Photosynthetic Behavior of Microalgae in Response to Environmental Factors

Microalgae are prokaryotic or eukaryotic photosynthetic microorganisms that can grow rapidly and live in a wide range of ecosystems extending from terrestrial to aquatic environment. Environmental factors such as temperature, UV-light, irradiance, drought and salinity are known to affect their photosynthesis. Photosynthesis was the most sensitive process in microalgae, leading to numerous changes in structure and function of the photosynthetic apparatus under various conditions. The photosynthetic response may be involved in the modification of energy collector complexes, antennas, or reaction centers and in the distribution of excitation energy between the two photo systems (PSII and PSI). Photosynthesis of micro algae is often inhibited by salt stress (Kirst, 1990). Such an inhibition may be explained by a decrease in PSII activity. Indeed, in the green microalgae, salt stress inhibits PSII activity in Dunaliella tertiolecta and Chlamydomonas reinhardtii that is associated with a state-2 transition (Gilmour et al., 1985, Endo et al., 1995). In the cyanobacteria, it has been suggested that the decreased PSII activity in salt-stressed cells is associated with the state-2 transition (Schubert et al., 1993; Schubert and Hagemann, 1990). It has been reported that salt stress significantly inhibit the maximal efficiency of PSII photochemistry in Spirulina platensis cells and this inhibition is increased with increasing light intensity ( Lu et al., 1999; Lu & Zhang, 1999, 2000). The decreased maximum quantum yield of primary photochemistry (Fv/Fm), interpreted as photodamage (Powles, 1984; Torzillo et al., 1998; Maxwell & Johnson 2000), may be due to an inactivation of PSII reaction centers, an inhibition of electron transport at both donor and acceptor sides of PSII, and a distribution of excitation energy transfer in favor of PSI (Lu & Vonshak, 1999; Lu et al., 1999; Lu & Vonshak, 2002), increasing the cyclic electron flow around PSI (Gilmour et al., 1982; Joset et al., 1996). This decrease is often identified as an adaptive acclimation process of down regulation of PSII, a protective mechanism that helps dissipate excess energy from the photosynthetic apparatus (Allen & Ort, 2001; Tsonev et al., 2003; Hill et al., 2004; Kramer et al., 2004; Hill & Ralph, 2005). In Synechocystis, it has been reported that the decreased PSII activity by salt stress can be explained the fact that salt stress inhibits the repair of photo-damaged PSII by by concealing


Introduction
Microalgae are prokaryotic or eukaryotic photosynthetic microorganisms that can grow rapidly and live in a wide range of ecosystems extending from terrestrial to aquatic environment. Environmental factors such as temperature, UV-light, irradiance, drought and salinity are known to affect their photosynthesis. Photosynthesis was the most sensitive process in microalgae, leading to numerous changes in structure and function of the photosynthetic apparatus under various conditions. The photosynthetic response may be involved in the modification of energy collector complexes, antennas, or reaction centers and in the distribution of excitation energy between the two photo systems (PSII and PSI). Photosynthesis of micro algae is often inhibited by salt stress (Kirst, 1990). Such an inhibition may be explained by a decrease in PSII activity. Indeed, in the green microalgae, salt stress inhibits PSII activity in Dunaliella tertiolecta and Chlamydomonas reinhardtii that is associated with a state-2 transition (Gilmour et al., 1985, Endo et al., 1995. In the cyanobacteria, it has been suggested that the decreased PSII activity in salt-stressed cells is associated with the state-2 transition (Schubert et al., 1993;Schubert and Hagemann, 1990). It has been reported that salt stress significantly inhibit the maximal efficiency of PSII photochemistry in Spirulina platensis cells and this inhibition is increased with increasing light intensity Lu & Zhang, 1999. The decreased maximum quantum yield of primary photochemistry (Fv/Fm), interpreted as photodamage (Powles, 1984;Torzillo et al., 1998;Maxwell & Johnson 2000), may be due to an inactivation of PSII reaction centers, an inhibition of electron transport at both donor and acceptor sides of PSII, and a distribution of excitation energy transfer in favor of PSI Lu & Vonshak, 2002), increasing the cyclic electron flow around PSI (Gilmour et al., 1982;Joset et al., 1996). This decrease is often identified as an adaptive acclimation process of down regulation of PSII, a protective mechanism that helps dissipate excess energy from the photosynthetic apparatus (Allen & Ort, 2001;Hill et al., 2004;Kramer et al., 2004;Hill & Ralph, 2005). In Synechocystis, it has been reported that the decreased PSII activity by salt stress can be explained the fact that salt stress inhibits the repair of photo-damaged PSII by by concealing Fig. 1. Chlorophyll-a concentration of Arthrospira platensis exposed to different NaCl concentrations. Data are mean ± SEM (n = 3).
