## 3 Results and Discussion
### Carbohydrate Composition  
Table 1 shows the carbohydrate levels expressed on an organic matter base. In line with literature data [12; 39], carbohydrates were a major constituent, with initial total carbohydrate concentrations close to 50% of the organic matter (Table 1).  
At the start (\(t_{0}\)), no large differences were observed between the two storage approaches in terms of carbohydrate levels. As expected, glucose was initially the main monosaccharide followed by galactose and xylose. Indeed, floridean starch, a glucose-based polymer, is known to be the main energy storage polymer in _Porphyridium_ sp. [40], while the cell wall polysaccharides and the extracellular polymeric substances have glucose, galactose, and xylose as main monosaccharides [15]. Glucose concentrations decreased during storage, particularly during concentrate storage and during 20 \({}^{\circ}\)C storage. The concentration of galactose, xylose, and glucuronic acid remained equal or increased, where increases were probably due to a loss of other organic compounds during preservation. To better understand the evolution of carbohydrate levels independently from changes of other organic compounds, results were also calculated on ash basis (Supplementary Materials Table S3) as ash can be considered as an inert internal standard in the case of concentrate storage. In this instance, glucose levels dropped drastically during concentrate storage, even at 4 \({}^{\circ}\)C. The other monosaccharides and uronic acid levels decreased to a smaller extent or remained equal. The net decrease of glucose is probably due to the consumption of floridean starch since algae were forced to call on their reserve resources during dark preservation. Carbohydrate consumption can occur fast as significant carbohydrate losses have been observed for other algae species after one night [41] and even after 30 min [42] of dark incubation. The cell wall and extracellular polysaccharide concentrations appeared to be much more stable in the current study as the amounts of galactose and xylose, two of their main building blocks, were little or not affected by cooled concentrate storage (Supplementary Materials Table S3). Hence, this observation cannot explain the changing rheological properties seen for concentrates during cooled storage (Figures 5B and 6B). Although conclusive evidence is not provided here, it is likely that physical interactions including EPS entanglement are needed for viscosity build-up and the formation of a (weak) gel in a _P. purpureum_ concentrate immediately after centrifuge harvesting. Such inter-molecular interactions can be expected to occur slower at low temperatures as they depend on polysaccharide solubility and Brownian motion.  
During dilute culture storage, no or limited changes were observed in carbohydrate composition during 4 and 8 \({}^{\circ}\)C storage (Table 1). During 20 \({}^{\circ}\)C storage, glucose levels had decreased after 14 days, albeit to a much smaller extent than during concentrate storage. It can be concluded that glucose was preferentially degraded during storage, especially during uncooled concentrate storage, and that this has consequences for the final organic matter yield.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Stage Temperature** & **Storage Time** & **Gluces (\%)** & **Calcatose (\%)** & **Xylose (\%)** & **Glucronic Acid (\%)** & **Manuronic Acid (\%)** & **Sum (\%) Organic** \\
**C** & **(Days)** & **Organic Matter)** & **Organic Matter)** & **Organic Matter)** & **Organic Matter)** & **Organic Matter)** & **Matter)** \\ \hline Concontrate storage & 0 & \(33.1\pm 1.0\) & \(8.6\pm 0.3\) & \(6.7\pm 0.2\) & \(0.82\pm 0.01\) & \