Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

How Are Strong Acids or Strong Bases Substituted by Weak Acids or Weak Bases in Aerosols?

<p>Due to significant influence on global climate and human health, atmospheric aerosols have attracted numerous interests from the atmospheric science community. To provide insight into the aerosol effect, it is indispensable to investigate the aerosol properties comprehensively.</p><p>Since atmospheric aerosols are surrounded by substantial gas phase and have high specific surface area, the composition partitioning between particle phase and gas phase must be considered as a key aerosol property, which is termed as volatility for volatile organic/inorganic components. Recent studies show that the aerosol volatility can also be induced by the reaction of components in addition to the volatile compositions. Herein, we summarize four types of volatility induced by reaction, namely chloride depletion, nitrate depletion, ammonia depletion and volatility induced by salt hydrolysis. For chloride depletion and nitrate depletion, these processes can be regarded as reactions that strong acids are substituted by weak acids. The high volatility of the formed HCl or HNO<sub>3</sub> drives the reaction continuously moving forward.</p><p>For ammonium depletion, we observed the reaction occurs between (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and organic acid salts during dehydration process by ATR-FTIR. For example, when molar ratio is 1:1, significant depletion of ammonium was observed in the disodium succinate/(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> particles, indicating the evaporation of NH<sub>3</sub>. Besides, the hygroscopicity of the aerosol particles decreased after the dehydration, which should be attributed to the formation of less hygroscopic succinic acid and ammonium depletion. By regarding organic acid salts as weak bases, the ammonium depletion is a reaction that strong base substituted by weak base, driving by the continuous release of NH<sub>3</sub>. In addition to volatility induced by reactions within multi-component aerosols, we also found that the salt hydrolysis can also cause the formation of volatile product. For magnesium acetate (MgAc<sub>2</sub>) aerosols, we found significant water loss of the aerosol particles under constant relative humidity condition, while the amount of acetate was also decreased. We infer that the acetic acid (HAc) evaporation is caused by the hydrolysis of MgAc<sub>2</sub>, leading to the volatility and declined hygroscopicity. Two factors contribute to the volatility of MgAc<sub>2</sub> aerosols. One is the volatile acid donner (Ac<sup>2-</sup>), which can lead to the formation of volatile HAc. The other is the residual ion accepter (Mg<sup>2+</sup>), which can combine residual OH<sup>-</sup> after the proton is depleted by the evaporation of HAc. The formation of insoluble Mg(OH)<sub>2</sub> effectively maintains the aqueous pH in a suitable range, keeping the reaction moving forward. It should be noted that the co-exist of volatile acid donner and residual ion accepter is indispensable for the volatility induced by hydrolysis.</p><p>Generally, for the volatile species present in atmosphere, the aerosol volatility induced by the reaction of components can be an important pathway for their recycling processes. Due to the substantial composition modification, the hygroscopicity is also affected by such reaction. Therefore, this partitioning behavior of aerosols needs to be considered in the future atmospheric aerosol study, which may prevent the underestimate of particle volatilization or overestimate of hygroscopicity.</p>