Para Prestadores de Servicio Social (PSS)

The identification of arsenic effects in our biofilms was especially focused on diatoms. Contrary to what several authors highlight about the high sensitivity of diatoms to toxicants among the aerobic photosynthetic microorganisms (e.g. Hill 1996; Corcoll 2011; Prieto et al. 2016c), we have found diatoms to evolve arsenic adaptation under arsenic “acute” exposure (Chapter 1), and becoming more resistant to this metalloid than other microalgal groups (green algae and cyanobacteria). However, this resistance had a cost: a clear decrease of real cell size or cell biovolume and a slight loss of species richness (S) (Chapter 1). Changes in diatom sizes were already announced by Rodriguez-Castro et al. (2015) as an expected effect of arsenic exposure. Cell size decrease in diatoms exposed to high metals was previously detected (e.g. Cattaneo et al. 1998; Cattaneo et al. 2004; Morin and Coste 2006; Luís et al. 2011; Menció et

al. 2016, see Fig. 2) but in some studies theoretical biovolume data are used instead of

performing real cell measurements. Taking real measurements for cell biovolume is time consuming but, in this thesis (Chapter 1), we demonstrated clear differences comparing real with theoretical measurements. We also demonstrated that, contrary to what some authors

suggested (e.g. Lavoie et al. 2006), real cell size provides strong additional and clear information about diatom responses to environmental toxicity. The reasons of this size decrease could lie in a higher cellular division rate during vegetative reproduction, typically under stressed conditions (Morin et al. 2012). However, several environmental factors could also contribute to the cell size decrease on diatoms and other microorganisms, as already commented in the

General Introduction (sub-section 2.5).

Figure 2 Polynomic fitting curve for environmental arsenic concentrations and diatom biovolume (μm3_{) per}

cell parameters is shown, in a field study where aquatic arsenic was gradually decreasing from the natural Can Verdaguer spring. From Menció et al. 2016.

Ecological impact of cell size decrease

It has been suggested that the size of organisms at any trophic level in the aquatic environment can be a determining factor in the ecological efficiency of energy transfer, as well as in the type of organisms living at the highest trophic level. For instance, the yield of fish from a marine ecosystem predominated by phytoplankton with large cells was found to be much greater than that from areas predominated by phytoplankton with small cells (Parsons and Takahashi 1973). In this thesis, we observed that arsenic may decrease the cell size of the diatom community, a main component of the biofilm (Chapter 1), as well as to transform biofilm from a N-rich to a poorer (high C/N ratio) composition (Chapter 3). Regarding the review of Finkel et al. (2010), cell size and elemental stoichiometry often respond predictably to abiotic conditions and follow biophysical rules that link environmental conditions to growth rates, and growth rates to food web interactions and, consequently, to the biogeochemical cycling of elements. Moreover, it was observed that the size structure and elemental composition of the phytoplankton community may have a cascading influence on the proportion of organic material transferred to the microbial loop and higher trophic levels (Finkel et al. 2010). Therefore, since a shift is predicted towards smaller phytoplankton species caused by the global change, leading to a cascading negative effect on the productivity and size structure of the benthic food web (Finkel et al. 2010), it could be also predicted similar consequences on As-affected ecosystems regarding, for instance, the effects of the diatom cell size decrease.

As (pp b) Bi ov /c el l ( µm 3 )

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2.3. Arsenic toxicity to fish

Fish can uptake trace metals by two main routes (Farkas et al. 2003; Terra et al. 2008; Rozon-Ramilo et al. 2011), either by adsorption from water through the gills, and from food absorbed through the digestive tract. The predominant pathways for metal uptake appear to be highly variable over the range of metals, fish species and levels of contamination. The bioavailability and further bioaccumulation of metals in fish depends, thus, on the concentrations in water and the rest of the ecosystem, such as biofilm, invertebrates and sediment, being the last one frequently ingested with food by bottom feeders. Nevertheless, direct proportionality does not necessarily exist between water concentrations and bioaccumulation levels in aquatic organisms (Andres et al. 2000; Yi and Zhang 2012). Arsenic toxicity to fish may be studied using a wide variety of biomarkers ranging from, for instance, molecular analyses such as enzyme activity determination (e.g. Tuulaikhuu et al. 2016) to analyses related to fish physiology and behavior (e.g. Chapter 2). In this thesis (Chapter 2), fish exposed to dissolved arsenic (130 μg L-1 _{or ppb) have become more aggressive and also have increased their weight,}

as well as their arsenic tissue content (around 600 μg g-1 _{or ppb). However, the highest arsenic}

bioaccumulation (almost 800 μg g-1_{) and strongest aggression in fish (leading to a decrease of}

their weight gain) were detected in our study when analyzing their interaction with biofilms, showing the importance of including different trophic levels together on As-impact studies (see below in sub-section 3, about interactions). Several studies have detected biochemical changes and genotoxicity effects on fish due to arsenic exposure (e.g. Castro et al. 2009; Ventura-Lima

et al. 2009; Kumar et al. 2014; Tuulaikhuu et al. 2016), with concentrations ranging from 10 to

100 μg As L-1_{. Under lower aquatic concentrations (around 2 μg As L}-1 _{in water) but with higher}

values in sediments (ranging from 10 to 14 mg kg-1 _{or ppm), consistent negative relationships}

between fish size and environmental arsenic concentrations was detected in different fish species (the small-sized bleak Alburnus alburnus and the Languedoc gudgeon Gobio

occitaniae, as well as the large-sized Ebro barbel Luciobarbus graellsii), which are gregarious

species feeding both on plant material and macroinvertebrates, searching for food mainly on the river bottom (Ebro barbel, gudgeon) and in the water column (especially bleak), and which have reached to bioaccumulate up to 5.6 mg As kg-1 or ppm (average value) in their muscles (Merciai

et al. 2014). Therefore, we may conclude that arsenic may cause toxicity to different fish

species at aquatic concentrations from 2 to 130 μg L-1_{, but the presence of other ecosystem}

compartments such as microorganisms and sediments (e.g. Merciai et al. 2014; Tuulaikhuu et

al. 2016) influences on the fish responses to this arsenic exposure. Regarding arsenic species,

values for fish from the ECOTOX database are set at higher concentrations (Tuulaikhuu 2016), setting the LC50 values at 40.9 mg AsV _L-1 _{and 24.5 mg As}III _L-1_.