The photosynthetic efficiency and the light saturated maximal photosynthetic activity were also increased with the increase of salt concentration; however, dark respiration and compensation points were decreased as shown in (Table 1). The light state transition regulates the distribution of absorbed excitation energy between the two photo-systems of photosynthesis under varying environmental conditions. In cyanobacteria, there is evidence of the redistribution of energy absorbed by both chlorophyll and phycobilin pigments. Proposed mechanisms differ in the relative involvement of the two pigment types. Changes in the distribution of excitation energy were assessed using 77K fluorescence emission spectroscopy under excitation of both phycobilin at 570 nm and chlorophyll at 440 nm.  2. Fluorescence emission spectra of excitation at 440 nm (a) and of excitation at 570 nm (b) on Arthrospira platensis exposed to different NaCl concentration. Spectra are normalized at 800 nm The ratios between the peak heights at 685 or 695 and 730nm assigned respectively to Photosystem II (PS II) and photo-system I (PS I) (F685/F730) or (F695/F730) were used to obtain qualitative characteristics. These ratios are considered as a relative indicator of the distribution of excitation energy between PSII and PSI and therefore as an indicator of the state of the cells. The increase of NaCl concentration to 500 mM was accompanied with an enhancement of PSII activity per report to PSI under excitation of both chlorophyll and phycobilin (Fig 2). The ratio between the heights of the peaks at 600 nm and 440 nm, calculated from the emission spectra of PSII at 695 nm (F695,600/F695,440), increased simultaneously with salt concentration of the growth media (Fig 3-a). This effect can be attributed to an increase of www.intechopen.com the energy transfer between phycobilisomes and PSII, which therefore enhanced the PSII fluorescence emission peak at 695 nm. This may also explain the elevated F695/F730 ratio in salt adapted cells of Arthrospira platensis. Additionally, the highest phycobilin/Chlorophyll-a ratio in salt adapted cultures allows the increase of light absorption by phycobilisomes which therefore increases PSII/PSI ratios. Fig. 3. Excitation spectra of PSII emission at 695 nm normalized at 670 nm (a) and PSI emission at 730 nm normalized at 710 nm (b) on Arthrospira platensis exposed to different NaCl concentrations A direct energy transfer from phycobilisomes to PSI was also observed as shown by the ratio between the heights of the peaks at 600 nm and 440 nm calculated from emission spectra of PSI at 730 nm (F730,600/F730,440) (Fig 3-b). These ratios were close to unity, indicating that the cross section of energy transfer from phycobilisomes and chlorophyll-a to PSI was probably equivalent. These ratios remained relatively unchanged with the increase of NaCl concentration in growth media, resulting in a constant fluorescence emission of PSI. Data of maximal efficiency of PSII photochemistry were not in agreement with the above mentioned results. Indeed, Fv/Fm ratio decreased in salt adapted cultures. However, the trapping flux per PSII reaction center (TR 0 /RC) and the probability of electron transport beyond Q A showed change neither at the donor nor at the acceptor sides of PSII. In plants, Fv/Fm was well defined as an index of the maximal photochemical efficiency of PSII (Bjorkman & Demming, 1987). But this interpretation depended on both F 0 and F v originating predominantly from sides of PSII ( This assumption was not valid for cyanobacteria (Buchel & Wilhem, 1993;Papageorgiou & Govindjee, 1968;Papageorgiou, 1996;Schreiber et al., 1986), since phycobilin fluorescence interfered with Chlorophyll fluorescence leading to an increase of F 0 , and therefore to a www.intechopen.com decrease of Fv/Fm values. Consequently, PSII contributed only a small proportion of total chlorophyll (Campbell et al., 1998). These data showed that phycobilin content, compared to the chlorophyll content as assessed by (phycobilin/Chlorophyll-a) ratio, increased with salt concentration and therefore affected the Fv/Fm measured values. Several works studying the effect of salt concentration on physiological behavior of Arthrospira platensis showed different results. Vonshak et al., (1988Vonshak et al., ( , 1995Vonshak et al., ( , 1996, Zeng & Vonshak (1998), Lu et al. (1998 and , 2002 have shown that an increase in salt concentration led to the decrease of the specific growth rate, photosynthetic efficiency, maximum rate of photosynthesis, phycobilin/Chlorophyll-a ratio and PSII activity. Conversely, dark respiration activity, compensation points and PSI activity were