Prochlorococcus, the smallest but most abundant marine primary producer, plays an important role in carbon cycling of the global ocean. As a phototroph, Prochlorococcus is thought to be confined to the euphotic zone, with commonly observed maximum depths of ∼150–200 m. But here we show, using flow cytometry and cellular ribosomal content, for the first time the presence of abundant and active Prochlorococcus in the dark ocean ("deep Prochlorococcus" hereafter). Intensive studies at the Luzon strait in the western Pacific Ocean show that the deep Prochlorococcus populations are exported from the euphotic zone. Multiple physical processes including internal solitary waves could be responsible for the transportation. The unexpected abundance of the tiny phototrophs in the dark ocean reveals a novel mechanism for picoplankton carbon export other than the known mechanisms such as sinking of phytodetritus and aggregates or grazing-mediated transportation. Such direct transportation of picoplanktonic phototrophs from surface to deep waters is poorly understood, but could significantly contribute to both the biological pump (through particulate organic carbon) and the microbial carbon pump (through release of dissolved organic carbon from microbial processes) for carbon sequestration in the ocean

Prochlorococcus, the smallest but most abundant marine primary producer, plays an important role in carbon cycling of the global ocean. As a phototroph, Prochlorococcus is thought to be confined to the euphotic zone, with commonly observed maximum depths of ∼ 150-200 m, but here we show for the first time the substantial presence of Prochlorococcus populations in the dark ocean ("deep Prochlorococcus" hereafter). Intensive studies at the Luzon Strait in the western Pacific Ocean show that the deep Prochlorococcus populations are exported from the euphotic zone. Multiple physical processes including internal solitary waves could be responsible for the transportation. These findings reveal a novel mechanism for picoplankton carbon export other than the known mechanisms such as sinking of phytodetritus and aggregates or grazing-mediated transportation.

No abstract

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are performed to investigate the asymmetric secondary electron emission (SEE) effects when electrons strike two different material electrodes in low pressure capacitively coupled plasmas (CCPs). To describe the electron-surface interactions, a realistic model, considering the primary electron impact energy and angle, as well as the corresponding surface property-dependent secondary electron yields, is employed in PIC/MCC simulations. In this model, three kinds of electrons emitted from the surface are considered: (i) elastically reflected electrons, (ii) inelastically backscattered electrons, and (iii) electron induced secondary electrons (SEs, i.e., δ-electrons). Here, we examined the effects of electron-surface interactions on the ionization dynamics and plasma characteristics of an argon discharge. The discharge is driven by a voltage source of 13.56 MHz with amplitudes in the range of 200–2000 V. The grounded electrode material is copper (Cu) for all cases, while the powered electrode material is either Cu or silicon dioxide (SiO2). The simulations reveal that the electron impact-induced SEE is an essential process at low pressures, especially at high voltages. Different electrode materials result in an asymmetric response of SEE. Depending on the instantaneous local sheath potential and the phase of the SEE, these SEs either are reflected by the opposite sheath or strike the electrode surface, where they can induce δ-electrons upon their residual energies. It is shown that highly energetic δ-electrons contribute significantly to the ionization rate and a self-bias forms when the powered electrode material is assumed to be made of SiO2. Complex dynamics is observed due to the multiple electron-surface interaction processes and asymmetric yields of SEs in CCPs.

The electrical asymmetry effect combined with the magnetic asymmetry effect in a geometrically symmetric argon discharge is investigated using a one-dimensional particle-in-cell simulation with a Monte Carlo collision model. Both the asymmetry effects can induce an asymmetric plasma response with a consequent dc self-bias. It is found that these two asymmetry effects work independently of each other to some extent, which greatly enhances the flexibility for controlling the ion properties of interest, e.g. ion flux Γi and mean ion energy E i at electrodes on the weak magnetic field side. On one hand, Γi can be modulated by tuning the gradient of the magnetic field, while the angle distribution on electrodes remains approximately unaffected for a fixed phase angle θ on the weak magnetic field side with a small shift in ion energy peak. On the other hand, E i can be modulated by adjusting θ, while Γi only slightly fluctuates at a fixed gradient of magnetic field. Besides, the confinement effect of magnetic field on electron motion induces enhanced ionization rate and plasma density near the sheath edge on the strong magnetic field side.

