Nitrogen availability improves the physiological resilience of coral endosymbiont Cladocopium goreaui to high temperature

Nitrogen source type modulates heat stress response in coral symbiont (Cladocopium goreaui)

Ocean warming due to climate change endangers coral reefs, and regional nitrogen overloading exacerbates the vulnerability of reef-building corals as the dual stress disrupts coral-Symbiodiniaceae mutualism. Different forms of nitrogen may create different interactive effects with thermal stress, but the underlying mechanisms remain elusive. To address the gap, we measured and compared the physiological and transcriptional responses of the Symbiodiniaceae Cladocopium goreaui to heat stress (31°C) when supplied with different types of nitrogen (nitrate, ammonium, or urea). Under heat stress (HS), cell proliferation and photosynthesis of C. goreaui declined, while cell size, lipid storage, and total antioxidant capacity increased, both to varied extents depending on the nitrogen type. Nitrate-cultured cells exhibited the most robust acclimation to HS, as evidenced by the fewest differentially expressed genes (DEGs) and less ROS accumulation, possibly due to activated nitrate reduction and enhanced ascorbate biogenesis. Ammonium-grown cultures exhibited higher algal proliferation and ROS scavenging capacity due to enhanced carotenoid and ascorbate quenching, but potentially reduced host recognizability due to the downregulation of N-glycan biosynthesis genes. Urea utilization led to the greatest ROS accumulation as genes involved in photorespiration, plant respiratory burst oxidase (RBOH), and protein refolding were markedly upregulated, but the greatest cutdown in photosynthate potentially available to corals as evidenced by photoinhibition and selfish lipid storage, indicating detrimental effects of urea overloading. The differential warming nitrogen-type interactive effects documented here has significant implication in coral-Symbiodiniaceae mutualism, which requires further research.IMPORTANCERegional nitrogen pollution exacerbates coral vulnerability to globally rising sea-surface temperature, with different nitrogen types exerting different interactive effects. How this occurs is poorly understood and understudied. This study explored the underlying mechanism by comparing physiological and transcriptional responses of a coral symbiont to heat stress under different nitrogen supplies (nitrate, ammonium, and urea). The results showed some common, significant responses to heat stress as well as some unique, N-source dependent responses. These findings underscore that nitrogen eutrophication is not all the same, the form of nitrogen pollution should be considered in coral conservation, and special attention should be given to urea pollution.

The diversity, community dynamics, and interactions of the microbiome in the world's deepest blue hole: insights into extreme environmental response patterns and tolerance of marine microorganisms

IMPORTANCE

This study comprehensively examined the community dynamics, functional profiles, and interactions of the microbiome in the world's deepest blue hole. The findings revealed a positive correlation between the α-diversities of Symbiodiniaceae and archaea, indicating the potential reliance of Symbiodiniaceae on archaea in an extreme environment resulting from a partial niche overlap. The negative association between the α-diversity and β-diversity of the bacterial community suggested that the change rule of the bacterial community was consistent with the Anna Karenina effects. The core microbiome comprised nine microbial taxa, highlighting their remarkable tolerance and adaptability to sharp environmental gradient variations. Bacteria and archaea played significant roles in carbon, nitrogen, and sulfur cycles, while fungi contributed to carbon metabolism. This study advanced our understanding of the community dynamics, response patterns, and resilience of microorganisms populating the world's deepest blue hole, thereby facilitating further ecological and evolutional exploration of microbiomes in diverse extreme environments.

Comparative growth characteristics and interspecific competitive interaction of two cyanobacteria, Phormidium autumnale and Nostoc sp.

This study examined the growth characteristics and competitive interaction of two cyanobacteria, Phormidium autumnale GJ_2B_I1 and Nostoc sp. DS_2B_I1, which were newly isolated from a southeast river (Nakdong) during the cyanobacterial harmful algal bloom (CyanoHAB) season in Korea. As major environmental parameters, water temperature (25 and 30 °C) and alkalinity (19-78 mg CaCO₃ L^-1 ) and nitrate concentration (1.5-3.5 mg NO₃ -N L^-1 ) were selected based on the water environmental monitoring data during the CyanoHAB season. Unlike P. autumnale, Nostoc sp. has a relatively high growth rate under both monoculture and co-culture and prefers the maximum environmental conditions (30 °C and 78 mg CaCO₃ L^-1 ; pH 9) during the CyanoHAB season. In addition, the growth of P. autumnale is relatively unaffected by alkalinity. Nitrogen (N) stress also has a limiting effect in the interspecific interactions of both cyanobacterial strains. All other cases except for Nostoc sp. in a co-culture showed a considerable increase in growth rate with increasing N content (1.5-3.5 mg NO₃ -N L^-1 ), showing 20-64% under the minimum field conditions (25 °C and 19 mg CaCO₃ L^-1 ; pH 7) and 18-140% under the maximum field conditions. The results show that the growth of P. autumnale can be stimulated by enhanced N stress. On the other hand, Nostoc sp. is less affected by N stress compared with P. autumnale. Therefore, it has excellent potential to be a major group of CyanoHABs because of their relatively high growth rate, particularly in the range of N tested.

Physiological and transcriptomic analyses reveal the threat of herbicides glufosinate and glyphosate to the scleractinian coral Pocillopora damicornis

The amino acid metabolism-related herbicides glufosinate and glyphosate are used worldwide and have flowed into the oceans, threatening the marine organisms. In the present study, physiological activities and transcriptomic profiles of the scleractinian coral Pocillopora damicornis and symbiotic Symbiodiniaceae were determined during a 48 h-exposure to the two herbicides with the final concentration of 10 μmol L-1. Coral samples were collected at 0, 12, 24, and 48 h after exposure to determine symbiont density, chlorophyll content, as well as activities of superoxide dismutase (SOD), catalase (CAT), nitric oxide synthetase (NOS) and phenoloxidase (PO), and the caspase-3 levels, and the samples collected at 24 h were employed in the transcriptomic analysis. Specifically, the symbiont densities did not change significantly in response to the two herbicides, while the chlorophyll content increased significantly at 24 h post glufosinate exposure. SOD and CAT activities in the coral host increased significantly at 12 h after glufosinate and glyphosate exposure, while the activity of NOS in symbionts decreased significantly at 48 h after glufosinate exposure. Caspase-3 levels in the coral host declined significantly at 24 h after exposure to the two herbicides. In the transcriptomic analysis, glufosinate triggered the expression of genes related to the response to stimuli and immunoregulation in the coral host, and suppressed the expression of genes related to coral nitrogen-related metabolism, symbiont cell cycle, and response to nutrient levels. Furthermore, glyphosate activated the expression of genes involved in coral calcification and symbiont nutrient export and suppressed the expression of genes involved in coral meiosis and symbiont cell communication. These results suggest that although the coral-Symbiodiniaceae symbiosis is not disrupted, short-term glufosinate and glyphosate exposures alter several essential physiological processes including metabolism, calcification, and meiosis in the coral host, as well as the cell cycle and nutrient export in the symbiont. SUMMARY: Glufosinate and glyphosate herbicide exposures can disturb several essential physiological processes, including metabolism, calcification, and meiosis in the coral host as well as the cell cycle and nutrient export in the symbiont, threating the survival of scleractinian corals.