ALBERT

Description: Marine bacteria are the main consumers of freshly produced organic matter. Many enzymatic processes involved in the bacterial digestion of organic compounds were shown to be pH sensitive in previous studies. Due to the continuous rise in atmospheric CO2 concentration, seawater pH is presently decreasing at a rate unprecedented during the last 300 million years but the consequences for microbial physiology, organic matter cycling and marine biogeochemistry are still unresolved. We studied the effects of elevated seawater pCO2 on a natural plankton community during a large-scale mesocosm study in a Norwegian fjord. Nine Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) were adjusted to different pCO2 levels ranging initially from ca. 280 to 3000 µatm and sampled every second day for 34 days. The first phytoplankton bloom developed around day 5. On day 14, inorganic nutrients were added to the enclosed, nutrient-poor waters to stimulate a second phytoplankton bloom, which occurred around day 20. Our results indicate that marine bacteria benefit directly and indirectly from decreasing seawater pH. During the first phytoplankton bloom, 5-10% more transparent exopolymer particles were formed in the high pCO2 mesocosms. Simultaneously, the efficiency of the protein-degrading enzyme leucine aminopeptidase increased with decreasing pH resulting in up to three times higher values in the highest pCO2/lowest pH mesocosm compared to the controls. In general, total and cell-specific aminopeptidase activities were elevated under low pH conditions. The combination of enhanced enzymatic hydrolysis of organic matter and increased availability of gel particles as substrate supported up to 28% higher bacterial abundance in the high pCO2 treatments. We conclude that ocean acidification has the potential to stimulate the bacterial community and facilitate the microbial recycling of freshly produced organic matter, thus strengthening the role of the microbial loop in the surface ocean.

Description: This data is part of the BMBF projects CUSCO (Coastal Upwelling Systems in a Changing Ocean) and BioTip subproject Humboldt Tipping. Data was collected during cruise number MSM80 with research vessel Maria S. Merian from 23.12.2018 - 30.01.2019 (from Panama to Valparaiso) in the Humboldt Upwelling system off the Eastern Tropical south Pacific. Samples were taken by CTD- rosette sampler from different depths and analysed onboard for dissolved inorganic nutrients and total dissolved nutrients. Triplicate nutrient samples were analysed for concentrations with an autosampler (XY-2 autosampler, SEAL Analytical) and a continuous flow analyzer (QUAAtro Autoanalyzer, SEAL Analytical) using standard colorimetric and flourometric methods by Kastriot Qelaj. Dissolved organic nutrients were calculated as the difference of the two for respective nitrogen and phosphorous nutrients. Phosphate was measured according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). An empty cell means that corresponding nutrient samples were not taken for the respective depth.

Description: Marine dissolved organic matter (DOM) represents one of the largest active carbon reservoirs on Earth. Changes in pool size or composition could have major impacts on the global carbon cycle. Ocean acidification is a potential driver for these changes because it influences marine primary production and heterotrophic respiration. Here we show that ocean acidification as expected for a 'business-as-usual' emission scenario in the year 2100 (900 µatm) does not affect the DOM pool with respect to its size and molecular composition. We applied ultrahigh-resolution mass spectrometry to monitor the production and turnover of 7,360 distinct molecular DOM features in an unprecedented long-term mesocosm study in a Swedish Fjord, covering a full cycle of marine production. DOM concentration and molecular composition did not differ significantly between present-day and year 2100 CO2 levels. Our findings are likely applicable to other coastal and productive marine ecosystems in general.

Description: We investigated the effects of ocean acidification on the DOM pool in the open subtropical North Atlantic Ocean off the coast of Gran Canaria during an oligotrophic phase and after artificial upwelling. Future ocean conditions were simulated in large-scale mesocosms with step-wise increasing pCO2 levels from 450 to 1030 µatm. While we observed no significant effect of ocean acidification on the concentration and molecular composition of DOM we found a pool of compounds which show similar dynamics in all treatments over phytoplankton blooms.