increased. These contradictory results might be attributed to different genetic and environmental factors. Indeed, according to Berry et al. (2003), bioenergetic processes in the cytoplasmic membrane, the thylakoid membrane and the cytoplasm exhibited special adaptation strategies of strains like Arthrospira platensis which were mainly realized by using available components from the "tool-box" with different expression levels. In this study, the Compere strain of Arthrospira platensis was used, whereas , 2002 and Pogoryelov et al. (2003) have used, respectively, the M2 and Mayse strains. Changes in the morphology of the trichome (from the helicoidal to the straight form) seem to be accompanied with modifications of physiological behavior of Arthrospira in response to the increase of NaCl concentration in growth media. Indeed, according to Jeeji Bai (1985) and Lewin (1980), the comparative behavior of the two morphones (straight and helicoïdal) in pure cultures showed that NaCl addition over and above the basal level inhibits the growth of the helicoidal morphone, while the straight morphone's growth behavior remained unaffected. Minor variations in culture conditions might induce differences on physiological responses. Indeed, in our study a light intensity of 20 μmol photon m -2 s -1 was used, which was lower than the one used by Vonshak (1999, 2002: 50 μmol photon m -2 s -1 ) and Pogoryelov et al. (2003: 60 μmol photon m -2 s -1 ). These experimental light conditions appeared to be appropriated to maintain growth and full activity of the cells under elevated salinity conditions. Indeed, Zeng & Vonshak (1998) have shown that, at a higher light intensity, growing cells showed lower photosynthetic activity after photo-inhibition under salinity stress, compared with cells growing under lower light intensity conditions. In addition, it has been observed that a 12 h salt stress inhibits electron transport at both the donor and acceptor sides of PSII. However, 2 weeks-old adapted cells showed a down-regulation of PSII reaction centers without any inhibition of electron transfer on the donor and acceptor sides of PSII (Lu & Vonshak, 2002). Moreover, the effect of salt stress on PSII function in Arthrospira cells might be attributed to a direct interaction of high salt with PS II or more complex interaction through unknown cell components, which remain to be studied further. Indeed, the time course of adaptation to salinity stress induced different results.

Combined effect of light intensity and glucose concentration on Arthrospira platensis growth and photosynthetic responses
Arthrospira platensis is a photosynthetic cyanobacterium which is able to convert the energy of sunlight into chemical compounds usable by the cell to fix carbon dioxide and release oxygen. This cyanobacterium was also shown to be able of using organic carbon sources in heterotrophic and mixotrophic culture conditions. (Marquez et al., 1993;Chen et al., 1996; www.intechopen.com Zhang et al., 1999;Vonshak et al., 2000;Chojnacka & Noworyta, 2004;Lodi et al., 2005;Andrade & Costa 2007). Mixotrophic growth offers a possibility of greatly increasing microalgal cell concentration in batch culture (Richmond, 1988;Marquez et al., 1993Marquez et al., , 1995Zhang et al., 1999). In mixotrophic growth, there are two distinctive processes within the cell, photosynthesis and aerobic respiration. The former is influenced by light intensity and the latter is related to the organic substrate concentration. The interaction of light and glucose on specific growth rate was found to follow multiplicative growth kinetics (Chojnacka & Noworyta, 2004). The level of light intensity and glucose concentration and their interaction may influence both autotrophic (photosynthesis) and heterotrophic (oxidative metabolism of glucose) processes and therefore influence cell growth.
In this section, we expose primarily the combined effects of light intensity and glucose concentration on maximal biomass concentration, maximum specific growth rate, maximum net photosynthetic rate, and dark respiration rate and secondly comparative analysis on growth and photosynthetic responses between photoautotrophic and mixotrophic cultures (Ben Dhiab et al., 2010). The effect of light intensity and glucose concentration on growth and photosynthesis was investigated using designs of response surface modelling (RSM) as shown in Table 3.

Experiment Factors Responses
Light (µmol photons m -2 s -1 )  Table 3. Maximal specific growth rate (µ max ), maximal biomass concentration (X max ), net photosynthetic rate (P n ) and dark respiration rate (R d ) for various culture conditions of light intensity and glucose concentration.
