Search

Dust is produced primarily in desert regions and transported long distances through the atmosphere to the oceans. Upon deposition of dust, its dissolution can provide an important source of a range of nutrients, particularly iron, to microbes living in open ocean surface waters. The dust supply is greatest nearest to deserts, hence in the Northern Hemisphere. The Southern Ocean region is farthest from these dust sources and shows clear evidence that phytoplankton primary production is limited, at least in part, by the rate of supply of iron. Iron is also essential for nitrogen fixation. In regions of high atmospheric iron supply, such as the tropical North Atlantic, stimulation of nitrogen fixation drives the phytoplankton population toward a state in which phosphorus supply rates limit primary production. Atmospheric deposition is also an important source of nitrogen to the low latitude ocean, where it stimulates primary production. In this review we consider the sources, transport, and deposition of atmospheric dust/iron and nitrogen to the oceans and their impacts on plankton systems. In conclusion, we suggest key areas for future research. [ABSTRACT FROM AUTHOR]

Species belonging to the filamentous cyanobacterial genus Trichodesmium are responsible for a significant fraction of oceanic nitrogen fixation. The availability of phosphorus (P) likely constrains the growth of Trichodesmium in certain regions of the ocean. Moreover, Trichodesmium species have recently been shown to play a role in an emerging oceanic phosphorus redox cycle, further highlighting the key role these microbes play in many biogeochemical processes in the contemporary ocean. Here, we show that Trichodesmium erythraeum IMS101 can grow on the reduced inorganic compound phosphite as its sole source of P. The components responsible for phosphite utilization are identified through heterologous expression of the T. erythraeum IMS101 Tery-0365- 0368 genes, encoding a putative adenosine triphosphate (ATP)-binding cassette transporter and nicotinamide adenine dinucleotide (NAD)-dependent dehydrogenase, in the model cyanobacteria Synechocystis sp. PCC6803. We demonstrate that only combined expression of both the transporter and the dehydrogenase enables Synechocystis to utilize phosphite, confirming the function of Tery-0365-0367 as a phosphite uptake system (PtxABC) and Tery-0368 as a phosphite dehydrogenase (PtxD). Our findings suggest that reported uptake of phosphite by Trichodesmium consortia in the field likely reflects an active biological process by Trichodesmium. These results highlight the diversity of phosphorus sources available to Trichodesmium in a resource-limited ocean. [ABSTRACT FROM AUTHOR]

Trichodesmium is a biogeochemically important marine cyanobacterium, responsible for a significant proportion of the annual ‘new’ nitrogen introduced into the global ocean. These non-heterocystous filamentous diazotrophs employ a potentially unique strategy of near-concurrent nitrogen fixation and oxygenic photosynthesis, potentially burdening Trichodesmium with a particularly high iron requirement due to the iron-binding proteins involved in these processes. Iron availability may therefore have a significant influence on the biogeography of Trichodesmium. Previous investigations of molecular responses to iron stress in this keystone marine microbe have largely been targeted. Here a holistic approach was taken using a label-free quantitative proteomics technique (MSE) to reveal a sophisticated multi-faceted proteomic response of Trichodesmium erythraeum IMS101 to iron stress. Increased abundances of proteins known to be involved in acclimation to iron stress and proteins known or predicted to be involved in iron uptake were observed, alongside decreases in the abundances of iron-binding proteins involved in photosynthesis and nitrogen fixation. Preferential loss of proteins with a high iron content contributed to overall reductions of 55–60% in estimated proteomic iron requirements. Changes in the abundances of iron-binding proteins also suggested the potential importance of alternate photosynthetic pathways as Trichodesmium reallocates the limiting resource under iron stress. Trichodesmium therefore displays a significant and integrated proteomic response to iron availability that likely contributes to the ecological success of this species in the ocean. [ABSTRACT FROM AUTHOR]

We present a unified conceptual framework describing the competition between diazotrophs and non-nitrogen-fixing marine plankton and their interaction with three essential nutrient elements: nitrogen (N), phosphorus (P), and iron (Fe). The theory explains the global biogeography of diazotrophs and the observed large-scale variations in surface ocean nutrient concentrations. The ratios in which N, P, and Fe are delivered to the surface ocean, relative to the demands of the phytoplankton community, define several biogeochemical provinces in terms of the limiting nutrients and the presence or absence of diazotrophs. Nutrient supply ratios provided by a global ecosystem model support the theoretical view that diazotroph biogeography is dominated by the Fe : N supply ratio, with the P : N supply ratio taking an important secondary role. The theory yields robust predictions for which strong empirical support is found in global observations of surface nutrient concentrations and diazotroph abundance. [ABSTRACT FROM AUTHOR]

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high ironflux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean. [ABSTRACT FROM AUTHOR]

Measurements of the intra-tidal and spring-neap variation in the vertical flux of nitrate into the base of the sub-surface chlorophyll maximum (SCM) were made at the shelf edge of the Celtic Sea, a region with strong internal mixing driven by an internal tide. The neap tide daily mean nitrate flux was 1.3 (0.9-1.8, 95% confidence interval) mmol m-2 d-1. The spring tide flux was initially estimated as 3.5 (2.3-5.2, 95% confidence interval) mmol m-2 d-1. The higher spring tide nitrate flux was the result of turbulent dissipation occurring within the base of the SCM as compared to deeper dissipation during neap tides and was dominated by short events associated with the passage of internal solitons. Taking into account the likely under-sampling of these short mixing events raised the spring tide nitrate flux estimate to about 9 mmol m-2 d-1. The neap tide nitrate flux was sufficient to support substantial new production and a considerable fraction of the observed rates of carbon fixation. Spring tide fluxes were potentially in excess of the capacity of the phytoplankton community to uptake nitrate. This potential excess nitrate flux during spring tides may be utilized to support new production during the lower mixing associated with the transition toward neap tide. The shelf edge is shown to be a region with a significantly different phytoplankton community as compared to the adjacent Celtic Sea and northeast Atlantic Ocean, highlighting the role of gradients in physical processes leading to gradients in ecosystem structure. [ABSTRACT FROM AUTHOR]