceanographic regimes of the southern California-Baja California coast in the Northeast Pacific margin. Although vitamin concentrations ranged from below the detection limits of our technique to 30 pM for B2 and B12 and to ∼500 pM for B1, B6, and B7, each vitamin showed a different geographical and depth distribution. Vitamin concentrations were independent of each other and of inorganic nutrient levels, enriched primarily in the upper mesopelagic zone (depth of 100–300 m), and associated with water mass origin. Moreover, vitamin levels were below our detection limits (ranging from ≤0.18 pM for B12 to ≤0.81 pM for B1) in extensive areas (100s of kilometers) of the coastal ocean, and thus may exert important constraints on the taxonomic composition of phytoplankton communities, and potentially also on rates of primary production and carbon sequestration.

Temporal variations in lead concentrations and isotopic composition in the Southern California Bight

Lead concentrations in surface waters of the Southern California Bight appear to have decreased threefold (from >170 to <60 pM) since they were initially measured by Clair Patterson and his associates in the 1970s. The decrease parallels a threefold decline in anthropogenic inputs of industrial lead to the bight over the past two decades. Moreover, mass balance calculations indicate that the primary source of lead to the bight now is upwelling. This is evidenced by the isotopic compositions of surface waters in the bight, which are most characteristic of Asian industrial lead aerosols (0.4793 [le] [sup 206]Pb/[sup 208]Pb [le] 0.4833) deposited in oceanic waters of the North Pacific. While the decrease in surface water lead concentrations in the bight reflects the reduction in industrial lead emissions from the United States, the isotopic compositions of surface waters in the southern reach of the bight reflect a concurrent increase in industrial lead emissions from Mexico (0.4852 [le] [sup 206]Pb/[sup 208]Pb [le] 0.4877). The isotopic composition ([sup 208]Pb/[sup 207]Pb [approximately] 2.427) of elevated lead concentrations of surface waters in San Diego Bay indicate that lead is being remobilized from contaminated sediments within that bay.