Caffeine’s potential impact on aquatic ecosystems
Caffeine reaches the environment via several means: purification plants, sludge spreading, organic waste composting and coffee-that-has-been-transformed waste treatment. The alkaloid is now found in almost all freshwater environments and was the subject of several researches so as to be utilized as a pesticide against frogs in the Hawaiian Islands between 2006 and 2012 (Marr et al., 2010). This authorization was eventually withdrawn due to the lack of knowledge about side effects on the environment and human health. Coffee grounds, however, continue to be used by individuals as a slug and snail repellent (Hollingsworth, Armstrong, and Campbell 2002). Insecticidal and acaricidal effects are also reported (Russell et al., 1991).
Caffeine degradation is rather fast under the effect of light, but slows down as UV exposure decreases. Despite its ability to photodegrade, increased caffeine concentration is found in most aquatic environments due to ever-growing emissions over time. Concentrations observed in natural environments commonly range between 0.1 and 900 ng / L (Siegener and Chen 2002, Benotti and Brownawell 2007, Rodriguez del Rey, Granek, and Sylvester 2012).
In 2003, scientists raise the alarm, publishing articles on the possible effects of caffeine on living species. Researchers (Sanderson et al., 2003) point out that the concentrations observed in the environment – and their potential impact on living species – ought to go through an official Food and Drug Administration risk assessment. Conversely, in 2008, a publication asserts caffeine’s environmental innocuousness (Moore et al., 2008), but the experiment concentrates on only three species (Ceriodaphnie, Chironome and a fish) which are particularly resistant to caffeine. In 2009, a thesis conducted on the effects of three substances (including caffeine) on different species (Palenske 2009) indicates that in certain Xenopus larval stages, caffeine concentrations commonly found in the environment can result in a heart rate decrease in 20 % of the population. Previous studies already brought the sensitivity of Xenopus to caffeine to the fore (Gaudet-Hull et al 1994, Fort et al 1998). In the end, amphibian-centered studies lay stress on the high sensitivity of this taxonomic group to caffeine, while fish, insect or crustacean-centered studies are less alarming.
The comparative method allows to embrace more data and to put the results into perspective with additional species and substances (Jolliet et al., 2003, Pennington, Payet, and Hauschild 2004, Henderson et al., 2011). AiiDA*’s results analysis (Payet and Hugonnot 2014) makes it possible to clear up scientific studies’ divergent outcomes. These results (reported below) show aquatic species’ average toxicological response. The results of chronic toxicity tests conducted on twenty-one different species are presented and show a significant variability ranging from 0.1 ug / L for the most sensitive species (the Xenopus laevis amphibian), to more than 3g / L for the most resistant species (the Elliptio complanata mollusc).
Figure 1: Average chronic results - Caffeine toxicity tests conducted on 21 species (extracted from AiiDA)
While concentrations commonly reported in the scientific literature are presented in the pink zone, the only amphibians for which we currently have tests are circled in red. The figure shows that amphibians is the most sensitive taxonomic group for caffeine and that its sensitivity is close to the concentrations observed in freshwater nowadays.
We must bear in mind that the tests are performed in laboratories, usually with only one substance (no mixing) and homogenous environment conditions. In real life, species are exposed to a cocktail of substances in highly heterogeneous and variable environments. During risk assessment, safety factors are brought to play to fully take into account this aspect. The figures presented above show that applying safety factors can potentially bring several species closer to the risk zone. This is in line with more recent studies which ask to investigate the issue further by taking into account both heterogeneous ecosystems (e.g. the marine environment) (Dafouz et al., 2018) and mixed substances (Lee et al. Wang 2015, Luo et al., 2019).
All these figures, although public, are not widely spread. This may seem surprising at the hour of environment protection and biodiversity preservation. There are two reasons for that: on the one hand, our societies face major environmental challenges, some of which are clearly identified and necessitate a strong engagement. There is very little room for obscure or of-secondary-importance questions. On the other hand, experience shows that environmental regulations have not operated on waste substances as yet. This type of action is now common for (non-commercial) substances like dioxin when impacts relate to human health or for (commercial) substances like pesticides when impacts relate to the natural environment. In contrast, a derived-from-waste-substance-centered and biodiversity-friendly regulation has not yet seen the light.
This being said, should we advocate coffee grounds composting for all that, at the risk of increasing caffeine concentration in natural environments? That’s a very good question. Damage to biodiversity is invariably multifactorial; this is the case for the global decline of amphibians which has now been observed for three decades (Bishop et al., 2012); no one can earnestly attribute the disappearance of amphibians to caffeine alone. Yet the (specific) sensitivity of amphibians to this substance is established and coffee grounds do not bring compost anything. In this case, it could make sense to incinerate coffee dregs or investigate other uses like mushroom production (see https://grocycle.com/growing-mushrooms-in-coffee-grounds/), ensuring that they do not return to the environment.
*Aquatic Impact Indicator Database
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