## 4 Discussion  
The cell density of _Phaeodactylum tricornutum_ decreased with exposure to increasing concentrations of glyphosate-based herbicide concentrations with an EC\({}_{50}\) of 225.9 \(\upmu\)g L\({}^{-1}\). A comparable effect was observed with similar glyphosate concentrations in freshwater diatoms _Gomphonema parulum_, _Achannthe minutissima_, and _Amphora veneta_[19] and some cyanobacteria such as _Planktonix agardhii_ and _Microcyystis aeruginosa_[70]. Besides the potential direct effect of this herbicide, it is also possible that, due to its chelating properties, the bioavailability of macro- and micronutrients (e.g., divalent cations including calcium, magnesium, manganese, and iron) is reduced, limiting growth [71]. Current  
Figure 11: Canonical analysis plot based on the Kautsky plot (**A**), pigment composition (**B**), and fatty acid profile (**C**) of _Phaeodactylum tricornutum_ following a 48 h glyphosate-based herbicide formulation in different concentrations. Ellipses group samples with lower statistical distance based on Euclidean resemblances. environmental concentrations and the unrestrained use of glyphosate-based products pose a continued threat to diatom populations and likely bottom-up effects in trophic webs that need to be further explored. The effect of the glyphosate-based mixture on _P. tricrontutum_ was very clear in several photochemical-related variables and showed lower photosynthetic efficiency, particularly at the highest concentrations of the glyphosate-based herbicide. However, exposure to the highest concentration led to an increase in all pigments, particularly in chlorophyll content, in opposition to what has been described for higher plants [72] and is likely a compensatory mechanism by which the diatoms counteract the lower efficiency of the affected reaction centers. In other works [73; 74; 75; 76; 77], an increase in chlorophyll \(a\) concentration was observed in algae communities exposed to low concentrations of atrazine (10 to 32 \(\upmu\)g L\({}^{-1}\)) but not at higher concentrations. Thus, this rise in chlorophyll content associated with the increase in fucoxanthin content may provide cells with the ability to counteract the decrease in the number of RC/CS, and therefore an increase in the efficiency of each RC available to trap photons occurs. In _P. tricrontutum_, the PS II light-harvesting complex is composed of the fucoxanthin-chlorophyll protein (FCP) complex, which may deregulate the light-harvesting capacity, increasing both the ability to trap photons and the number of RC available for reduction [46]. However, there was an increase in photoprotective pigments, including \(\beta\)-carotene, diadinoxanthin, and diatoxanthin, which often allows the cells to increase the energy dissipation through non-photochemical processes, preventing possible photoinhibitory events. Moreover, a decrease in the contribution of the dark reactions from Q\({}_{\text{A}}\)\({}^{-}\) to plastocyanin reinforces the hypothesis that glyphosate at higher concentrations may have nefarious effects on the responses studied, mainly at the ETC level in diatoms [45]. Regarding the fatty acid analyses, the most striking result was the significant increase in the total lipid content, which was nearly twice that in control when cells were exposed to the two highest herbicide concentrations. These increases in microalgae lipid contents are commonly associated with stress conditions, such as nutrient limitation or temperature increase [16; 46]. Additionally, exposure to these higher glyphosate-based herbicide concentrations also induced changes in the fatty acid profile of the diatom. It is interesting to note that concomitantly with the increase in the contents of photosynthetic pigments there was also a rise in the relative amounts of 16:3 present in plastitial lipids, namely monogalactosidiacylglycerol, one of the most abundant thylakoid lipids [46]. Changes in chloroplastidial FA composition can lead to alterations of the redox potential [76].