Koji Sugie

Effects of pCO2 and iron on the elemental composition and cell geometry of the marine diatom Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae)1

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 Earths history.

Size of Dominant Diatom Species Can Alter Their Evenness

Traditionally, biodiversity has often been estimated on the basis of abundance partly due to the need for complicated measurements of biomass. Here, we conducted robust measurements of the community composition and of the size structure of diatoms in the North Pacific to evaluate the importance of biomass on the biodiversity. We found that the two most useful evenness indices increased in most cases where small species were numerically dominant when calculations were based on biomass compared with those on abundance. Size-abundance spectra of diatoms revealed that numerically dominant small species rarely dominated in terms of biomass. On the other hand, intermediate to large diatom species generally played a dominant role in terms of biomass in diatom community. The results suggest that the size of the dominant species is a crucial factor in determining the role of diatoms in the ecosystem functioning. Because such size variability can also be observed in other organisms, we need to pay attention to the effect of size structures on biodiversity.

Response of Spring Diatoms to CO2 Availability in the Western North Pacific as Determined by Next-Generation Sequencing.

Next-generation sequencing (NGS) technologies have enabled us to determine phytoplankton community compositions at high resolution. However, few studies have adopted this approach to assess the responses of natural phytoplankton communities to environmental change. Here, we report the impact of different CO2 levels on spring diatoms in the Oyashio region of the western North Pacific as estimated by NGS of the diatom-specific rbcL gene (DNA), which encodes the large subunit of RubisCO. We also examined the abundance and composition of rbcL transcripts (cDNA) in diatoms to assess their physiological responses to changing CO2 levels. A short-term (3-day) incubation experiment was carried out on-deck using surface Oyashio waters under different pCO2 levels (180, 350, 750, and 1000 μatm) in May 2011. During the incubation, the transcript abundance of the diatom-specific rbcL gene decreased with an increase in seawater pCO2 levels. These results suggest that CO2 fixation capacity of diatoms decreased rapidly under elevated CO2 levels. In the high CO2 treatments (750 and 1000 μatm), diversity of diatom-specific rbcL gene and its transcripts decreased relative to the control treatment (350 μatm), as well as contributions of Chaetocerataceae, Thalassiosiraceae, and Fragilariaceae to the total population, but the contributions of Bacillariaceae increased. In the low CO2 treatment, contributions of Bacillariaceae also increased together with other eukaryotes. These suggest that changes in CO2 levels can alter the community composition of spring diatoms in the Oyashio region. Overall, the NGS technology provided us a deeper understanding of the response of diatoms to changes in CO2 levels in terms of their community composition, diversity, and photosynthetic physiology.

A new marine araphid diatom, Thalassionema kuroshioensis sp. nov., from temperate Japanese coastal waters

A new marine planktonic diatom Thalassionema kuroshioensis sp. nov., was described based on morphological comparison with five other extant Thalassionema species. Thalassionema kuroshioensis was found in coastal to offshore regions of the Kuroshio current in the temperate to subtropical western North Pacific. Morphological characteristics of T. kuroshioensis were identified using light and scanning electron microscopic observations and include isopolar valves, high density of areolae, cross- to club-shaped occluding bars, apical excavations, and sternum traversed across both ends of apices on the valve face. One rimoportula is located at the end of both valve apices connecting with the external opening slit in the deeply excavated apex. The outline of the valve is linear or slightly inflated. We provide size ranges and a key to the species of all known extant Thalassionema species, including the new species.

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.