Analysis of the results was performed by MODDE. 7.0. The effect of each factor and their interactions was obtained by ANOVA with confidence interval of 90%. Growth was characterized by two responses: maximum biomass concentration (X max ) and maximum specific growth rate (μ max ), whereas photosynthesis was evaluated by maximum net photosynthetic rate (P n ) and dark respiration rate (R d ).
The results showed that Arthrospira platensis grew in the presence of organic substrate (glucose) in the light. Independently of light intensity and glucose concentration, rates of the instantaneous relative growth, net photosynthesis, and dark respiration demonstrated two different phases (Fig. 4). The first phase occurred during the first 3 to 4 days. It was characterized by the highest rates of instantaneous relative growth and net photosynthesis, as well as the preponderance of photosynthetic activity even in mixotrophic cultures, as seen by the increase in pH until the third day. This result might be supported by the data reported by Yang et al. (2000) who found that light was the major source for ATP production in the early phase of mixotrophic cultivation.
The second phase occurred from the fourth day onwards. It was characterized by the decrease of the instantaneous relative growth rates. In photoautotrophic cultures, rates of maximal net photosynthesis and dark respiration were maintained at constant values which are lower than those observed during the first phase. However, in mixotrophic cultures, net photosynthetic rate was reduced to negative values and dark respiration rate increased. Thus, we suggest that metabolic activity was based essentially on photoheterotrophy. This suggestion is supported by the decrease of pH during this phase, due to the release of carbon dioxide, caused by the heterotrophic component of mixotrophic metabolism as reported by Hase et al. (2000). These results are in contrast with those observed by Chen & Zhang (1997) who showed that heterotrophic metabolism dominated in the first phase, then decreased subsequently as the glucose was consumed, and later photosynthesis became predominant. The same comment was signaled by Andrade & Costa (2007). Indeed, these suggestions were based only on the glucose consumption and phycocyanin content. Our results showed effectively that the total glucose in the medium was consumed during the first 3 days, considered as the photoautotrophic phase and characterized by the highest net photosynthetic rate. Thus, we hypothesise that glucose was stored as reserve carbohydrate (glycogen) to be metabolized in the second phase considered as heterotrophic. Indeed, Pelroy et al. (1972) showed in Synechocystis sp. that exogenous glucose is mainly stored as glycogen under illumination before being metabolized for the maintenance of cells . Our hypothesis is supported by results of Martinez and Orus (1991) who noted that respiratory rate was noticeably enhanced in mixotrophic cultures, reflecting the increasing rate of glucose metabolism after the induction of glucose uptake ability. Moreover, in the second phase and additional to the decrease of the net photosynthetic rate, we noted a reduction of the photochemical efficiency of photosystem II accompanied by lower electron transport rate. Therefore, organic carbon sources reduced the photosynthetic efficiency in this phase, and the enhancement of biomass of Arthrospira platensis implied that organic sources had more pronounced effects on respiration than on photosynthesis. These conclusions are in agreement with results obtained with Phaeodactylum tricornutum under mixotrophic culture (Liu et al., 2009). All the above results confirmed that, in the second phase, Arthrospira platensis might use and metabolize glucose and then shift to the heterotrophic nutrition mode. Comparison between mixotrophic and autotrophic cultures showed that the former were characterized by the highest values of the instantaneous relative growth rates and maximal biomass concentration. These results are consistent with those obtained by Marquez et al. (1993Marquez et al. ( , 1995, Vonshak et al. (2000), Zhang et al. (1999), and Andrade and Costa (2007). Marquez et al. (1993) and Hata et al. (2000) suggested that in mixotrophic culture, a simultaneous uptake of organic compounds and CO2 takes place as carbon sources for cell synthesis, and it is then expected that CO 2 will be released via respiration and will be rapidly trapped and reused under sufficient light intensity. Thus, mixotrophic cells acquire the energy by catabolizing organic compounds via respiration and converting light energy into chemical energy via photosynthesis (Hata et al., 2000). Effectively, both photosynthetic and dark respiration rates were the highest in mixotrophic cultures, as observed by Vonshak et al. (2000) in Arthrsopira platensis, Kang et al. (2004) in Synechococcus sp., Yu et al. (2009) in Nostoc flagelliforme, and Orus et al. (1991) in Chlorella vulgaris. The biomass gain recorded in mixotrophic cultures was achieved in the second phase characterized by the nutritive salt reduction and the decrease of the available photons for cells as a consequence of shading caused by the increase in cell density. Such conditions seem to stimulate heterotrophic growth which gives the possibility to increase biomass concentration (Chen et al., 1996;Chen & Zhang, 1997). Martinez et al. (1997) indicated in Chlorella pyrenoidosa that light contributes the energy needed for growth and cell maintenance, while glucose is used for the formation of biomass. Furthermore, the results of Yu et al. (2009) demonstrated that, during the first 4 days, the cell concentration in mixotrophic culture of Nostoc flagelliforme was lower than the sum of those in photoautotrophic and in heterotrophic culture. However, from the fifth day, the cell concentration in mixotrophic culture surpassed the sum of those obtained from the other two trophic modes. The reason for this phenomenon requires further investigation. Growth and photosynthetic activity of mixotrophic cultures were evaluated with respect to light intensity and glucose concentration using response surface methodology as shown in Fig 5. The experimental design showed that polynomial models dependent on light intensity and glucose concentration could describe relatively accurately the maximal biomass concentration, maximum specific growth rate, and maximum net photosynthetic rate.
