To assess the effects of strong solar radiation on microbial carbon and phosphorus metabolism, the functional genes were grouped according to the time of accumulation of solar radiation and analyzed to determine whether there was a significant difference in the functional genes in different time periods.
3.4.1 carbon metabolism
Central carbon metabolism is a major source of energy for all organisms (Eylem et al., 2022) through a complex series of enzymatic reactions that convert sugars into metabolic precursors, which are subsequently used to generate cellular biomass (Noor et al., 2010). Central carbon metabolism mainly consists of the pentose phosphate pathway and the TCA cycle, glycolysis,which also requires the involvement of cofactors such as adenosine triphosphate (ATP), nicotinamide adenine dinucleotide (NADH), and nicotinamide adenine dinucleotide phosphate (NADPH) (Werner et al., 2016). The analysis of the expression of genes involved in central carbon metabolism can help to gain insights into the associations between different carbon metabolic processes in microorganisms under the accumulation of strong solar radiation and to analyze which pathways microorganisms mainly use to metabolize organic matter. It has been suggested that environmental pressures and demands may ultimately determine the use of pathways (Sudarsan et al., 2014).
The related functional genes were predicted based on the PICRUSt2 function, and the data were analyzed in conjunction with the KEGG database (Kyoto Encyclopedia of Genes and Genomes), and the metabolic maps and related gene abundance bubble plots under different solar radiation accumulation are shown in Fig. 7 Fig. 8. This system has functional genes for all enzymes in the Glycolysis,Pentose phosphate pathway, and TCA cycle process.
Glycolysis (M00001) is a series of reactions that occurs in all living cells, converting one molecule of glucose into two molecules of pyruvate while producing ATP. In this system, genes K01624 (FBA), K00134 (GAPDH), K00927 (PGK), and K00873 (PK) were subjected to the inhibitory effect of the accumulation of solar radiation, and their relative abundance tended to be highest prior to the absence of solar irradiation. Except for the genes K01624, K00134, K00927, and K00873, the rest of the genes involved in glycolysis The abundance of genes was promoted by the accumulation of solar radiation, of which K00845 (glk), K00850 (pfkA), and K15634 (gpmB) were important genes encoding glucokinase, 6-phosphofructokinase, and phosphoglycerate mutase, which showed high levels in all the samples and were significantly (p < 0.05) and positively correlated with solar radiation accumulation. In conclusion, functional genes involved in the EMP pathway were significantly affected by solar radiation in different samples, showing significant promotion or repression, and all of them exhibited high absolute abundance (> 40,000).
In the pentose phosphate pathway (M00004), when glucose-6-phosphate is oxidized to ribose-5-phosphate and CO2, NADPH is produced, and this five-carbon sugar and its derivatives are the building blocks of important biomolecules such as ATP, CoA, NAD+, FAD, RNA, and DNA. Thus, the pentose phosphate pathway plays a crucial role in regulating cell growth (Werner et al., 2016; Dashty et al., 2013; Ramos-Martinez et al., 2017). This system predicted all the key genes involved in the pentose phosphate pathway, and genes K07404 (pgl), K01808 (rpiB), and K00615 were significantly up-regulated with increasing time of solar exposure. The significantly up-regulated gene K00615 encodes transketolase, and the rise of its product Glucose-6P also further contributed to the up-regulation of K00850 (pfkA), which was consistent with the changes in gene abundance in the EMP pathway, along with a high absolute abundance (> 70,000).
The EMP pathway produces 2-fold more ATP than the ED pathway (M00008), and most bacteria still prioritize the ED pathway because the ED pathway requires 3-5-fold less enzyme proteins than the EMP pathway for the same flux of sugar metabolism, and it can support a large amount of microbial growth. It has been shown that bacteria choose sugar metabolism pathways with a trade-off between energy production and the amount of protein consumed and usually take the least consumptive pathway to metabolize carbon sources (Yang et al., 2022). Energy-poor anaerobes overwhelmingly rely on higher ATP production from the EMP pathway, and the ED pathway is common in parthenogenetic anaerobes and even more so in aerobic bacteria (Flamholz et al., 2013). Some researchers have found that high levels of metabolites from carbon catabolism can increase the environmental tolerance of bacteria, so it is hypothesized that the adoption of a non-traditional central carbon metabolism pathway (ED instead of EMP) by modern bacteria is a result of their continuous evolution, degradation of environmental pollutants, and constant adaptation to the environment (Sudarsan et al., 2014). In this system, the absolute abundance of genes involved in the ED pathway ranged from 20,000–30,000, with a clear predominance of the EMP pathway (40,000–50,000). Therefore, it was inferred that most bacteria in this system still preferred the traditional EMP pathway.
The next step in the production of energy from glucose under aerobic conditions is the oxidative decarboxylation of pyruvate to produce Acetyl-CoA, which is then fully oxidized to CO2 in the mitochondria in a series of reactions known as the TCA cycle (M00009). The TCA cycle is the ultimate common pathway for oxidizing compounds such as amino acids, fatty acids, and carbohydrates. Its metabolic process generates large amounts of NAD (P)H, FADH2, GTP, and ATP, which serve as energy hubs to supply energy for life activities. The absolute abundance of genes involved in the TCA cycle ranged from 30,000 to 50,000, with significant down-regulation of genes: K01681 (ACO), K00031 (IDH1), K00240 (sdhB), K00242 (sdhD), and K01676, which were not affected by the down-regulation of these genes and did not affect the overall effectiveness of the TCA cycle, as there were alternative genes with the same functions as alternative genes.
Overall, solar radiation accumulation did not significantly alter the ability of microbial communities to degrade organic pollutants; however, the relative abundance of functional genes was significantly different compared to no solar exposure. The integrity of carbon metabolic pathways ensured the degradation of organic pollutants and was elevated after solar irradiation.