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: The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N2O, CH4 and CO2. From 3 to 23 December 2012, R/V Meteor (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4°S, and 82.0 to 77.5°W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (S) values and excitation-emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope S varied between 0.012 to 0.043 1 nm-1 and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher S values were observed in the ULW of the southern stations below 15°S. By EEMs, we identified five fluorescent components (F1-5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m-1. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air-sea gas exchange in this area.

Description: Chromophoric dissolved organic matter (CDOM) is the photo-reactive fraction of the marine dissolved organic matter (DOM) pool. Changes in CDOM quality and quantity have impacts on marine microbial dynamics and the underwater light environment. One major source of CDOM is produced by marine bacteria through their alteration of pre-existing DOM substrates. In a series of microcosm experiments in controlled marine conditions, we explored the impact of polystyrene microplastics on the quality and quantity of microbial CDOM, observing an increased production of CDOM with changes in its molecular weight, which resulted from either an increased microbial CDOM production or an enhanced transformation of DOM from lower to higher molecular weight CDOM. This open dataset reports CDOM, bacteria, DOC and oxygen data collected in the series of microcosm experiments recently published. They refer to Experimental Set-up 1 and Experimental Set-up 2 (ES1, ES2). The CDOM data from the blank control experiment are also reported.

Description: These data accompany the perspective paper "Plastic Accumulation in the Sea Surface Microlayer: An Experiment-Based Perspective for Future Studies" authored by L. Galgani and S. A. Loiselle. The data reflect the results obtained from a small pilot laboratory experiment used to support the hypothesis that plastic accumulation in the sea surface microlayer (SML) might have effects on organic matter cycling in the surface ocean. Bacterial abundance, chromophoric dissolved organic matter (CDOM), and oxygen concentrations were measured.

Description: The interface layer between ocean and atmosphere is only a couple of micrometers thick but plays a critical role in climate relevant processes, including the air-sea exchange of gas and heat and the emission of primary organic aerosols (POA). Recent findings suggest that low-level cloud formation above the Arctic Ocean may be linked to organic polymers produced by marine microorganisms. Sea ice harbors high amounts of polymeric substances that are produced by cells growing within the seaice brine. Here, we report from a research cruise to the central Arctic Ocean in 2012. Our study shows that microbial polymers accumulate at the air-sea interface when the sea ice melts. Proteinaceous compounds represented the major fraction of polymers supporting the formation of a gelatinous interface microlayer and providing a hitherto unrecognized potential source of marine POA. Our study indicates a novel link between sea ice-ocean and atmosphere that may be sensitive to climate change.

Description: This dataset supports the recent publication "Microplastics increase the marine production of particulate forms of organic matter" by L. Galgani et al (2019) in ERL. It comprises all data presented in the manuscript and Supplementary Information. In the file, Control = mesocosm without plastic addition (1,2,3) and MP = mesocosm with plastic addition (1,2,3). The data proceed from the MESO-Plastic experiment in 2017 conducted at the Hellenic Center for Marine Research, a member of the European Aquacosm network of mesocosm facilities.

Description: The present dataset relates to an experiment studying the impacts of particles on chromophoric dissolved organic matter (CDOM) dynamics by comparing three scenarios: a particle-free environment, a particle-enriched system with polystyrene microplastics, and a particle-enriched system with inorganic particles (water insoluble SiO₂). In experimental part 1 of the experiment, natural marine organic matter was obtained by culturing a non-axenic strain of Chaetoceros socialis in 2 L flasks under each of three scenarios (1C = control, 2PS = polystyrene, 3S = silica). The first dataset reports cell numbers per each 2 L flask for experimental part 1. Subsequently, in experimental part 2, filtered samples from the three flasks containing dissolved organic matter and bacteria were incubated separately in the dark in separate cuvettes (4 replicates per each scenario) for 5 days to monitor changes in dissolved organic matter. In this phase, Chromophoric dissolved organic matter (CDOM), a bulk optical property, was monitored daily to examine changes in its quality and quantity and to compare degradation dynamics in the three systems (1C, 2PS and 3S). The dataset for part 2 reports CDOM absorption coefficient at 355 nm (a355, m^-1^), estimated Dissolved Organic Carbon (DOC, µM), as well as spectral slope parameters in different wavelengths as indicators of CDOM degradation processes and microbial activity: spectral slope Sbetween 302 and 322 nm (S302-322, nm^-1^), between 275 and 295 nm (S275-295, nm^-1^), and between 350 and 400 nm (S350-400, nm^-1^), as well as slope ratio SR, as SR = S275-295:S350-400. At the beginning of experimental part 2 (on day t7), oxygen concentrations were also measured in each cuvette of all of the three scenarios 1C, 2PS and 3S.