The results showed clearly that all these responses were influenced by both factors. Furthermore, the interaction of both factors showed that at low light intensity, glucose had a low effect on maximum biomass concentration and maximum net photosynthetic rate.

www.intechopen.com
However, at the highest light intensity (150 μmol photons m -2 s -1 ), the effect of glucose was positive and the responses were more sensitive to it. The effect of this organic carbon substrate might be attributed to the protective role of glucose or to the shift in light intensity at which photo-inhibition occurs, as explained by Chojnacka & Marquez-Rocha (2004). Thus, cells growing heterotrophically might use part of the O 2 produced by cells growing photoautotrophically, decreasing dissolved oxygen concentration; this can help reduce photooxidative damage. As has been observed in the response surface plot, the maximum biomass concentration (1.33 gL -1 ) was obtained at the highest light intensity (150 μmol photons m -2 s -1 ) and glucose concentration (2.5 gL -1 ). The same conditions improved maximum specific growth rate (0.49 day -1 ) and maximum net photosynthetic rate (139.89 μmol μmol O 2 mg Chl -1 h -1 ). Conditions favouring high biomass production and maximum net photosynthetic rate were, however, not optimal. Indeed, the optimal conditions of light intensity and glucose concentration are not achieved in the experimental range used in this www.intechopen.com study. Therefore, further studies that extend the experimental range of light intensity and glucose concentration might be required to reveal optimal conditions that maximize growth and photosynthesis of mixotrophic cultures. For the same species, Zhang et al. (1999), using a number of mathematical models, found that the optimal initial glucose concentration and light intensity were 2.5 g L -1 and 48 μmol photons m -2 s -1 (4 klx), respectively. However, Chojnacka (2003) determined optimal growth parameters to be 2.5 gL -1 glucose concentration and 126 μmol photons m -2 s -1 (10.5 klx) light intensity. This difference in the optimal light intensity, as commented by Chojnacka & Marquez-Rocha (2004), could be due to the different methods of light intensity measurement and distribution of cells inside the culture vessels. Considering all the findings drawn from the experimental design, it is also recommended that data from batch cultures should be further examined to develop much more accurate models.

Combined effect of temperature, light intensity and C/N on photo system II in Cosmarium. sp isolated from Tunisian geothermal using polyphasic rise of fluorescence transients
Hot spring microalgae are exposed to daily and seasonal fluctuations in temperature and light, which are the major factors determining photosynthetic and growth rates. These factors intensities may disturb the balance between the absorption of energy through photosynthesis and the ability to utilize this energy. Such conditions may cause higher excitation pressure on PSII that result in damage to the photosynthetic apparatus (Huner et al., 1998;Yamamoto, 2001;Wilson et al., 2006). Within the photosynthetic apparatus, photo-system II (PSII) is the most sensitive component of the electron transport chain (Cajanek et al., 1998). Damage to PSII is often the first manifestation of stress. Among partial reactions of PSII, the oxygen evolving complex (OEC) is particularly heat sensitive (Georgieva et al., 2000). Numerous studies have shown that high temperature stress has various effects on PSII function. Several major regulatory mechanisms may be involved in the protection of the PSII apparatus from photo-damage under stress conditions: photochemical processes related to electron transport, non photochemical processes by which excess energy is dissipated as heat fluorescence or transferred to other systems, and modification in D1 protein turnover under excess energy stress (Prasil et al., 1992;Campbell et al., 1998;Huner et al., 1998;Melis, 1999;Adams et al., 2001;Yamamoto, 2001;Tsonev & Hikosaka, 2003). Polyphasic rise in Chlorophyll-a fluorescence (OJIP test) has been used as a tool to evaluate modifications in PSII photochemistry in a wide range of studies, not only in higher plants (Tomek et al., 2001;Ban Dar & Leu, 2003;Force et al., 2003;Zhu et al., 2005;Strauss et al., 2006) but also in algae (Hill et al., 2004;Hill & Ralph, 2005;Kruskopf & Flynn, 2006) and cyanobacteria (Strasser et al., , 2004Qiu et al., 2004;Lazar, 2006). This tool has been used in a variety of studies, including structure and function of the photosynthetic apparatus, characterization of vitality and physiological condition, and selection of species tolerant to stress conditions (Hermans et al., 2003, Goncalves & Santos, 2005. Fluorescence induction kinetics (Kautsky curve) of all photosynthetic organisms show a polyphasic rise between initial (F0) and maximum (F m ) fluorescence yields Srivastava et al., 1999). These phases were designed as O, J, I and P, and can be visualized using a logarithmic time scale. By monitoring www.intechopen.com fluorescence transients and quenching, it is possible to obtain information on the absorption, transfer and dissipation of energy by PSII.
