Contrasting effects of ocean acidification on the microbial food web under different trophic conditions

Abstract

We investigated the effects of an increase in dissolved CO2 on the microbial communities of the Mediterranean Sea during two mesocosm experiments in two contrasting seasons: winter, at the peak of the annual phytoplankton bloom, and summer, under low nutrient conditions. The experiments included treatments with acidification and nutrient addition, and combinations of the two. We followed the effects of ocean acidification (OA) on the abundance of the main groups of microorganisms (diatoms, dinoflagellates, nanoeukaryotes, picoeukaryotes, cyanobacteria, and heterotrophic bacteria) and on bacterial activity, leucine incorporation, and extracellular enzyme activity. Our results showed a clear stimulation effect of OA on the abundance of small phytoplankton (pico- and nanoeukaryotes), independently of the season and nutrient availability. A large number of the measured variables showed significant positive effects of acidification in summer compared with winter, when the effects were sometimes negative. Effects of OA were more conspicuous when nutrient concentrations were low. Our results therefore suggest that microbial communities in oligotrophic waters are considerably affected by OA, whereas microbes in more productive waters are less affected. The overall enhancing effect of acidification on eukaryotic pico- and nanophytoplankton, in comparison with the non-significant or even negative response to nutrient-rich conditions of larger groups and autotrophic prokaryotes, suggests a shift towards medium-sized producers in a future acidified ocean.

Influence of Ocean Acidification and Deep Water Upwelling on Oligotrophic Plankton Communities in the Subtropical North Atlantic: Insights from an In situ Mesocosm Study

“…plankton communities can occur under prolonged low-nutrient conditions, where observed variability in biomass is generally low. This conclusion is in line with recent studies in different oceanic regions, which reported most prominent effects of OA when inorganic nutrients were depleted (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016). However, our study is the first to demonstrate this for oligotrophic waters of the subtropical North Atlantic.…”

“…The few reported community-level studies mostly focused on rather eutrophic environments at higher latitudes, such as the Arctic Ocean or temperate waters, since these regions are commonly assumed to be most vulnerable to ongoing changes in carbonate chemistry (Orr et al, 2005;Yamamoto-Kawai et al, 2009). However, recent evidence from the Baltic Sea, North Sea, and Mediterranean Sea indicated that OA effects might be most pronounced when inorganic nutrient concentrations are low (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016;Hornick et al, 2016). How plankton communities in the vast oligotrophic regions of the subtropical gyres might respond to OA is presently unknown.…”

Individual and interactive effects of warming and CO<sub>2</sub> on <i>Pseudo-nitzschia subcurvata</i> and <i>Phaeocystis antarctica</i>, two dominant phytoplankton from the Ross Sea, Antarctica

“…However, major nutrients sometimes become depleted late in the season on McMurdo Sound, the origin of our culture isolates, as Fe inputs are somewhat higher in these nearshore waters (Bertrand et al, 2015). Experiments using nutrient-limited phytoplankton frequently find differing responses to CO 2 and temperature compared to those of nutrient-replete cells, including sometimes enhanced effects of these global change factors on elemental ratios (Taucher et al, 2015;Sala et al, 2016). Our experiments under high-nutrient, Fe-replete conditions thus are likely to best predict possible biological effects of future high CO 2 and temperature during the first half or more of the Ross Sea growing season.…”

Ocean Acidification-Induced Restructuring of the Plankton Food Web Can Influence the Degradation of Sinking Particles

“…Recent mesocosm experiments showed that OA effects can be particularly pronounced during extended periods of low inorganic nutrient concentrations (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016;Thomson et al, 2016). We therefore conducted an in situ mesocosm experiment in the eastern subtropical Atlantic Ocean off the coast of Gran Canaria (Canary Islands)-a region characterized by low concentrations of inorganic nutrients in particular during fall when the study took place (Arístegui et al, 2001).…”