Respiration

The CO 2 Balance of Unproductive Aquatic Ecosystems Carlos M. Duarte, * Susana Agustí Science 10 July 1998: Vol no. 5374, pp DOI: /science

Respiration represents the major area of ignorance in our understanding of the global carbon cycle (del Giorgio P.A., Williams P.J. le B. 2005) While there is a considerable body of information on marine productivity (including growth rates for autotrophs and heterotrophs), there is a paucity of data regarding respiration. This disparity is glaring considering that, unlike photosynthesis, respiration is common to all aerobic organisms and occurs at all depths in aquatic environment.

The use of ETS activity as an estimate of respiration rates is based on the reduction of the tetrazolium salt 2-para (iodophenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride (INT), a water-soluble, membrane-permeable salt, which passively penetrates into the cell (Dufour and Colon 1992), by dehydrogenase enzymes present in the ETS, forming insoluble formazan crystals (INT-F). These dense, stable, colored, intracellular formazan granules can be detected spectrophotometrically or observed with bright-field microscopy (Zimmermann et al. 1978; Posch et al. 1997).

Dissolved Oxygen Analysis First devised in 1889, the Winkler method is considered the "gold standard" for measuring the concentration of dissolved oxygen in a sample of water. Through a series of chemical reactions, the O 2 combines with iodine to form a golden yellow chemical. Therefore each oxygen molecule is associated with an iodine molecule, and we can measure oxygen by measuring the iodine. When the iodine is neutralized by the addition of sodium thiosulfate, the golden color disappears, and we can determine how much iodine (hence oxygen) was in the sample. Once the water sample is collected, it is important to "fix" the sample immediately. Phytoplankton, bacteria, and other organisms in the sample can quickly change the oxygen content of the sample through photosynthesis and respiration.

How to fix Important: water for Oxygen analysis has to be sampled as first taking care not to produce any bubble. The first step of the Winkler method is the addition of manganous sulfate (a source of manganese ions) to the sample, quickly followed by the addition of lithium hydroxide (a strong base) and potassium iodide (a source of iodine). Manganese(II) ions liberated from the manganese sulfate are loosely bound with excess hydroxide. Manganese(II) is oxidized to Manganese(III) in the presence of a strong base and binds the dissolved oxygen.

In the presence of the strong base, each oxygen atom binds with a manganese ion to form a manganous hydroxide complex. This reaction creates a pale precipitate that will eventually sink to the bottom of the sample container. Sulfuric or sulfamic acid is added to the solution to reduce the pH and dissolve the precipitate. When this occurs, free iodine is produced at a rate of one iodine molecule per manganese ion. This produces one iodine molecule for each oxygen molecule in the sample. At this point, the sample is "fixed" (all the oxygen converted to iodine) and can be set aside for several hours before final analysis. Free iodine is produced upon acidification of the sample at a rate of one I 2 molecule for each atom of oxygen. Free iodine complexes with excess iodide ions

The final step of the dissolved oxygen measurement is a titration. Titration is a method of determining the concentration of a substance in a solution by adding a second chemical of a known concentration to produce a controlled chemical reaction. In the titration step, sodium thiosulfate is slowly added to the solution until all the iodine is neutralized (color disappears). We can determine how much iodine was in the solution from the amount of thiosulfate added. The iodine/iodide complex is reduced to iodide with thiosulfate Furthermore, because each iodine molecule was produced by the reaction of a single oxygen atom, the amount of thiosulfate added also tells us how much oxygen was in the sample.

In the past a burette was used and the point of color change was detected by eyes. Now we use this automatic device (Titrino)

Dissolved gasses in seawater The gasses dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amount of gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperature increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount of gas dissolved. Inert gasses like Nitrogen and Argon do not take part in the processes of life and are thus not affected by plant and animal life. But non-conservative gases like Oxygen and Carbon dioxide are influenced by sea life. Plants reduce the concentration of Carbon dioxide in the presence of sunlight, whereas animals do the opposite in either light or darkness.

Solubility of Oxygen in the water (table a) as function of partial pressure in each of the two compartments (atmosphere - water). The same for Nitrogen (table b). Usually gas concentration are expressed as % saturation = 100G G 1 a b where G is the observed concentration of the gas and G 1 is the saturation value corresponding to the temperature and salinity.

Temperature0° 12°24° Chlorinity (‰) O 2 N 2 CO 2 O 2 N 2 CO 2 O 2 N 2 CO 2 ml/Lml/Lml/Lml/Lml/Lml/Lml/Lml/L ml/L As temperature increases solubility decreases as well as salinity increases solubility decreases. This means that diluted cold water can absorb more gasses.

Anoxia

Generally at 2 mgL -1 of Oxygen marine organisms flee from the zone (if they can!) at 1 mg L -1 who couldn ’ t escape dies.

Oxygen in the Gulf of Trieste: Usually oversaturated, occasionally at the bottom under-saturated, usually at the end of summer.