In this section we study the response of photo-system II under combined environmental factors of temperature, light intensity and Carbon/Nitrogen ratio in Cosmarium sp isolated from Tunisian hot spring using Polyphasic rise of fluorescence transients. Analysis of fluorescence transients in Cosmarium sp might be provide information about changes taking place in the structure, conformation and functional of the photosynthetic apparatus, especially in PSII under these environmental factors. The increase of temperature from 20°C to 60°C causes a significant decrease in the minimal (F 0 ) and maximal (F m ) fluorescence (Fig 6). A decrease in the fluorescence yield in Cosmarium cells can be attributed to an inhibition of electron flow at oxidizing site of PSII (Lu & Vonshak, 2002). The decrease in F m and fluorescence at J, I, P may be due to two reasons, first by inhibition of electron transport at the donor side of the PSII which results in the accumulation of P680 + (Govindjee, 1995; and second due to a decrease in the pool size of Q A -. The same results were observed in Synechocystis cells exposed to Sb at 10 mg L -1 . In fact F m was decreased and the shape of J-I-P phase became flat with the increase of Sb concentration, indicating that PSII were partially inactivated and could not be closed, and the reduction of PQ (non-photochemical phase) was inhibited (Zhang et al., 2010). In Spirulina platensis, Zhao et al. (2008) showed that F m decreased but F 0 increased significantly with increasing temperature. A loss of the JI-phase at higher temperatures suggests that heat stress resulted in the destruction of the oxygen-evolving complex which may be due to a loss of the manganese cluster activity. The reduction in the J and P steps observed in the Kenya strain of Arthrospira platensis grown at low temperature reflects a bigger decline in the concentration of Q A , Q B 2-, and plastoquinol (PQH 2 ) and therefore a decrease in the accumulation of reduced Q A (Govindjee, 2004;Zhu et al., 2005;Lazar 2006) as well as a decrease in the concentration of active reaction centers (Govindjee, 2004;Zhu et al., 2005;Lazar, 2006). At the donor side, the quenching effect of the variable fluorescence yield at J, I and P was also ascribed to the deterioration of the water splitting system (Strasser, 1997). Moreover, the efficiency of the water-splitting complex on the donor side of PSII as expressed from (F v /F 0 ) (Schreiber et al., 1994) is the most sensitive component in the photosynthetic electron transport chain. In this study the increase of temperature from 20°C to 60°C decreased this ratio from 2.71 to 0.14. This decrease results from photosynthetic electron transport impairment as indicated by Pereira et al. (2000). The same results were also observed in barely under salt concentration (Kalaji et al., 2011) and in Synechocystis sp. exposed to Sb (Zhang et al., 2010). Area over the fluorescence induction curve between F 0 and F m is proportional to the pool size of the electron acceptor Q A on the reducing side of PSII. If the electron transfer from reaction center to quinone pool is blocked, this area will be dramatically reduced (Strasser et al., 2000). As compared to the initial inoculums, the area over the fluorescence curve was decreased with increasing temperature at 60°C to reach ratios of 0.28 and 0.65. However these ratios were augmented to 3.5 and 4.24 times in the lowest temperature at 20°C. This decrease in area over the fluorescence curve suggests that high temperature inhibits the electron transfer rates at the donor side of PSII. This inhibition becomes most considerable under the interaction of the highest temperature and light intensity (60°C/130µmol photons m -2 s -1 ) which lead to the blockage of the electron transfer from reaction center to quinone www.intechopen.com To localize the action of high temperature in electron transport chain on acceptor side of PSII, the kinetics of relative variable fluorescence (V j ), was calculated. V j is equivalent to (F j -F 0 /F m -F 0 ). F j is the fluorescence at J step at 2 ms, relative variable fluorescence (V j ) at 2 ms for unconnected PS II units, equals to the fraction of closed RCs at J step expressed as proportion of the total number of the RCs that can be closed (Force et al., 2003). Efficiency with which a trapped exciton can move an electron in to the electron transport chain further than Q A -( 0 = ET 0 /TR 0 ) was also measured. Increasing temperature from 20°C to 60°C provoke an increase of V j from 85% to 147% and a decrease of  0 from 112% to 62%. Thus, revealed a loss of Q A -reoxidation capacity and an inhibition of electron transport at the acceptor side of PSII and also beyond Q A -. The same results were showed in wheat leaves , Spirulina platensis (Lu & Vonshak, 2002) and Synechocystis sp. (Zhang et al., 2010) treated respectively with 0.5 M NaCl, 0.8M NaCl and antimony potassium tartrate (Sb).
