Abstract
Changes in iron (Fe) bioavailability influence diatom physiology and community composition, and thus have a profound impact on primary productivity and ecosystem dynamics. Iron limitation of diatom growth rates has been demonstrated in both oceanic and coastal waters of the Northeast Pacific Ocean and is predicted to become more pervasive in future oceans. However, it is unclear how the strategies utilized by phytoplankton to cope with low Fe bioavailability and resupply differ across these ocean provinces. We investigated the response of diatom communities to variable Fe conditions through incubation experiments performed in the Fe mosaic of the California Upwelling Zone and along a natural Fe gradient in the Northeast Pacific Ocean. Through coupling gene expression of two dominant diatom taxa (Pseudo-nitzschia and Thalassiosira) with biological rate process measurements, we provide an in-depth examination of the physiological and molecular responses associated with varying Fe status. Following Fe enrichment, oceanic diatoms showed distinct differential expression of gene products involved in nitrogen assimilation, photosynthetic carbon fixation and vitamin production compared to diatoms from low-Fe coastal sites, possibly driven by the chronic nature of Fe stress at the oceanic site. Genes of interest involved in Fe and N metabolism additionally exhibited divergent expression patterns between the two diatom taxa investigated, demonstrating that diverse diatoms may invoke alternative strategies when dealing with the identical changes in their environment. We report here several mechanisms used distinctly by coastal or oceanic diatom communities as well as numerous taxa-specific strategies for coping with Fe stress and rearranging nutrient metabolism following Fe enrichment.
Highlights
Phytoplankton growth is limited by iron (Fe) availability in ∼30–40% of the ocean (Moore et al, 2001, 2004)
Supplementary Table 1) was substantially below the predicted threshold ratio for eventual FeCl3 addition (Fe) stress of 12 μmol:nmol for phytoplankton in this region as proposed by King and Barbeau (2007), albeit this ratio is subject to variation as a function of phytoplankton iron demands (Bruland et al, 2001), suggesting this phytoplankton community was not likely to be driven into Fe limitation prior to complete NO−3 utilization
The gene expression patterns presented here demonstrate that the cosmopolitan diatom genera Pseudo-nitzschia and Thalassiosira rely on diverse sets of strategies to handle Fe stress, and that oceanic diatoms from both groups are highly responsive to changes in Fe availability with a greater degree of differentially expressed genes involved in nitrate assimilation, carbon fixation, and vitamin production compared to their coastal counterparts
Summary
Phytoplankton growth is limited by iron (Fe) availability in ∼30–40% of the ocean (Moore et al, 2001, 2004). Productivity in the NE Pacific Ocean remains low as a result of low Fe concentrations regardless of sufficient nitrate (NO−3 ) levels and is typically dominated by small cells such as the cyanobacterium Synechococcus and eukaryotic picophytoplankton (Varela and Harrison, 1999). In this region, Fe is supplied to surface waters mainly through atmospheric deposition of dust from arid continental regions and volcanic emissions, with Fe inputs from continental margin sediments fueling winter phytoplankton blooms when atmospheric deposition is low (Lam et al, 2006; Lam and Bishop, 2008). Low Fe levels result from the lack of Fe inputs from rivers and narrow continental shelves that prevent mixing of upwelled water with Fe derived from Fe-rich shelf sediments (Johnson et al, 1999; Bruland et al, 2001) and the primary Fe source in the CUZ is winter river sediment deposition (Hutchins et al, 2002; Chase et al, 2005)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
