## 1 Introduction  
In recent years, the concern over emerging pollutants and their effects on the marine biota metabolism has grown exponentially. Man-made contaminants invariably present new challenges in monitoring efforts and risk prevention [1]. These chemicals, including pesticides, pharmaceuticals, and personal and household care products, are used daily worldwide and their presence in the environment stems from an exponential increase in human activities related to their usage [2]. In the first decade of the 21 st century, more than 50% of the total production of chemicals included environmentally harmful compounds [3]. Furthermore, the speed of technological advances made in synthetic chemical production continues to increase the list of these novel substances [4], raising important questions and concerns about their ecotoxicity and efficient monitoring methodologies. Glyphosate (N-(phosphonomethyl) glycine) is a phosphonate herbicide and the main active ingredient in the commercial mixture Roundup(r), the most used chemical herbicide worldwide [5; 6]. Glyphosate is a broad-spectrum herbicide with a unique mode of action: It acts as a glycine analogue, inhibiting the enzyme 5-enolpyruvateyl-shikimate-3-phosphate synthase (EPSPS) of the shikimate pathway, thus affecting the synthesis of aromatic amino acids [7]. The dramatic increase in its use globally is also associated with the development of glyphosate-tolerant crops that present a tolerant EPSPS synthase and/or a glyphosate metabolism gene [8]. Being relatively unsusceptible to chemical- and photodecomposition, glyphosate can easily reach coastal and marine areas via a multitude of direct and/or diffuse pathways [9]. While pesticides are mainly used in both agriculture and weed control, their persistence in any given aquatic environment can allow them to be carried into remote marine environments, although the exact means by which this occurs is still a subject under debate [10; 11]. Marine phytoplankton is a key biomonitor in many marine trophic webs which, under natural conditions, responds to a wide range of environmental disturbances and contamination events [12; 13; 14; 15; 16] and any impacts at this level are highly likely to lead to bottom-up impacts. Glyphosate was shown to impair cyanobacteria growth at concentrations as low as 50 \(\mu\)g L\({}^{-1}\)[17], and in freshwater, glyphosate influences microbial community structure, changing the community from green algae and diatoms (glyphosate sensitive) to cyanobacteria (glyphosate tolerant) [18; 19]. Moreover, structural changes to marine communities have been associated with glyphosate application [20]. Ultimately, ecotoxicological studies on marine organisms are of the utmost importance to establish guidelines to safeguard local biodiversity and the functioning of estuarine, coastal, and marine environments [21; 22]. Diatoms, as part of phytoplankton, are among the first organisms to be affected by contaminants, quickly responding to suspended toxicants due to their small size (0.2-200 \(\mu\)m) and high uptake rates [13; 23; 24; 25; 26]. Diatoms are also constantly re-adjusting the equilibrium between energy production from photosynthesis and energy consumption under environmental stress conditions, an ability that undermines their success in highly dynamic coastal and estuarine environments [27]. While they are responsible for half of all the photosynthesis on Earth and thus have an important role in the global biological carbon pump and the silica cycle [28], our knowledge of carbon allocation regulation in diatoms is very limited, particularly because of these present distinct metabolic compartments and additional metabolic pathways in comparison to the more widely studied green algae [29; 30; 31; 32]. Diatoms are also major marine producers of specific fatty acids [32], including essential fatty acids (EFA), linoleic acid (omega-6 [u-6] class), and \(\alpha\)-linolenic acid (ALA).