Impacts of ocean acidification and iron enrichment on photosynthetic ability of diatoms in the Bering Sea as estimated from their rbcL gene expressions

第2回極域科学シンポジウム　共通セッション「海氷圏の生物地球化学」 11月16日（水） 統計数理研究所 3階リフレッシュフロ

Resting spore formation in the marine diatom Thalassiosira nordenskioeldii under iron- and nitrogen-limited conditions

Resting spore formation was investigated in the neritic and oceanic strains of Thalassiosira nordenskioeldii under iron- and nitrate-depleted conditions at 5°C and 10°C. Both strains immediately formed resting spores under nitrate-depleted conditions with almost 100% composition after 4–8 and 3–6 d cultivation periods at 5°C and at 10°C, respectively. However, resting spore formation in both strains under iron-depleted conditions increased with incubation time more gradually, and after 15 d of cultivation, spore composition ranged from 60% in the neritic strain at 5°C to 1% in the oceanic strain at 10°C. In addition, chlorotic cells with smaller cell volume compared with vegetative cells were observed under iron-depleted conditions. Sinking rates of vegetative cells, iron-limited cells and spores and nitrate-limited resting spores cultivated at 5°C were 1.24 ± 0.14, 3.41 ± 0.43 and 9.22 ± 1.04 m d−1, respectively, slightly faster than those at 10°C. The faster sinking rates in iron-limited resting cells and resting spores than in vegetative cells may prevent their habitat from expanding to high-nitrate low-chlorophyll oceanic regions with low iron concentrations

Change in the elemental composition and cell geometry of the marine diatom Attheya longicornis under nitrogen- and iron-depleted conditions

The morphology of the siliceous cell wall (frustule) is fundamental to the identification of diatom species. One of the fundamental questions is the ecophysiological role of the diatom frustule, which often shows morphological plasticity under different growth conditions. In this study, the morphology and elemental composition of the diatom Attheya longicornis were investigated under nutrient-replete (control), iron-depleted and nitrogen-depleted conditions. This cylindrical, unicellular species has four siliceous horns per cell. The horns are each formed from a hoop-like structure with a supporting rod, which greatly increases the surface area (SA) of the cell. Under the iron-depleted conditions, relative to the controls, the SA to cell volume ratio, silicon cell quota and siliceous horn length increased 2.3-, 2.3- and 1.4-fold, respectively. Under the nitrogen-depleted conditions, the cell size decreased without an increase in horn length, and the cellular biogenic silica (BSi) content was the highest between the three growth media. The change in cell geometry and elemental composition modified the sinking behaviour of A. longicornis. Estimated sinking rate was fastest in the nitrogen-depleted cells, followed by the controls and iron-depleted cells. The data suggest that the biogeochemical processes of BSi could show vertically opposite direction depending on the growth-limiting factors through a change in the elemental composition and cell morphology of diatoms. Such plastic responses to nitrogen and iron depletion may contribute to the relatively wide distribution of this species from the coastal to open ocean in the subarctic region

Effects of pCO2 and iron on the elemental composition and cell geometry of the marine diatom Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae)

Partial pressure of CO2 (pCO2) and iron availability in seawater show corresponding changes due to biological and anthropogenic activities. The simultaneous change in these factors precludes an understanding of their independent effects on the ecophysiology of phytoplankton. In addition, there is a lack of data regarding the interactive effects of these factors on phytoplankton cellular stoichiometry, which is a key driving factor for the biogeochemical cycling of oceanic nutrients. Here, we investigated the effects of pCO2 and iron availability on the elemental composition (C, N, P, and Si) of the diatom Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle by dilute batch cultures under 4 pCO2 (~200, ~380, ~600, and ~800 μatm) and five dissolved inorganic iron (Fe′; ~5, ~10, ~20, ~50, and ~100 pmol · L−1) conditions. Our experimental procedure successfully overcame the problems associated with simultaneous changes in pCO2 and Fe′ by independently manipulating carbonate chemistry and iron speciation, which allowed us to evaluate the individual effects of pCO2 and iron availability. We found that the C:N ratio decreased significantly only with an increase in Fe′, whereas the C:P ratio increased significantly only with an increase in pCO2. Both Si:C and Si:N ratios decreased with increasing pCO2 and Fe′. Our results indicate that changes in pCO2 and iron availability could influence the biogeochemical cycling of nutrients in future oceans with high- CO2 levels, and, similarly, during the time course of phytoplankton blooms. Moreover, pCO2 and iron availability may also have affected oceanic nutrient biogeochemistry in the past, as these conditions have changed markedly over the Earth's history

Effects of increased pCO2 on phytoplankton community compositions in the NW subarctic Pacific and Bering Sea in summer

