Understanding of Microbial Processes on Organic Matter Aggregates: A hidden world of extraordinary complexity
In this talk I will present the guiding hypotheses of my DFG Eigene Stelle and some of the result that led to them, with the intention to stimulate possible collaboration on the topic within the IGB.
Downward fluxes of particulate organic matter (POM) are the major process for sequestering atmospheric CO2 into aquatic sediments with budget calculations heavily based on the ratio between carbon export and remineralization. Currently microbial dynamics on POM is determined using closed vessels, which are strongly biased towards heterotrophy. We developed a flow-through rolling tank for long term studies that continuously maintains POM at near in-situ conditions. There, bacterial communities resembled in-situ communities and greatly differed from those in closed systems. Photosynthesis and respiration on individual aggregates in the open system were high for 9 days, double than in a closed system. Using transcriptomics we assessed initial microbial colonization of aggregates and whether gene expression corroborates rapid changes in carbon-quality. Current understanding is that aggregate composition, structure and surface properties dictate initial microbial colonization followed by rapid succession events as organic matter lability and nutrient content change during degradation. We used replicate samples of 3–4 aggregates of identical source and age. The active microbial communities were highly heterogeneous despite an identical aggregate source, suggesting random initial colonization. This phenomenon is under further investigation comparing large numbers of individual aggregates of different sources. Expression of carbon utilization genes didn’t change after 8-9 days of incubation. Consequently, we suggest that in nature, changes in aggregate-associated community related to carbon availability are much slower (days to weeks) due to constant supply of labile, easily degradable organic matter, as also suggested by the prolonged high-respiration period. Initial, random aggregate colonization seems to be followed by multiple organismic interactions shaping community structural and functional dynamics. These results call for reevaluating our mechanistic concepts on microbial activity on POM as well as any experimental data on microbial activity on aggregates derived from closed systems.
…and in less fancy words
It is important to correctly estimate how much carbon sinks to the deep ocean and how much is respired and release back to the atmosphere. The methodology we choose has a large effect on our results and the way we interpret them. As such, experiments conducted in closed bottles are known to heavily bias the results. We developed a new tool to study organic matter aggregates that allows us to conduct long term, controlled experiments, while continuously exposing the aggregates to water from their natural environment without losing them from the incubation chamber. We could show that in such open system the microbial degradation of aggregates is at least two times slower than in closed environments, suggesting that more carbon can sink than suggested by similar experiments conducted in closed bottles. Additionally, we show that even similar aggregates differ a lot between each other in the nature of the microbial community that colonizes them. Last, we suggest that the first rounds of microbial succession on aggregates are not driven by the type of available resources, i.e. quality of food, but rather by a general battle over resources between all parties.
Host: Hans-Peter Grossart