Limitaciones, recomendaciones y reflexión final

Contrary to the results of Nägeli and Schanz (1991) and Slauenwhite and Johnson (1996), who found decreases in the surface tension of water samples containing phytoplankton monocultures (range of 8.9 mN m-1 for Clamydomonas rheinhardii and 5.1 mN m-1 for

Oscillatoria agardhii see Table 4.3) and a reduction in bubble surface tension for seawater

containing phytoplankton (range of 5.3 mN m-1 for Nitzschia pungens see Table 4.3), no significant reduction in surface tension was detectable with the SITA f60 tensiometer over the different stages of growth for experiments 2 (range of 0.9 mN m-1) and 4 (range of 0.4 mN m-1), for which surface tension measurements were carried out. This included the investigation of accumulation of surface active substances with time, which was achieved using the auto-mode of the instrument (see section 2.5.8) and which revealed no significant reduction in surface tension within the timeframe of 60 seconds. It can be argued, that most likely this timeframe is not long enough to accumulate sufficient surface active material, especially if it is compared to the potential lifetime of bubbles at increasing BRT (assuming that BRT is a representative value of the lifetime of a bubble). However, test measurements were carried out with dense monocultures of Nitzschia closterium (4.3 ×105 cells ml-1 ± 4.7×104) and Phaeocystis (1.4 × 106 cell ml-1 ± 2.1× 105 cells ml-1) using a different surface tension measurement instrument, the Profile Analysis Tensiometer PAT-1 (Sinterface Technologies), which enables monitoring of surface tension over time spans of several minutes. The principle of the PAT-1 is based on the analysis of the shape of a pendent and sessile drop via a profile fitting technique. The general theory of this measurement method is based on a liquid meniscus which is subjected to gravity, taking a shape which corresponds to the minimum of the total energy of the system (i.e. sum of the bulk plus interfacial energy). The interfacial energy depends on the interfacial tension. The fitting software of the instrument fits a Laplacian curve to the observed drop profile. The experimental profile is compared with the calculated Laplacian curve. From the difference between experimental and theoretical profile, the surface tension can be calculated. Further details on this method are given in Loglio et al. (2001). Results of these test measurements are shown in Figure 4.5 and do not reveal significant changes in surface tension with increasing adsorption time. However, the surface tension of Nitzschia closterium was lower (mean surface tension ~ 72 mN m-1) than the surface tension of Phaeocystis and F/2 nutrient medium (mean surface tension ~ 74-75 mN m-1), indicating that possibly some

surface active substances were produced by Nitzschia closterium which initially lowered the surface tension but did not lead to a further decline in surface tension with increasing adsorption time.

To date few studies have investigated the surface tension of algal samples and Nägeli and Schanz (1991) and Slauenwhite and Johnson (1996) have used different species (including freshwater species) to the ones investigated in this study (see Table 1.3) as well as different measurement methods of surface tension. Nägeli and Schanz used a ring tensiometer and Slauenwhite and Johnson investigated bubble surface tension by spinning single bubbles in a rotating cell. For both studies (Nägeli and Schanz and Slauenwhite and Johnson) the range between minimum and maximum surface tension was significantly greater than for any of the experiments from this study (Table 4.3). Due to the larger sample volume needed for the ring tensiometer method, larger quantities of surfactant may have been available during Nägeli’s and Schanz’ investigations. The influence of algae on surface tension may be species specific, this being another possibility why Nägeli and Schanz and Slauenwhite and Johnson detected an influence. However, especially with regards to

Phaeocystis, it is remarkable and unexpected that no surface activity could be detected, as

this organism is well known for its foaming capacity, described by Lancelot (1995) that in turn is attributed to the release of surface active polysaccharides (Lancelot and Rousseau, 1994; Lancelot, 1995) resulting in regularly occurrences of foam accumulation along Dutch and German North Sea beaches as reported by Bätje and Michaelis (1986). Additionally, Hoagland et al. (1993) state that diatoms are well known to release large amounts of polysaccharides during all stages of growth, many of which are known to be surface active, as discussed by Zutic et al. (1981), Mopper et al. (1995) and Zhou et al. (1998). In all growth experiments DOC concentrations were high but evidently the organic material present in the tank water was not sufficiently surface active to significantly

influence surface tension. One explanation for this may be that the surface active

polysaccharides produced during phytoplankton blooms, that, according to Gershey (1983) should be predominantly of high molecular weight, were rapidly consumed by

heterotrophic bacteria as has been described by Amon and Benner (1994 and 1996). However, Zutic et al. (1981) detected significant surface activity in various marine phytoplankton cultures that were non-axenic. It is possible that despite the lack of change in surface tension, some surfactants were produced during the growth experiments conducted in study that contributed to some extent to the changes in BRT in the tank system but were beyond the detection limits of the tensiometers used.

C h a p te r F o u r - D is cu

Table 4.3 Summary table of minimum and maximum surface tension values and range for experiments 2 and 4 and from the literature.

Experimental results of surface tension measurements Surface tension measurements from literature

Exp. No.

Algal species Temp.

(°C) Sal. Min. surface tension (mN/m) Max. surface tension (mN/m) Range Mean standard deviation

Algal species Min.

surface tension (mN/m) Max. surface tension (mN/m) Range Temp. (°C) Sal. Reference Oscillatoria agardhii 66.5 71.6 5.1 20 0 Nägeli and Schanz (1991) 2 Thalassionema nitzschioides; Skeletonema costatum; Thalsassiosira sp.; Nitzschia closterium 12°C 16.6 73.3 74.2 0.9 0.3 Clamydomonas rheinhardii 62.5 71.4 8.9 20 0 4 Chaetoceros muelleri

18°C 31.0 73.36 73.72 0.4 0.07 Nitzschia pungens 67 72.3 5.3 ? ? Slauenwhite

and Johnson (1996)

Figure 4.5 Changes in surface tension with time for filtered Phaeocystis and (unfiltered) Nitzschia closterium determined with the PAT-1.

PAT-1 measurements were carried out by the Department of Analytical Chemistry – University of Geneva.