Results of specific energy fluxes demonstrated that an increase of temperature from 20°C to 60°C provoked the raise of the effective antenna size per reaction center (ABS/RC) (from 33% to 74.5%), the trapping energy flux per reaction center (from 78% to 493%) and dissipated energy flux per reaction center (from 13% to 936%). The ratio of ABS/RC demonstrates average antenna size and expresses the total absorption of PS II antenna chlorophylls divided by the number of active (in the sense of Q A reducing) reaction centers. Therefore, the antenna of inactivated reaction centers are mathematically added to the antenna of the active reaction centers. Consequently, the increase of this ratio was justified by an inactivation of some active reaction centers. TR 0 /RC ratio represents the maximal rate by which an exciton is trapped by the RC resulting in the reduction of Q A . So it refers only to the active (Q A to Q A -) centers (Force et al. 2003). The increase in this ratio indicates that all the Q A has been reduced but it is not able to oxidize back due to stress, as a result the reoxidation of Q A -is inhibited so that Q A cannot transfer electrons efficiently to Q B . DI 0 /RC corresponds to the ratio of the total dissipation of untrapped excitation energy from all RCs with respect to the number of active RCs. Dissipation may occurs as heat, fluorescence and energy transfer to other syste m s . I t i s i n f l u e n c e d b y t h e r a t i o s o f active/inactive RCs. The ratio of total dissipation to the amount of active RCs increased (DI 0 /RC) due to the high dissipation of the active RCs. These ratios conclusively describe that the number of inactive centers have increased due to high temperature in Cosmarium sp. These results were confirmed by the decrease in the concentration of active PSII reaction centers (RC/CSm) from 315% to 5% at 60°C, resulted in a similar decrease in maximum quantum yield for primary photochemistry (φ P0 ) and in the quantum yield of electron transport (φ E0 ) respectively from 239% to 50% and from www.intechopen.com 264% to 33%. Furthermore the increase of temperature from 20°C to 60°C provokes the decrease in the performance index (PI ABS ) from 2361% to 122% which indicates the sample vitality. Fig. 8. Spider plot of specific energy fluxes parameters All these results were in line with those observed by Mathur et al. (2010) and Mehta et al. (2010) in wheat leaves of Triticum aestivum as a response to high salt stress. According to Zhao et al. (2008), heat stress in Spirulina platensis resulted in a decrease in RC/ABS, suggesting further a decrease in the total number of active reaction center. It should be pointed out that the expression RC/ABS refers to Q A reducing reaction centers of PSII Strasser et al., 2000). The decrease in RC/ABS in heat-stressed Spirulina cells also indicates a decrease in the total number of Q A reducing reaction centers of PSII. These results further indicate that heat stress caused a shift of the equilibrium towards Q A , making the electrons tend to stay longer on Q A , suggesting that heat stress induced an increase in the Q B non-reducing reaction centers of PSII. Dissipation in this context refers to the loss of absorbed energy through heat, fluorescence and energy transfer to other systems (Strasser et al., 2000), and is represented by the equation DI 0 /RC= (ABS/RC)-(TR 0 /RC). Therefore, dissipation can be thought of as the absorption of photons in excess of what can be trapped by the reaction center. In illuminated Monstera leaves, the excitation energy in the antenna of the active reaction centers was in excess of that required for trapping and some energy was dissipated.

