Impact of ocean acidification on microbial degradation of organic matter

Marine bacteria play a key role in carbon and nutrient cycles of the ocean. The majority of organic carbon fixed during photosynthesis is rapidly recycled in the upper ocean via microbial degradation. The remaining organic carbon is sequestrated in the deep sea making the ocean a net sink for atmospheric carbon dioxide (CO2). Organic matter in the ocean is a complex mixture of organic compounds and a major portion is continuously transformed chemically from dissolved small molecules to polymers and gels, up to particles and broken back down to small dissolved monomers by microbial activities. Gel particles, in particular, are hotspots of microbial degradation as they provide food and a surface to attach to. Furthermore gel particles accelerate aggregation of particulate organic matter (POM) and play a major role in sedimentation processes in the sea. In order to utilize organic matter, bacteria have to produce extracellular enzymes that operate outside the cell to degrade polymers into smaller compounds. Many enzymes involved in the bacterial hydrolysis of organic matter were shown to be pH-sensitive. As CO2 concentrations in the ocean rise due to anthropogenic CO2 emissions, seawater pH decreases – a process known as ‘ocean acidification’. Since 1750, the ocean’s pH decreased by 0.1 units and a further decrease of around 0.15 - 0.35 units in the surface oceans by the year 2100 is projected. Despite their importance, relatively little is known about in what way marine bacteria may be influenced by ocean acidification. This doctoral thesis addresses the question how microbial degradation of organic matter and organic matter composition may change in the acidifying ocean. The first part of the thesis investigates the response of a natural plankton community to CO2 enrichment during a mesocosm study in a Norwegian fjord. The second part focuses specifically on the filamentous diazotrophic cyanobacterium Nodularia spumigena, which plays an important role in nitrogen and phosphorus cycling in the Baltic Sea. In both studies, elevated CO2 concentrations induced higher bacterial abundances as well as an enhanced accumulation of exopolymer substances such as gel particles and mucus. Stimulated carbon and nitrogen fixation of Nodularia spumigena resulted in higher biomass. Faster enzyme hydrolysis rates increased the microbial turn-over of organic matter in the natural plankton community as well as in the Nodularia spumigena cultures. Recycling of nitrogen or phosphorus from organic compounds by the extracellular enzymes alkaline phosphatase and leucine aminopeptidase was significantly accelerated with decreasing seawater pH, especially after nutrient depletion. This additionally supported microbial growth. During the Nodularia spumigena study, mainly uncharacterized components dominated the decrease in the dissolved organic phosphorus (DOP) pool. Potential compositional changes in the DOP pool under elevated CO2 concentrations could therefore not be detected due to methodological constraints. During the mesocosm study, small compositional changes in amino acids under different CO2 conditions were detected which may be related to stimulated bacterial degradation. These CO2-induced changes in dissolved organic matter (DOM) composition may persist in the future ocean and change the bioavailability of DOM in the long-term. Results of both studies indicate that marine microbial biomass and organic matter cycling will change due to ocean acidification. Enhanced enzymatic hydrolysis of organic matter and increased presence of gel particles may support higher bacterial cell numbers and degradation activities in the acidifying ocean. Generally, ocean acidification may improve the ability of bacteria to use various alternative nutrient sources by increasing nitrogen fixation or enzymatic recycling of organic nutrients. Particularly blooms of diazotrophic cyanobacteria such as Nodularia spumigena may intensify or expand in nitrogen-limited regions or periods in the future. This would provide additional bioavailable nitrogen and enhance the turnover of phosphorus within these regions. Enhanced microbial acquisition of nutrients by hydrolytic enzymes at lowered seawater pH in the surface ocean may decrease the flux of organic nutrients to the deep ocean. In conclusion, I infer that ocean acidification will increase bacterial abundance and degradation activities resulting in faster heterotrophic turn-over of organic matter. This may increase the fraction of organic matter which is degraded at the surface and lower the efficiency of particle export and carbon sequestration fluxes to the ocean depth. However, changes in gel particle formation in the acidifying ocean may counteract this trend increasing particle export and supporting carbon storage in the ocean

A new perspective at the ship-air-sea interface: the environmental impacts of exhaust gas scrubber discharge

Shipping emissions are likely to increase significantly in the coming decades, alongside increasing emphasis on the sustainability and environmental impacts of the maritime transport sector. Exhaust gas cleaning systems (“scrubbers”), using seawater or fresh water as cleaning media for sulfur dioxide, are progressively used by shipping companies to comply with emissions regulations. Little is known about the chemical composition of the scrubber effluent and its ecological consequences for marine life and biogeochemical processes. If scrubbers become a central tool for atmospheric pollution reduction from shipping, modeling, and experimental studies will be necessary to determine the ecological and biogeochemical effects of scrubber wash water discharge on the marine environment. Furthermore, attention must be paid to the regulation and enforcement of environmental protection standards concerning scrubber use. Close collaboration between natural scientists and social scientists is crucial for progress toward sustainable shipping and protection of the marine environment

Marvelous Marine Microgels: On the Distribution and Impact of Gel-Like Particles in the Oceanic Water-Column

Three-dimensional hydrogels of organic polymers have been suggested to affect a variety of processes in the ocean, including element cycling, microbial ecology, food-web dynamics, and air-sea exchange. However, their abundance and distribution in the ocean are hardly known, strongly limiting an assessment of their global significance. As a consequence, marine gels are often disregarded in biogeochemical or ecosystem models. Here, we demonstrate the widespread abundance of microgels in the ocean, from the surface to the deep sea. We exhibit size spectra of two major classes of marine gels, transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP) for three different ocean regimes: (a) Polar Seas, (b) Eastern Boundary Upwelling Systems, and (c) the oligotrophic open ocean. We show the variations of TEP and CSP over the water-column, and compare them to dissolved organic carbon (DOC). We also discuss how the observed distributional patterns inform about productivity and particle dynamics of these distinct oceanic regimes. Finally, we exploit current research topics, where consideration of microgels may give new insight into the role of organic matter for marine biogeochemical processes

