ALBERT

Description: Mixotrophic organisms are increasingly recognized as important components of ecosystems, but the factors controlling their nutrition pathways (in particular their autotrophy-heterotrophy balance) are little known. Both autotrophy and heterotrophy are expected to respond to density-dependent mechanisms but not necessarily in the same direction and/or strength. We hypothesize that the autotrophy-heterotrophy balance of mixotrophic organisms might therefore be a function of population densities. To investigate this relationship, we sampled mixotrophic jellyfish holobionts (host, Mastigias papua etpisoni; symbiont, Cladocopium sp.) in a marine lake (Palau, Micronesia) on six occasions (from 2010 to 2018). Over this period, population densities varied similar to 100 fold. We characterized the nutrition of the holobionts using the delta C-13 and delta N-15 signatures as well as C:N ratios. delta C-13 values increased and delta N-15 values decreased with increasing population densities (respectively, R-2 = 0.86 and 0.70, P 〈 0.05). Although less distinct, C:N ratios increased with increasing population densities (R-2 = 0.59, 0.1 〉 P 〉 0.05). This indicates that the autotrophy-heterotrophy balance tends toward autotrophy when population densities increase. We propose that the availability of zooplanktonic prey is the main driver of this pattern. These results demonstrate that the autotrophy-heterotrophy balance of mixotrophic jellyfishes can be tightly regulated by density-dependent mechanisms.

Description: The major part of dissolved iron (DFe) in seawater is bound to organic matter, which prevents iron from adsorptive removal by sinking particles and essentially regulates the residence time of DFe and its availability for marine biota. Characteristics of iron‐binding ligands highly depend on their biological origin and physico‐chemical properties of seawater which may intensely alter under ongoing climate change. To understand environmental controls on the iron binding, we applied a function of pH and dissolved organic carbon (DOC) to describe changes in the binding strength of organic ligands in a global biogeochemical model (REcoM). This function was derived based on calculations using a thermodynamic chemical speciation model NICA. This parameterization considerably improved the modeled DFe distribution, particularly in the surface Pacific and the global mesopelagic and deep waters, compared to our previous model simulations with a constant ligand or one prognostic ligand. This indicates that the organic binding of iron is apparently controlled by seawater pH in addition to its link to organic matter. We tested further the response of this control to environmental changes in a simulation with future pH of a high emission scenario. The response of the binding potential shows a complex pattern in different regions and is regulated by factors that have opposite effects on the binding potential. The relative contributions of these factors can change over time by a continual change of environmental conditions. A dynamic feedback system therefore needs to be considered to predict the marine iron cycle under ongoing climate change.

Description: Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W m−2, or -2.0 to -0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds. Key Points: - An assessment of multiple lines of evidence supported by a conceptual model provides ranges for aerosol radiative forcing of climate change - Aerosol effective radiative forcing is assessed to be between -1.6 and -0.6 W m−2 at the 16–84% confidence level - Although key uncertainties remain, new ways of using observations provide stronger constraints for models