## 3 Results and Discussion
### PHA Production from Xylan and Agarose  
The ability of _Sde_ 2-40, _B. cereus_, and co-culture of _Sde_ 2-40 and _B. cereus_ to produce PHA utilizing xylan and agarose was evaluated using two approaches (i) one step culture in nutrient balanced medium and (ii) two step culture (transfer of cells grown on nutrient balanced medium to nitrogen free medium). The production of PHA using 2% (\(w/v\)) xylan and agarose as carbon sources was first attempted by growing the cells on nutrient balanced conditions in minimal media at 30 \({}^{\circ}\)C. The samples were collected for PHA analyses at 120 h. The PHA content from agarose for _Sde_ 2-40 was 18.1% of dry cell weight (Figure 8A) and the PHA concentration was 0.240 g/L (Figure 8B). For co-culture,  
Figure 7: Gas Chromatography–Mass Spectrometry (GC–MS) analysis of biopolymer obtained from co-culture of _Sde_ 2-40 and _B. cereus_. (**A**) Ion chromatogram; the peaks at 5.806 min are 3-hydroxybutyric acid methyl ester (3 HB-Me); (**B**) Mass spectrum for the sample; (**C**) Reference mass spectrum for 3 HB-Me from the MS library; (**D**) Mass spectrum of the commercial poly(3-hydroxybutyrate) (PHB) (Sigma 363502). The presence of the ions 51, 77, and 105 in the sample indicates PHB synthesis. the PHA content from agarose was 19.7% of dry cell weight and the PHA concentration was 0.295 g/L (Figure 8A,B). Agarose is a neutral linear polysaccharide composed of alternating residues of 3-O-linked \(\beta\)-d-galactopyranose and 4-O-linked 3,6-anhydro-\(\alpha\)-l-galactopyranose. It has been predicted that the agardiytic system of _Sde_ 2-40 harbours agarose breakdown pathway consisting of five agarases [4; 55]. The collective hydrolysis of agarose to simplest sugar units such as 3,6-anhydro-L-galactose and \(\mathcal{D}\)-galactose happens due to these enzymes [55; 56]. A study conducted by [PERSON] et al. [6] using 0.2% agarose demonstrated the production of high levels of 3,6-anhydrogalactose by _Sde_ 2-40. Therefore, we set a control experiment to test the efficiency of _B. cereus_ to utilize galactose, which revealed that _B. cereus_ can grow well on galactose containing media and produce low amounts of PHA (5.3% of dry cell weight) (Figure 9). This suggests that most of the galactose produced in the medium might be used by _Sde_ 2-40 to produce PHA. This may be one of the possible and strong reasons for low PHA content from agarose by co-culture. Figure 8: Production of PHA in one step culture in nutrient balanced medium. (**A**) PHA contents of _Sde_ 2-40, _B. cereus_, and co-culture on agarose and xylan; (**B**) PHA concentrations of _Sde_ 2-40, _B. cereus_, and co-culture on agarose and xylan. Figure 9: PHA production by _B. cereus_ from galactose and xylose. The PHA content from xylan at 120 h by _Sde_ 2-40 was 22.7% of dry cell weight (Figure 8A), and the PHA concentration was 0.198 g/L (Figure 8B). In co-culture, the PHA content at 120 h was 34.5% of dry cell weight (Figure 8A), and the PHA concentration was 0.267 g/L (Figure 8B). _Sde_ 2-40 has the ability to use xylans as a carbon source for growth and energy [1]. _Sde_ 2-40 has a complete system for the degradation of xylan down to monomeric sugars [5; 57; 58]. The high amount of PHA production from xylan in co-culture made us to think that monomeric sugars may be produced extracellularly by _Sde_ 2-40, which might be serving as carbon source for PHA production by _B. cereus_. If this is correct, _B. cereus_ may have the ability to grow on xylose, a monomeric sugar produced by _Sde_ 2-40 using xylan. Therefore, a control experiment was set in order to check the ability of _B. cereus_ to grow and produce PHA using xylose as a carbon source. To our surprise, _B. cereus_ displayed good ability to grow and produce 23.8% PHA of dry cell weight from xylose (Figure 9). This suggests that the released sugars in broth may probably be utilized by _B. cereus_ in co-culture, thereby resulting in high content of PHA from xylan.