Do bacteria thrive when the ocean acidifies? Results from an off-­shore mesocosm study

Marine bacteria are the main consumers of the freshly produced organic matter. In order to meet their carbon demand, bacteria release hydrolytic extracellular enzymes that break down large polymers into small usable subunits. Accordingly, rates of enzymatic hydrolysis have a high potential to affect bacterial organic matter recycling and carbon turnover in the ocean. Many of these enzymatic processes 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 with so-far unknown consequences for microbial physiology, organic matter cycling and marine biogeochemistry. We studied the effects of elevated seawater pCO2 on a natural plankton community during a large-scale mesocosm study in a Norwegian fjord. Nine 25m-long Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) were adjusted to different pCO2 levels ranging from ca. 280 to 3000 µatm by stepwise addition of CO2 saturated seawater. After CO2 addition, samples were taken 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 both phytoplankton blooms, more transparent exopolymer particles were formed in the high pCO2 mesocosms. The total and cell-specific activities of the protein-degrading enzyme leucine aminopeptidase were elevated under low pH conditions. The combination of enhanced enzymatic hydrolysis of organic matter and increased availability of gel particles as substrate supported 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

Effects of ocean acidification on the biogenic composition of the sea-surface microlayer: Results from a mesocosm study

The sea-surface microlayer (SML) is the ocean's uppermost boundary to the atmosphere and in control of climate relevant processes like gas exchange and emission of marine primary organic aerosols (POA). The SML represents a complex surface film including organic components like polysaccharides, proteins, and marine gel particles, and harbors diverse microbial communities. Despite the potential relevance of the SML in ocean-atmosphere interactions, still little is known about its structural characteristics and sensitivity to a changing environment such as increased oceanic uptake of anthropogenic CO2. Here we report results of a large-scale mesocosm study, indicating that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in composition and dynamics of organic matter in the SML. Our results reveal a potential coupling between anthropogenic CO2 emissions and the biogenic properties of the SML, pointing to a hitherto disregarded feedback process between ocean and atmosphere under climate change

Stimulated Bacterial Growth under Elevated p Co2: Results from an Off-Shore Mesocosm Study

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

The Level of Awareness on the Green ICT Concept and Self Directed Learning among Malaysian Facebook Users

AbstractThe Green information and communication technology is introduced to support the implementation of the green environment. Recent initiatives of promoting green technology and green economy which include “green manufacturing hub “green infrastructure”, low carbon emission, efficient use of resources and a healthy, well- educated populace. For this study, the independent variable is the self directed learning readiness while the dependent variable is the level of awareness on Green ICT. The sample size is seventy seven student adult learners. Random sampling is the sampling method used for this study. The study is to highlight the level of awareness among Malaysian Facebook users

Bacterial Colonization and Vertical Distribution of Marine Gel Particles (TEP and CSP) in the Arctic Fram Strait

Gel particles—a class of abundant transparent organic particles—have increasingly gathered attention in marine research. Field studies on the bacterial colonization of marine gels however are still scarce. So far, most studies on respective particles have focused on the upper ocean, while little is known on their occurrence in the deep sea. Here, we report on the vertical distribution of the two most common gel particle types, which are polysaccharide-containing transparent exopolymer particles (TEP) and proteinaceous Coomassie stainable particles (CSP), as well as numbers of bacteria attached to gel particles throughout the water column, from the surface ocean down to the bathypelagial (< 3,000 m). Our study was conducted in the Arctic Fram Strait during northern hemispheres' summer in 2015. Besides data on the bacterial colonization of the two gel particle types (TEP and CSP), we present bacterial densities on different gel particle size classes according to 12 different sampling depths at four sampling locations. Gel particles were frequently abundant at all sampled depths, and their concentrations decreased from the euphotic zone to the dark ocean. They were colonized by bacteria at all sampled water depths with risen importance at the deepest water layers, where fractions of bacteria attached to gel particles (%) increased within the total bacterial community. Due to the omnipresent bacterial colonization of gel particles at all sampled depths in our study, we presume that euphotic production of this type of organic matter may affect microbial species distribution within the whole water column in the Fram Strait, down to the deep sea. Our results raise the question if changes in the bacterial community composition and functioning on gel particles occur over depth, which may affect microbial respiration and remineralization rates of respective particles in different water layers

Inter-annual variability of transparent exopolymer particles in the Arctic Ocean reveals high sensitivity to ecosystem changes

Transparent exopolymer particles (TEP) are a class of marine gel particles and important links between surface ocean biology and atmospheric processes. Derived from marine microorganisms, these particles can facilitate the biological pumping of carbon dioxide to the deep sea, or act as cloud condensation and ice nucleation particles in the atmosphere. Yet, environmental controls on TEP abundance in the ocean are poorly known. Here, we investigated some of these controls during the first multiyear time-series on TEP abundance for the Fram Strait, the Atlantic gateway to the Central Arctic Ocean. Data collected at the Long-Term Ecological Research observatory HAUSGARTEN during 2009 to 2014 indicate a strong biological control with highest abundance co-occurring with the prymnesiophyte Phaeocystis pouchetii. Higher occurrence of P. pouchetii in the Arctic Ocean has previously been related to northward advection of warmer Atlantic waters, which is expected to increase in the future. Our study highlights the role of plankton key species in driving climate relevant processes; thus, changes in plankton distribution need to be accounted for when estimating the ocean’s biogeochemical response to global change