Effects of secondary electron emission (SEE) on the plasma density and electron excitation dynamics in dual-frequency (2 MHz and 14 MHz) capacitively coupled Ar plasmas are investigated. The plasma density n p is measured with a hairpin probe, and the spatio-temporal distribution of electron excitation rate (ground state into Ar 2p 1 state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage f L increases, n p first decreases at low f L , due to compressed the plasma bulk length by the LF source, and then increases slightly with f L , suggesting that the plasma is dominated by the α mode. When f L exceeds some critical value, n p increases dramatically with f L , due to significantly enhanced ionization by secondary electrons, indicating a α-γ mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high f L . Under various conditions, including the high-frequency voltage f H , the pressure p, the electrode gap L, and the electrode material, different dependences of n p on f L are also discussed. It is found that the discharge turns into the γ mode at a lower f L when f H is higher. As p increases, the density peak moves axially towards the powered electrode, due to reduced sheath thickness, while its distance to the powered electrode is almost independent of other external conditions (f H , L, electrode material, etc). A higher p or higher L is favorable for the enhancement of n p in γ mode, because secondary electrons can contribute more to the ionization. Due to the higher SEE coefficient of the aluminum electrode, a more significant increase in n p in the γ mode can be seen than that with a stainless steel or copper electrode. Meanwhile, the spatio-temporal distributions of the electron excitation rate under the same conditions are analyzed to further understand the SEE effects.

The mechanism of nonlinear oscillations in symmetric capacitively coupled plasmas is studied by the particle-in-cell/Monte Carlo collisions approach. A physical origin of this nonlinear phenomenon is identified by spatiotemporal kinetic analysis of electron dynamics. It is found that multi-beams of high-energy electrons are stimulated at the sheath expansion phase, following with reversed electric field filaments. The instantaneous absence of the quasi-neutrality in the vicinity of the sheaths is responsible for the observed phenomenon. In addition, a simple theoretical model is introduced to qualitatively illustrate the numerical findings. Our simulations demonstrate that the frequency and intensity of this nonlinearity are very sensitive to the plasma density, sheath velocity, and sheath thickness. More nonlinear oscillations could be stimulated at the condition of high density and high sheath velocity, while a large sheath thickness normally induces large-amplitude oscillations. A simple relation of pressure and gap distance for nonlinear sheath oscillations has been built.

While seawater acidification induced by elevated CO 2 is known to impact coccolithophores, the effects in combination with decreased salinity caused by sea ice melting and/or hydrological events have not been documented. Here we show the combined effects of seawater acidification and reduced salinity on growth, photosynthesis and calcification of Emiliania huxleyi grown at 2 CO 2 concentrations (low CO 2 LC:400 µatm; high CO 2 HC:1000 µatm) and 3 levels of salinity (25, 30, and 35). A decrease of salinity from 35 to 25 increased growth rate, cell size and photosynthetic performance under both LC and HC. Calcification rates were relatively insensitive to salinity though they were higher in the LC-grown compared to the HCgrown cells at 25 salinity, with insignificant differences under 30 and 35. Since salinity and OA treatments did not show interactive effects on calcification, changes in calcification:photosynthesis ratios are attributed to the elevated photosynthetic rates at lower salinities, with higher ratios of calcification to photosynthesis in the cells grown under 35 compared with those grown at 25. In contrast, photosynthetic carbon fixation increased almost linearly with decreasing salinity, regardless of the pCO 2 treatments. When subjected to short-term exposure to high light, the low-salinitygrown cells showed the highest photochemical effective quantum yield with the highest repair rate, though the HC treatment enhanced the PSII damage rate. Our results suggest that, irrespective of pCO 2 , at low salinity Emiliania huxleyi up-regulates its photosynthetic performance which, despite a relatively insensitive calcification response, may help it better adapt to future ocean global environmental changes, including ocean acidification, especially in the coastal areas of high latitudes.