Diazotroph Diversity Associated With Scleractinian Corals and Its Relationships With Environmental Variables in the South China Sea

Abstract

Coral reef ecosystems cannot operate normally without an effective nitrogen cycle. For oligotrophic coral reef areas, coral-associated diazotrophs are indispensable participants in the nitrogen cycle. However, the distribution of these diazotrophs and the correlation with the physical and chemical variables of the surrounding seawater remain unclear. To this end, 68 scleractinian coral colonies were sampled from 6 coral reef areas with different environmental variables in the South China Sea to investigate the composition of associated diazotrophs based on nifH gene amplification using highthroughput sequencing. The six coral reefs can be clearly divided into two types (fringing reefs and island reefs), are affected by varying degrees of human activities and are located at different latitudes from 9 • 20'06"N to 22 • 34'55"N with different seawater temperatures. Alpha-and beta-diversity analyses showed that the distribution of diazotrophs among coral reefs exhibited significant geographical fluctuations (p ≤ 0.05) and non-significant interspecific fluctuations (p > 0.05). The predominant bacterial phyla included Proteobacteria, Chlorobi, Cyanobacteria, and two unclassified phyla. Chlorobi exhibited a relative abundance of 47-96% in coral samples from the highlatitude Daya Bay fringing reef affected by eutrophication. Unclassified bacteria II, with a relative abundance of 28-87%, was found in all coral samples from the midlatitude Luhuitou fringing reef affected by eutrophication. However, unclassified bacteria I and Proteobacteria dominated (>80% relative abundance) in most of the coral samples from the Weizhou Island fringing reef, which is far from land, and three island reefs (Huangyan Island, Xinyi Reef, and Sanjiao Reef) at relatively low latitudes. At the genus level, some core diazotrophs were found in different coral sample groups. In addition, correlation analysis with various environmental variables revealed that the variables were positively or negatively correlated with different diazotrophic genera. Coral-associated diazotrophs were common among coral individuals. However, their composition was closely related to the different environmental variables. These results provide insights into the geographical distribution characteristics of coral-associated diazotrophs and their evolutionary trends in response to environmental change in the South China Sea.

Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

“…S5 and S6 ). Given the prevalence of Cluster V nifH sequences across coral species, analyses of nifH diversity or gene copy number that do not differentiate between nifH clusters (e.g., [ 10 , 24 , 101 ]) could overestimate diazotrophic diversity and activity in coral. Moreover, taxa containing Cluster V chlorophyllide genes, such as Ostreobium , are known to bloom during coral bleaching events [ 86 ], which means that particular care must be exercised when analyzing nifH abundance in bleaching studies.…”

Unravelling the Importance of Diazotrophy in Corals – Combined Assessment of Nitrogen Assimilation, Diazotrophic Community and Natural Stable Isotope Signatures

“…A previous study also observed a mismatch with lower DDN assimilation rates in corals where genes involved in N 2 fixation were predicted (using PICRUSt2 ) to be enriched in comparison with other corals ( Lesser et al, 2019 ). Interestingly, Liang et al (2020) indicated that corals are not necessarily associated with diazotrophs for the acquisition of newly fixed nitrogen. They found for several coral species from the South China Sea that the nutrient availability in the reef water could completely meet the nitrogen demand of coral holobionts, even without biological N 2 fixation by coral-associated diazotrophs.…”

“…Such differences in N 2 fixation in coral holobionts may also be linked to changes in the coral-associated diazotrophic community. The composition of these communities is known to vary between coral species, environmental conditions and geographical location ( Lema et al, 2014b ; Zhang et al, 2016 ; Lesser et al, 2018 ; Liang et al, 2020 ). In particular, seasonal shifts of ammonium and nitrate in the surrounding seawater have been suggested to drive seasonal changes in diazotrophic communities associated with corals ( Zhang et al, 2016 ).…”

Remarkable Metabolic Reconfiguration due to N Deficiency and an Ammonium‐to‐Nitrate Shift in the Free‐Living Effrenium voratum (Symbiodiniaceae)

“…The key reason that coral reef ecosystems can flourish in oligotrophic oceans (Muscatine & Porter, 1997) is the efficient cycling of important biogenic elements such as carbon, nitrogen, sulfur, and phosphorus in the symbiotic microbiome (Nakajima et al, 2013). For example, coral-associated diazotrophs are considered indispensable participants in coral reef N cycle (Liang et al, 2020;Olson et al, 2009). In natural environments, the immediately bioavailable forms of N mainly include + is directly used for amino acid synthesis (Glibert et al, 2016).…”