More Light Please: Daphnia Benefit From Light Pollution by Increased Tolerance Toward Cyanobacterial Chymotrypsin Inhibitors

Abstract

Cryptochromes are evolutionary ancient blue-light photoreceptors that are part of the circadian clock in the nervous system of many organisms. Cryptochromes transfer information of the predominant light regime to the clock which results in the fast adjustment to photoperiod. Therefore, the clock is sensitive to light changes and can be affected by anthropogenic Artificial Light At Night (ALAN). This in turn has consequences for clock associated behavioral processes, e.g., diel vertical migration (DVM) of zooplankton. In freshwater ecosystems, the zooplankton genus Daphnia performs DVM in order to escape optically hunting predators and to avoid UV light. Concomitantly, Daphnia experience circadian changes in food-supply during DVM. Daphnia play the keystone role in the carbon-transfer to the next trophic level. Therefore, the whole ecosystem is affected during the occurrence of cyanobacteria blooms as cyanobacteria reduce food quality due to their production of digestive inhibitors (e.g., protease inhibitors). In other organisms, digestion is linked to the circadian clock. If this is also the case for Daphnia, the expression of protease genes should show a rhythmic expression following circadian expression of clock genes (e.g., cryptochrome 2). We tested this hypothesis and demonstrated that gene expression of the clock and of proteases was affected by ALAN. Contrary to our expectations, the activity of one type of proteases (chymotrypsins) was increased by ALAN. This indicates that higher protease activity might improve the diet utilization. Therefore, we treated D. magna with a chymotrypsin-inhibitor producing cyanobacterium and found that ALAN actually led to an increase in Daphnia’s growth rate in comparison to growth on the same cyanobacterium in control light conditions. We conclude that this increased tolerance to protease inhibitors putatively enables Daphnia populations to better control cyanobacterial blooms that produce chymotrypsin inhibitors in the Anthropocene, which is defined by light pollution and by an increase of cyanobacterial blooms due to eutrophication.

Negative Effects of Cyanotoxins and Adaptative Responses of Daphnia

“…Those enzymes include protein phosphatases, carboxypeptidases and serine proteases. The production of protein-cleaving enzymes is probably controlled by Daphnia's circadian clock since they show a rhythmic gene expression pattern over 24 h [56]. Interestingly, the enzyme activity is at a constant level over 24 h and does not follow its gene expression peak.…”

“…Besides climatic changes, further abiotic anthropogenic stressors have the potential to disrupt the general (the "normal") Daphnia-cyanobacteria interaction. It has already been demonstrated that artificial light at night / light pollution changes the expression of Daphnia protease genes and that increased protease activity leads to better control of a protease inhibitor-producing cyanobacterium [56]. Furthermore, increasing sound pollution might be problematic for the control of cyanobacterial blooms: A mesocosm study by Lürling and Tolman [138] showed that ultrasound strongly suppressed Daphnia, thus freeing phytoplankton from grazing control.…”

“…Since cyanobacteria lack both types of those fats, a salinization effect on Daphnia-cyanobacteria can be expected. Furthermore, road salts also disrupt Daphnia's circadian clock gene expression [148], which might influence Daphnia's ability to cope with cyanobacterial protease inhibitors since the gene expression of gut proteases is regulated by the circadian clock [56].…”

RNA sequencing indicates widespread conservation of circadian clocks in marine zooplankton

“…In Daphnia pulex , for example, it was demonstrated that the expression of the Period gene is suppressed in order to enhance salt tolerance caused by high levels of road salt, representing an unusual alteration to circadian functionality in response to environmental stress ( 66 ). Artifical light pollution also appears to affect the clock, with the expression of the Cryptochrome 2 (CRY2) gene being altered in Daphnia magna (with implications for growth and feeding) in response to exposure to artificial light at night ( 67 ). Altered circadian clock functionality can have potentially wide-ranging and far reaching consequences since zooplankton clocks have been implicated in influencing swimming behavior ( 68 ), feeding behavior ( 69 ), photoperiod-induced diapause (a survival strategy) ( 70 ), photoperiod adaptation ( 71 ), and, importantly, DVM ( 72 , 73 ).…”