Presumably, the processes in the ocean floor are similar to those in the water column 

All these data provide insights into the diverse world of long-lived organic molecules. In this world, microorganisms play a crucial role not only as a source of CO2, but also in its storage: they ingest organic matter and use their tools, highly specific enzymes, to break down certain molecular bonds and release new substances. Among others, the microbiology research groups in Oldenburg and Bremen also study these organisms and their enzymes. Together, the researchers provide valuabe insights into the world of molecules and microbes from different angles.

For example, a comparatively simple experiment conducted by the researchers of the Oldenburg-based Roseobacter Collaborative Research Centre, which recently concluded its activities, illustrates the complexity of the interactions between microorganisms and molecules: the results showed that a single species of bacteria feeding on a single sugar in a lab culture excretes tens of thousands of largely unknown substances.

On the basis of such experiments and observations the researchers have concluded that the long-lived substances are a molecular waste product of enzymatic degradation processes. “The cells actively excrete these substances because they cannot utilise them,” explains Dittmar. According to one hypothesis, some of these substances accumulate because their molecular structure prevents them from being broken down further. However, this hypothesis is called into question by the fact that there are hardly any substances on Earth that microorganisms cannot process.

Consequently, the researchers suspect there is another reason why microorganisms, and especially those in the deep seas, paradoxically do not utilise this abundant food supply. They posit that ingestion, processing and excretion processes produce more and more new compounds in ever lower concentrations. As a result, despite the abundance of molecules, it becomes increasingly difficult for microorganisms to find ones they are able to process.
 

Findings on processes that take place on a small scale cannot simply be extrapolated to global scales

However, with all these projects, the following challenge remains: findings on processes that take place on a small scale cannot simply be extrapolated to regional, let alone global scales, such as the world’s oceans. The interactions in the microbial network are too complex for that. But ultimately, this is the only way to find out what role dissolved organic matter plays in the carbon cycle, and thus for our climate. Given these limitations, modelling expert Sinnika Lennartz takes the findings from detailed studies and identifies the most important processes, then integrates only these simplified findings into her larger models.

This approach helps to shed light on the large-scale distribution patterns of dissolved organic matter in the ocean. The researchers know, for example, that dissolved organic matter accumulates in the nutrient-poor regions of subtropical oceans. Presumably, the microorganisms living in these areas are unable to break down these substances because they lack other nutrients such as nitrogen or phosphorus that are crucial for their growth. “If we factor this into the model, we can reproduce the observed patterns and thus locate large carbon reservoirs in the world’s oceans,” explains Lennartz.

By combining measurements, experiments and modelling, the researchers thus gradually move closer to their goal of better understanding the molecules and their cycling to be able to integrate this knowledge into global climate models. As the size of the dissolved organic carbon pool is enormous, even small changes could have a major impact on the ocean’s ability to store CO2. Whether this is really the case remains to be seen. For Dittmar, at any rate, the quest to understand the invisible traces of life in the deep blue sea continues.

The article was first published in the research magazine EINBLICKE.