spread effects are expected to occur over the coming de cades, resulting in an alteration of the planet's geological, biological, and ecological systems. In marine microbial communities, ongoing and future changes are expected due to global warming, increased CO 2 , increased anthropogenic nitrogen sup- ABSTRACT: We report on results of a microcosm experiment to study the interactive effects of elevated CO 2 , high organic and inorganic nutrient loading, and high irradiance on phytoplankton and bacterioplankton from the Mediterranean coastal ecosystem of the Alboran Sea. This experiment was part of the Group for Aquatic Productivity 9th international workshop and was conducted by Working Group 1 (WG1: Phytoplankton of coastal waters, www.gap9.uma.es). Over a 7 d period, we measured the variation in physical and chemical variables and the characteristics of phytoplankton and bacterioplankton in microcosms incubated under 8 treatments, representing full factorial combinations of 2 levels each of CO 2 supply, nutrient concentrations and solar radiation exposure. For each treatment combination, we incubated triplicate microcosms consisting of 20 l polyethylene bags which were transparent to ultraviolet radiation. Sustained growth of phytoplankton biomass (chl a) occurred in all treatments. The absence of mesozooplankton in the microcosms resulted in a trophic cascade. Picophytoplankton were initially stimulated but then decreased, apparently due to microzooplankton grazing, and were largely replaced by diatoms. Bacteria were also initially stimulated and then decreased, but eventually recovered. Responses were modified markedly by nutrient enrichment and light availability, with moderate effects of elevated CO 2 . Relative to ambient CO 2 , elevated CO 2 resulted in higher chl a under low irradiance, but lower chl a under high irradiance.

Solar radiation and nutrient pulses regulate the ecosystem’s functioning. However, little is known about how a greater frequency of pulsed nutrients under high ultraviolet radiation (UVR) levels, as expected in the near future, could alter the responses and interaction between primary producers and decomposers. In this report, we demonstrate through a mesocosm study in lake La Caldera (Spain) that a repeated (press) compared to a one-time (pulse) schedule under UVR prompted higher increases in primary (PP) than in bacterial production (BP) coupled with a replacement of photoautotrophs by mixotrophic nanoflagellates (MNFs). The mechanism underlying these amplified phytoplanktonic responses was a dual control by MNFs on bacteria through the excretion of organic carbon and an increased top-down control by bacterivory. We also show across a 6-year whole-lake study that the changes from photoautotrophs to MNFs were related mainly to the frequency of pulsed nutrients (e.g. desert dust inputs). Our results underscore how an improved understanding of the interaction between chronic and stochastic environmental factors is critical for predicting ongoing changes in ecosystem functioning and its responses to climatically driven changes.

Because the nature of the main resource that limits bacterioplankton (e.g. organic carbon [C] or phosphorus [P]) has biogeochemical implications concerning organic C accumulation in freshwater ecosystems, empirical knowledge is needed concerning how bacteria respond to these two resources, available alone or together. We performed field experiments of resource manipulation (2×2 factorial design, with the addition of C, P, or both combined) in two Mediterranean freshwater ecosystems with contrasting trophic states (oligotrophy vs. eutrophy) and trophic natures (autotrophy vs. heterotrophy, measured as gross primary production:respiration ratio). Overall, the two resources synergistically co-limited bacterioplankton, i.e. the magnitude of the response of bacterial production and abundance to the two resources combined was higher than the additive response in both ecosystems. However, bacteria also responded positively to single P and C additions in the eutrophic ecosystem, but not to single C in the oligotrophic one, consistent with the value of the ratio between bacterial C demand and algal C supply. Accordingly, the trophic nature rather than the trophic state of the ecosystems proves to be a key feature determining the expected types of resource co-limitation of bacteria, as summarized in a proposed theoretical framework. The actual types of co-limitation shifted over time and partially deviated (a lesser degree of synergism) from the theoretical expectations, particularly in the eutrophic ecosystem. These deviations may be explained by extrinsic ecological forces to physiological limitations of bacteria, such as predation, whose role in our experiments is supported by the relationship between the dynamics of bacteria and bacterivores tested by SEMs (structural equation models). Our study, in line with the increasingly recognized role of freshwater ecosystems in the global C cycle, suggests that further attention should be focussed on the biotic interactions that modulate resource co-limitation of bacteria.

photosynthetic organisms and the subsequent sinking of POC which is sequestered in deep waters (the so-called biological pump) are 2 of the main organism-mediated processes that are involved in this CO 2 sink effect. In pelagic ecosystems, a large proportion of the POC produced by the phytoplankton is re - ABSTRACT: We conducted a microcosm experiment aimed at studying the interactive effects of high CO 2 , nutrient loading and irradiance on the metabolism of a planktonic community sampled in the Western Mediterranean near the coast of Málaga. Changes in the metabolism of phytoplankton and bacterioplankton were observed for 7 d under 8 treatment conditions, representing the full factorial combinations of 2 levels each of CO 2 , nutrient concentration and solar radiation exposure. The initial plankton sample was collected at the surface from a stratified water column, indicating that phytoplankton were naturally acclimated to high irradiance and low nutrient concentrations. Nutrient addition combined with high irradiance resulted in a significant increase in primary production. Nitrate uptake by phytoplankton was also stimulated under high nutrient conditions. High nutrients, high irradiance and the combination of low CO 2 and high irradiance positively affected bacterial production. Light was the main factor affecting the respiration rates of the community, which were higher at the high light level. After 7 d of incubation, nutrient loading was the only factor that significantly affected the amount of particulate organic carbon (POC) accumulated in the microcosms. Therefore, the changes in metabolic rates produced at high CO 2 had no effect on net production of particulate organic matter. If these results are extrapolated to the natural environment, it could be hypothesized that high levels of CO 2 will have a limited impact on biological pump activity in the northern Alboran Sea since it is assumed that POC export towards deeper layers determines the potential for carbon sequestration.

Mixotrophy is a dominant metabolic strategy in ecosystems worldwide. Shifts in temperature (T) and light (i.e. the ultraviolet portion of spectrum (UVR)) are key abiotic factors that modulate the conditions under which an organism is able to live. However, whether the interaction between both drivers alters mixotrophy in a global-change context remains unassessed.To determine the T 9 UVR effects on relative electron transport rates, nonphotochemical quenching, bacterivory, and bacterial production, we conducted an experiment with Isochrysis galbana populations grown mixotrophically, which were exposed to 5°C of cooling and warming with respect to the control (19°C) with (or without) UVR over light-dark cycles and different timescales.At the beginning of the experiment, cooling inhibited the relative electron transport and bacterivory rates, whereas warming depressed only bacterivory regardless of the radiation treatment. By the end of the experiment, warming and UVR conditions stimulated bacterivory. These reduced relative electron transport rates (c. 50% (warming) and > 70% (cooling)) were offset by increased (35%) cumulative bacterivory rates under warming and UVR conditions.We propose that mixotrophy constitutes an energy-saving and a compensatory mechanism to gain carbon (C) when photosynthesis is impaired, and highlight the need to consider the natural environmental changes affecting the populations when we test the impacts of interacting global-change drivers.