Cuál es la materia de este Sacramento, y qué pan, puede consagrarse.

The rate of photosynthesis and phytoplankton cell growth is strictly controlled by the availability of the essential resources, and these are light, and dissolved organic and inorganic macro- and micronutrients. In addition, phytoplankton metabolic rates are also mediated by ambient temperature.

Energy for the photosynthesis comes from the absorption of components of the visible light spectrum (referred to as photosynthetically available radiation, PAR), where the level of the available radiation exponentially decays with the depth of the water column. The energy from photons is harvested in cell chloroplasts that include photosynthetic pigments. Depending on the intensity of the available radiation, phytoplankton modify the abundance of pigments in a cell. The pigment abundance increases in light lim- ited conditions in order to optimize the light absorption, and reduces in phytoplankton growing at the well-lit surface to avoid photoinhibition (Falkowski et al., 1985). This adaptation can be observed in the vertical structuring of phytoplankton communities through the formation of a deep chlorophyll maximum (Anderson, 1969; Hickman et al., 2010). Light is considered a main limiting factor for the high-latitude phytoplankton communities (Colijn and Cadée, 2003; Harrison and Li, 2007).

The macronutrients essential for phytoplankton growth are carbon, nitrogen, phos- phorus and silica. Carbon makes up the majority of the cell by forming major cell components, and controls the fundamental functions such as energy storing for repro- duction. Carbon is continuously supplied from the atmosphere and so phytoplankton cell growth is never limited by its insufficient concentration.

facilitating biochemical reactions through enzyme production. Both nitrogen and phos- phorus form nucleic acids that hold the genetic information about the cell and control its reproductive functions (Anderson, 1995). Nitrogen is generally found to be the main limiting macronutrient across oceanic environments (Tyrrell, 1999). Limiting nitrogen concentration in the low-latitude regions (Moore et al., 2013) leads to an increase in the rates of nitrogen fixation, that is an enzymatic reaction converting nitrogen gas into organic form, and therefore primary production in the oligotrophic gyres is controlled by the phosphate availability (Mather et al., 2008).

Silica is widely utilized by diatoms for production of the frustule, a hard cell wall composed mainly of silica. Therefore, in low concentrations, silica inhibits diatom blooms and can be an important factor controlling primary production at high latitudes (Jézéquel et al., 2000).

Iron is a pivotal micronutrient that facilitates the enzymatic reduction of oxidized ni- trogen compound, nitrate, to nitrite. Iron is often found to be a key factor inhibiting macronutrient uptake in the high-latitude ecosystems (Martin, 1990; de Baar et al., 1995; Boyd et al., 2000; Blain et al., 2007; Achterberg et al., 2013). In nitrogen-limited, oligotrophic gyres, low iron supply can limit the process of nitrogen fixation due to the increased iron requirement for nitrogen-fixing diazotrophs (Berman-Frank et al., 2001).

Additionally, other trace metals, such as manganese, zinc, cobalt, copper, cadmium and nickel, and vitamins are incorporated into some proteins and enable enzymatic reactions and thus control phytoplankton growth (Saito et al., 2002; Morel and Price, 2003; Hassler et al., 2012; Sinoir et al., 2012). The B-vitamins cofactor important cellular processes and can be a co-limiting factor of phytoplankton growth in coastal waters (Gobler et al., 2007) and open ocean habitats (Bertrand et al., 2007; Koch et al., 2011). Also, the biosynthesis of the vitamin B12 can be limited by low cobalt

concentrations (Panzeca et al., 2009).

Spatial and temporal variability in essential resources shape global phytoplankton dis- tribution and community structure. In an era of climate change, the environmental conditions that affect phytoplankton communities are predicted to alter through a pos- sible increase of stratification and expansion of the oligotrophic gyres (Sarmiento et al., 2004; Bopp et al., 2005; Irwin and Oliver, 2009). The resulting changes to the avail- ability of essential resources might have crucial implications for the future distribution of phytoplankton biomass and the efficiency of the biological pump (Bopp et al., 2005; Morán et al., 2010).

Diversity of phytoplankton community was found to increase in the environment where many resources limit cell growth simultaneously (Interlandi and Kilham, 2001). Nitro- gen, phosphorus, silica and light are key limiting macronutrients. Trace metals and vitamins indirectly inhibit phytoplankton growth through affecting the efficiency of util- ization of essential nutrients. Overall, there are maybe few tens of potentially limiting resources that may inhibit phytoplankton growth while there are hundreds of species coexisting throughout the year. There are tens to a hundred of dominant phytoplankton species that make up the majority of the community biomass, found to coexist across oceanic provinces (Cermeño et al., 2013) and freshwater lakes (Stomp et al., 2011). In addition, many background species coexist at very low concentrations which often inhibits the ability of detection.

Therefore, the research presented in this thesis implements the view that the num- ber of species coexisting in aquatic ecosystems exceeds the number of essential resources they compete for, and therefore the paradox remains. In reality, the ob- servational evidence whether the paradox occurs within the real world microbial com- munities remains challenging to obtain. The techniques applied for sampling of the

planktonic communities entail significant limitations, where a handful of phytoplankton samples taken at a particular location are taken as a representative of the local com- munity. The difficulty in detection of phytoplankton species surviving at low concen- trations or sampling at the location where some species are temporarily undetectable, makes the precise estimate of the number of coexisting species unfeasible to obtain. Similarly, verification of how phytoplankton growth and their competitive abilities are affected by climatic factors and availability of micro- and macronutrients, including fur- ther implications related to the chemical structuring of molecules, is a continuously developing field of research.