Background:

We thrive to understand the mechanisms regulating the flux of particulate organic carbon (POC) to the depths of the oceans within a changing world. The rate of increase in atmospheric CO₂ concentrations will depend, among other things, on the effectiveness of the biological pump, e.g. the vertical flux of POC from the surface waters to the deep ocean. This POC flux to depth depends both on the formation of particles (e.g. through photosynthesis) in the surface layer and on processes impacting the sinking of POC through deeper layers.

Global climate change will impact physical (temperature, mixing, stratification, dust and iron input), chemical (nutrient availability, pH) and biological (organism dominance, organism function) conditions in the surface ocean and thus impact food web structure, including phytoplankton composition and particle formation. The bioavailability of inorganic and organic macro-nutrients (e.g. nitrate and urea, respectively) and micronutrients (iron), for example determines phytoplankton succession and thus species composition, but details of such mechanisms are currently unknown. Increased UV radiation abiotically changes the concentration of transparent exopolymer particles (TEP), which form abiotically from phytoplankton exudation and are a necessary component of aggregates. Because single phytoplankton cells sink very slowly, the formation of aggregates from TEP, phytoplankton, detritus, fecal pellets and minerals is a prerequisite for high sedimentation rates. Only certain phytoplankton groups, e.g. diatoms are currently known to sediment en masse. The food web structure and environmental conditions determine which fraction of the POC that was  formed, sinks out of the euphotic zone.

The fraction of POC that arrives at the seafloor is a function of the degradation rate relative to the sinking velocity: e.g. POC of faster sinking particles will have a shorter residence and degradation time than more slowly sinking particles. The sinking velocity of large aggregates depends on the relative amounts of TEP and mineral particles (ballast) enclosed in the aggregate, but the exact interactions are not fully understood. Mineral availability as well as the capacity of POC, especially TEP and minerals to bind to each other depends on the pH with the consequence that mineral loading of organic aggregates will change in the future ocean. Whereas availability of biogenic minerals and stickiness of TEP are expected to decrease in the future ocean, dust input is thought to increase. These interactive effects will totally change the depth range of ballasting, and thus the sinking velocity of aggregates and, as a consequence the effectiveness of the biological pump. Changes in the rates and depths of degradation and dissolution (e.g. opal) will affect the recycling and thus the availability of nutrients in the surface ocean.

Project examples:

We work on changes in phytoplankton composition, focusing on the effects of nutrient dynamics, aggregation and on sedimentation of POC out of the upper layer. We also study the mechanisms affecting the flux of carbon below the surface layer, especially the effects of polysaccharides and minerals on sinking, degradation and dissolution.

We are studying the ability of coccolithophorids (important both for the production of POC and biogenic minerals) to grow on organic vs. inorganic macro-nutrients, as the expected increase of temperature will drastically increase stratification and therefore decrease the availability of inorganic nutrients. The addition of iron has been postulated to mediate rising atmospheric CO2 concentration as diatom growth is limited by iron in large areas of the ocean. We are investigating if TEP, released by diatoms may act as iron-ligands, retaining Fe in the surface layer and allowing diatoms to bloom after an iron input event. We are also studying the effect of UV radiation on TEP, as TEP are important both for iron binding and for aggregation. And we are looking at the ability of different phytoplankton groups, other than diatoms to form fast sinking aggregates.

Alga-bacteria interactions and the microbial loop and their effect on the biochemical cycling of elements are also being investigated. Dissolution rates of the opal frustules of diatoms within aggregates are being determined, as it has been shown that processes within aggregates differ to those found in freely suspended material. In a different project, respiratory turnover of organic carbon back to CO₂, and sinking velocity of sinking particles (aggregates or feces) are experimentally determined. Minerals are thought to increase sinking velocity while decreasing microbial degradation rate. Hence, the role of minerals (biogenic or clays) for sinking, degradation and dissolution of organic carbon (ballasting hypothesis) is also quantified by exploring aggregate formation and degradation as a function of phytoplankton species, TEP, and mineral particles available. Parameters on the effect of minerals as well as on degradation and dissolution rates are used in modeling exercises to explore their effect on future POC flux.

The measurements of carbon fluxes through aggregates or feces at a small-scale (≈ mm) are integrated with those of carbon sinking thousands of meters through the ocean interior (> 1000 m) and may be put into relation to measurements on the physiology of carbon flow on a cellular scale (< 10 μm), which are also conducted in our Sektion. All these experimental investigations are combined with modeling and paleoproxy work.