www.intechopen.com
The energy pipeline model as an effect of elevated temperature was deduced using biolyzer HP3 software. This model gives information about the efficiency of flow of energy from antennae to the electron transport chain components through the RC of PSII. The area or the width of the arrows for each of the parameters, ABS/CS 0 , TR 0 ⁄CS 0 , ET 0 ⁄CS 0 and DI 0 ⁄CS 0 , indicate the efficiency of light absorption, trapping, electron transport and dissipation per cross-section of PSII, respectively, in Cosmarium sp. treated in 20°C and 60°C. The effect of elevated temperature on Cosmarium cells was represented phenomenologically per excited cross-section (CS) area (Fig. 9). Results showed a decrease in ABS⁄CS 0 , ET 0 ⁄CS 0 under elevated temperature (60°C). Indeed, ABS⁄CS 0 describes the number of photons absorbed by antenna molecules of active and inactive PSII RCs over the excited cross-section of the tested sample and is represented by the dark-adapted F 0 . The ABS⁄CS 0 can be substituted as an approximation by the fluorescence intensity, F 0 . A decrease in ABS⁄CS 0 at high temperature indicates a decrease in the energy absorbed per excited cross-section. ET 0 ⁄CS 0 represents electron transport in a PSII cross-section and indicates the rate of reoxidation of reduced Q A via electron transport over a cross-section of active RCs (Force et al., 2003). A decrease in this ratio indicates inactivation of RC complexes and the Oxygen Evolving Complex (OEC) and also suggests that the donor side of PSII has been affected. DI 0 ⁄CS 0 , represents the total dissipation measured over the cross-section of the sample that contains active and inactive RCs. A decrease in the density of active RCs (indicated as open circles) and an increase in the density of inactive RCs (indicated as filled circles) was observed in response to elevated temperature. Dissipation occurs as heat, fluorescence and energy transfer to other systems. An increase in energy dissipation at high temperature indicates that energy available for photochemistry is reduced under stress conditions (Strasser, 1987;Strasser et al., 1996Strasser et al., , 2000Kruger et al., 1997;Force et al., 2003).

Conclusion
In conclusion, our findings showed that morphological change of Arthrospira platensis trichome from helicoidal to the straight morphone was accompanied with physiological modifications as response to salt stress. Indeed, the increase of NaCl concentration in growth media to 500mM enhances the growth, the photosynthetic efficiency and PSII activity as compared to 17mM cultures. The elevated PSII activity might be attributed to the increase of phycobilin content which led to an increase of light absorption by phycobilisomes relative to that of chlorophyll-a and therefore an increase of the energy transfer between phycobilisomes and PSII. Furthermore, the fluorescence emission spectra of phycobilin excitation showed that the energy in excess might be directly dissipated by fluorescence of phycobilin in order to protect the PSII against the photo inhibition. The straight form seems to be more tolerant to salt; it maintains full activity under 500 mM NaCl medium whereas the optimal growth medium for the helicoidal morphone is 17 mM NaCl. Therefore, a better understanding of salt stress on PSII may help optimize the productivity of the microalgal cultures grown outdoors. In order to increase and optimize the biomass productivity, this strain was performed under two trophic modes (autotrophic and mixotrophic) using factorial design. As revealed by the kinetics of maximal net photosynthetic, dark respiration rates and instantaneous growth rates, mixotrophic cultures showed two phases. The first was distinguished by the preponderance of the photoautotrophic mode while the second was based mainly on photoheterotrophy. The synergistic effect of photosynthesis and glucose oxidation enhanced the growth rate and the biomass concentration of Arthrospira platensis under mixotrophic mode as compared to autotrophic mode. The combined effect of temperature, light intensity and C/N ratio on Cosmarium sp demonstrate that Cosmarium cells isolated from hot spring water maintained their maximum photochemical efficiency under 20°C. Nevertheless this specie was acclimated and withstood under 60°C by down-regulating electron transport at both donor and acceptor sides of PSII. Otherwise, the conversion of some active reaction centers of PSII into inactive form provides a protective mechanism for quenching excessive energy. Indeed the excitation energy in excess would be dissipated as heat and fluorescence or transferred to other systems via electron transport pathways such respiration or state transition between PSII and PSI suggested to be a part of the acclimation mechanisms to environmental stress. Ultimately to withstand and grow under harsh environments or extreme conditions, several adaptive mechanisms might be developed. These mechanisms were accompanied with their ability to adjust their photosynthetic apparatus in order to acclimate to the prevailing environmental conditions.