Rising atmospheric CO, concentration have led to greater CO, uptake by the ocean with a decrease in pH of seawater (i.e. ocean acidification). To elucidate the effects of ocean acidification On natural phytoplankton assemblages, CO, manipulated incubation experiments were conducted in the NW subarctic Pacific and Bering Sea in summer of2008 and 2009, respectively. Phytoplankton pigments samples were analyzed by HPLC with the algorithm CHEMT AX. As a result, an increase in pCO, in seawater depressed a specific phytoplankton group at least and the effect differed among the sea areas. Since export flux of organic carbon from the surface to ocean interior (i.e. biological pump) largely depends on the relative abundance of each phytoplankton group. Ocean acidification has a significant impact on the CO, absorption ability in situ. I plan to do more detailed field and laboratory studies to examine physiological conditions in relation to CO, fixation of phytoplankton.「北海道大学総長賞 （最優秀賞）」受

Data from: Characterization of the synoptic-scale diversity, biogeography and size distribution of diatoms in the North Pacific

The diversity, biogeography, and size distribution of diatoms in the North Pacific and underlying mechanisms shaping those patterns have little been characterized despite their importance in marine ecosystems. Here, we examined the community structure of diatoms in the surface and subsurface chlorophyll a maximum (SCM) layers of the North Pacific using light and scanning electron microscopy. Diatom carbon biomass in both the subarctic and temperate coastal regions was higher than that in the open subtropical and tropical waters. Species density was high in the temperate coastal region and certain open ocean stations where coastal water could be intruded. Diversity was generally higher in the SCM layer than that in the surface layer without a clear latitudinal trend. All diatom species in the open subtropical waters were observed in the temperate coastal waters, suggesting that the coastal regions act as a species bank. Certain indigenous species in the subarctic waters were considered as cryophilic species. We found that the general patterns of the size spectrum were nearly identical among different geographical regions of the North Pacific, irrespective of the species composition, environmental conditions, and seasonality. Our results provide mechanistic insights indicating that the diatom biodiversity in the species bank around the coastal region and the following current systems from the coastal to oceanic regions could principally determine biodiversity patterns in the North Pacific. The species-specific ecophysiological traits of diatoms and environmental conditions may further modify the biodiversity patterns and size distribution of diatoms

Results of manipulation experiments during MR15 and MR16 cruises

The Arctic Ocean has been experiencing the rapid climate changes such as warming and sea ice melt. Further, the large area of sea ice retreat enhance ocean uptake of CO2. We conducted two shipboard experiments in September 2015 and 2016 to examine the effects of temperature, CO2, and salinity on phytoplankton dynamics to better understand the impacts of climate changes on the Arctic ecosystem. Two temperature (Control and 5°C above the Control), two CO2 (Control and 300/450 μatm above the Control), and two salinity (Control and 1.4 below the Control) conditions were fully factorially manipulated in eight treatments. Higher temperature enhanced almost all phytoplankton traits, whereas the experiment in 2015 demonstrated that diatom diversity decreased due to the replacement of chain-forming Thalassiosira spp. by solitary Cylindrotheca closterium. Higher CO2 levels significantly increased the growth of small-sized phytoplankton (<10 μm) in both years. Decreased salinity had marginal effects but significantly increased the growth of small-sided phytoplankton in terms of chl-a in 2015. This study demonstrates that the change in phytoplankton community structure in the shelf edge region of the Chukchi Sea in the western Arctic Ocean under multiple environmental perturbations are likely to lead to decreases in the efficiency of the biological carbon pump, trophic transfer and food supply for benthos

Lord Rosebery (CNews) : [photographie de presse] / [Agence Rol]

Référence bibliographique : Rol, 106101Appartient à l’ensemble documentaire : Pho20RolImage de press

Increase in Si:N drawdown ratio due to resting spore formation by spring bloom-forming diatoms under Fe- and N-limited conditions in the Oyashio region

Resting spore formation and Si:N drawdown ratios were investigated under iron (Fe)- and nitrogen (N)-limited conditions using a unialgal culture of Thalassiosira nordenskioeldii and natural phytoplankton assemblages during the spring bloom in the Oyashio region. In the unialgal culture of T nordenskioeldii, 20% and 100% of the cells formed resting spores under Fe- and N-limited conditions, respectively. The Si:N drawdown ratios were 2- and 14-fold higher in Fe- and N-limited conditions, respectively, compared to Fe- and N-sufficient conditions. At the start of the natural phytoplankton incubation, 18 among 47 identified diatom species were known resting spore-forming species. Approximately 15 common diatom species formed resting spores under Fe- and N-limited conditions. During the natural phytoplankton incubation, the percentage of the resting spores increased with time under both Fe- and N-limited conditions, reaching 25% and 40% of total diatom abundance, respectively. The Si: N drawdown ratios significantly increased with an increase in the contribution of resting spores in both the unialgal culture and natural phytoplankton incubations. These results suggest that if the bloom dominated by neritic, resting spore-forming diatom species decline by either Fe- or N-depletion, Si may be utilized preferentially to N in the upper mixed layer due to the formation of heavily silicified resting spores