The iron limitation mosaic in the California Current System: Factors governing Fe availability in the shelf/near-shelf region

The California Current System is a productive eastern boundary region off the coasts of Washington, Oregon, and California. There is strong seasonality to the region, with high levels of rainfall and river input to the coastal ocean during the winter season, and coastal and Ekman upwelling during the spring and summer. Iron (Fe) input to the coastal ocean during the winter months can be stored in the continental shelf mud belts and then be delivered to the surface ocean by upwelling in the spring and summer. There have been a number of studies providing strong evidence of Fe-limitation of diatom growth occurring in regions of the California Current System off of California, and the occurrence of Fe-limitation has been linked with narrow continental shelf mud belt width and low river input. We provide evidence for potential Fe-limitation of diatoms off the southern coast of Oregon in July 2014, just off the shelf break near Cape Blanco in a region with moderate shelf width and river input. Since eastern boundary regions account for a disproportionally large amount of global primary production, this observation of potential Fe-limitation in an unexpected near-shore region of the California Current System has implications for global models of primary productivity. In order to re-evaluate the factors impacting Fe availability, we utilize satellite imagery to compare with historical datasets, and show that unexpected levels of Fe can often be explained by eddies, plumes of upwelled water moving offshore, or lack of recent upwelling

Impaired viral infection and reduced mortality of diatoms in iron-limited oceanic regions

Diatom primary productivity is tightly coupled with carbon export through the ballasted nature of the silica-based cell wall, linking the oceanic silicon and carbon cycles. However, despite low productivity, iron (Fe)-limited regimes are considered ‘hot spots’ of diatom silica burial with enhanced carbon export efficiency, raising questions about the mechanisms driving the biogeochemistry of these regions. Marine viruses are classically recognized as catalysts of remineralization through host lysis, short-circuiting the trophic transfer of carbon and facilitating the retention of dissolved organic matter and associated elements in the surface ocean. Here we used metatranscriptomic analysis of diatoms and associated viruses, along with a suite of physiological and geochemical metrics, to study the interaction between diatoms and viruses in Fe-limited regimes of the northeast Pacific. We found low cell-associated diatom virus diversity and abundance in a chronically Fe-limited region of the subarctic northeast Pacific. In a coastal upwelling region of the California Current, transient iron limitation also substantially reduced viral replication. These observations were recapitulated in Fe-limited cultures of the bloom-forming, centric diatom, Chaetoceros tenuissimus, which exhibited delayed virus-mediated mortality in addition to reduced viral replication. We suggest Fe-limited diatoms escape viral lysis and subsequent remineralization in the surface ocean, providing an additional mechanism contributing to enhanced carbon export efficiency and silica burial in Fe-limited oceanic regimes

Divergent gene expression among phytoplankton taxa in response to upwelling

Frequent blooms of phytoplankton occur in coastal upwelling zones creating hotspots of biological productivity in the ocean. As cold, nutrient-rich water is brought up to sunlit layers from depth, phytoplankton are also transported upwards to seed surface blooms that are often dominated by diatoms. The physiological response of phytoplankton to this process, commonly referred to as shift-up, is characterized by increases in nitrate assimilation and rapid growth rates. To examine the molecular underpinnings behind this phenomenon, metatranscriptomics was applied to a simulated upwelling experiment using natural phytoplankton communities from the California Upwelling Zone. An increase in diatom growth following 5 days of incubation was attributed to the genera Chaetoceros and Pseudo-nitzschia. Here, we show that certain bloom-forming diatoms exhibit a distinct transcriptional response that coordinates shift-up where diatoms exhibited the greatest transcriptional change following upwelling; however, comparison of co-expressed genes exposed overrepresentation of distinct sets within each of the dominant phytoplankton groups. The analysis revealed that diatoms frontload genes involved in nitrogen assimilation likely in order to outcompete other groups for available nitrogen during upwelling events. We speculate that the evolutionary success of diatoms may be due, in part, to this proactive response to frequently encountered changes in their environment

Iron speciation and its biological availability in seawater: A workshop

This workshop brought together marine chemists with expertise in iron chemistry and biologists with expertise in the role of iron in phytoplankton production to discuss controversies regarding the role of iron in oceanic primary productivity and global climatic change. A new paradigm for marine iron biogeochemistry was generated. The five major new items within this paradigm included (1) the nature of iron inputs to the sea, (2) chemical speciation of iron in seawater, (3) relationships between iron chemistry and marine microbial community dynamics, (4) adaptations of marine microbes to iron input, and (5) ecological and biogeochemical implications of changes in iron supply to the sea

LEAD SPECIATION IN SURFACE WATERS OF THE EASTERN NORTH PACIFIC.

Titrations using differential pulse anodic stripping voltammetry (DPASV) to detect electroactive lead were carried out on fresh seawater samples and on samples stored acidified to determine the extent of lead complexation in the eastern North Pacific. Results of these analyses on surface water samples indicate total dissolved lead concentrations between 17 and 49 pM. The inorganic or DPASV labile fraction is 30–50%. Titration with lead yielded data consistent with one class of organic ligand(s), present at low concentration (between 0.2 and 0.5 nM) with a mean value for a conditional stability constant with respect to inorganic lead of log K′cond=9.7. The presence of this ligand, together with the various inorganic ligands in seawater, gives rise to a concentration of free ionic lead of ∼ 0.4 pM

The relationship between cadmium and phosphate in the Atlantic Ocean unravelled

Cadmium (Cd) is not generally considered a nutrient element, but behaves like a nutrient in the oceans and might play an important role in ocean biology after all. The relationship between Cd and the nutrient phosphate (PO₄) has been studied for over 40 yrs, but the debate on the driving mechanism and reason behind the ‘kink’, a change in the steepness of the slope is ongoing. Using new data of high accuracy and spatial resolution covering the West-Atlantic Ocean from north to south, in combination with a robust extended optimum multiparameter (eOMP) water mass model, we show that mixing between different water masses is the dominant factor explaining the observed correlation and its kink. Regeneration of Cd via remineralisation explains the smaller scale variability, notably in the surface ocean. Observations imply the availability of Cd in surface waters determines the Cd-uptake and thus the Cd:PO₄ remineralisation ratio. This ratio is variable between different ocean regions, notably between the northern and southern high latitude oceans. Due to their role in deep water formation, both the northern and southern high latitude oceans are a driving factor in the Atlantic and global Cd and PO₄ relation. Outside the Atlantic Ocean, the classical kink is not expected, but the relationship is by no means linear. Most likely, this is due to the interaction between low latitude surface waters and subsurface waters from high latitude origin, but more data are required to assess this in detail