Marine iron biogeochemistry under a changing climate: impact on the phytoplankton and the diazotroph communities

Diatoms constitute a major group of phytoplankton, accounting for ~20% of the world’s primary production. Biological dinitrogen (N2) fixation by diazotrophic cyanobacteria has great biogeochemical implications in nitrogen (N) cycling, being the major source of new N input to the oceans and thereby contributing significantly to carbon (C) export production. It has been shown that iron (Fe) can be the limiting nutrient for phytoplankton growth, in particular, in the HNLC (High Nutrient Low Chlorophyll) regions. Iron plays thus an essential role in governing the marine primary productivity and the efficiency of biological carbon pump. Oceanic systems are undergoing continuous modifications at varying rates and magnitudes as a result of changing climate. The objective of our research is to evaluate the effects of global climate change processes (changing dust deposition, ocean acidification and sea-surface warming) on phytoplankton growth, biological N2 fixation, biogeochemical cycles, and the controlling role of Fe within these impacts. Laboratory culture experiments using a marine diatom Chaetoceros socialis were conducted at two temperatures (13 ℃ and 18 ℃) and two carbon dioxide partial pressures (pCO2, 400 µatm and 800 µatm). The present study clearly highlights the effect of ocean acidification on enhancing the release of Fe upon dust deposition. Our results also confirm that being a potential source of Fe, mineral dust provides in addition a readily utilizable source of macronutrients such as phosphorus (P) and silicon (Si). However, elevated atmospheric CO2 concentrations and ocean acidification may also have an adverse impact on diatom growth, causing a decrease in cell size and possible further changes in phytoplankton composition. Meanwhile, increasing temperature and ocean warming may lead to the reduction of diatom production as well as cell size, inducing poleward shifts in the biogeographic distribution of diatoms. Numerous factors can affect the extent of N2 fixation. A better understanding of the major environmental and nutrient controls governing this process is highly required. Iron and/or phosphorus are thought to be limiting factors in most oceanic regions. Special attention has been given to studying the effects of mineral dust deposition which is believed to promote N2 fixation as it increases the availability of both Fe and P. Three laboratory bioassays (+Fe, +P, +Dust) via incubation experiments were performed on Trichodesmium IMS101, an important contributor to marine N2 fixation. Each addition of Fe, P or desert dust was found to stimulate the growth and the N2 fixation activity of Trichodesmium IMS101. Several adaptive nutrient utilization strategies were observed, such as a Fe luxury uptake mechanism, a P-sparing effect and colony formation. In addition, during a field study in the temperate Northeast Atlantic Ocean using natural phytoplankton assemblages, N2 fixation was remarkably stimulated through the addition of dissolved Fe under low temperature and depleted P conditions, highlighting the critical role of Fe. At the time of this study, no Trichodesmium filaments were found in the region of investigation. The diazotrophic community was dominated by the unicellular cyanobacteria symbiont (prymnesiophyte-UCYN-A1) and heterotrophic diazotrophs, therefore suggesting that Fe could be the ultimate factor limiting N2 fixation of these smaller diazotrophs as well. Recently, the effects of ongoing climate change (ocean warming and acidification) on N2 fixation drew much attention, but various studies led to controversial conclusions. Semi-continuous dilution growth experiments were conducted on Trichodesmium IMS101 under future high pCO2 and warming seawater conditions (800 µatm and 28 °C) and compared to the present-day situations (400 µatm and 24 °C). The results indicate that higher pCO2 and therefore ocean acidification may be beneficial for Trichodesmium growth and N2 fixation. However, the present study suggests that Fe or P limitation in oligotrophic seawaters may offset the stimulation induced on Trichodesmium IMS101 due to ocean acidification. In contrast, ocean warming may not play an important role in Trichodesmium growth and N2 fixation with a 4 °C increase from 24 °C to 28 °C. Nevertheless, ocean warming was previously predicted to cause a shift in the geographical distribution of Trichodesmium toward higher latitudes, extending its niche to subtropical regions and potentially reducing its range in tropical ocean basins.Overall, the biological responses of the marine diatom Chaetoceros socialis and the N2-fixing cyanobacteria Trichodesmium IMS101 to several key climate change processes were presented and discussed in this study. These processes included dust deposition, and ocean acidification and warming, which were shown to have a significant impact on oceanic phytoplankton growth, cell size and primary productivity, biological N2 fixation, phytoplankton distribution and community composition. They would thus affect the C, N, P, Si and Fe biogeochemical cycles in various ways. Iron, as one of the most crucial micronutrients for marine phytoplankton, has in particular strong links to climate change and biogeochemical feedback mechanisms.

Li X., 2018. Marine iron biogeochemistry under a changing climate: impact on the phytoplankton and the diazotroph communities. PhD thesis, Université libre de Bruxelles. Thesis (restricted access).

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