The Perennial Blooming of MGII and Their Correlation with MGI in the Pearl River Estuary, China

Abstract:

Marine Group (MG) I and MG II Archaea were first reported over two decades ago. While significant progress has been made on MG I, the progress on MG II has been noticeable slower. The common understanding is that while MG I mainly function as chemolithoautotrophs growing on ammonia and live predominantly in deeper water, MG II live heterotrophically and reside mostly in the photic zone. While some MG I lineages that could conduct ammonium oxidation are frequently found in terrestrial environments, MG II are exclusively found in marine environments and thus named Thalassoarchaea. A few studies showed MG IIs were sporadically blooming in coastal waters and may be influenced by the level of eutrophication between seasons, which inhibited the enrichment and cultivation for MGII. In this study, we quantified the abundance of planktonic MGI (represented by archaeal amoA gene) and MGII (16S rRNA gene) using qPCR in the water column of different salinities (A: 0.8‰; B: 18.1‰; C: 23.9‰: D: 31‰) in the Pearl River Estuary over a 12-month period. The results showed that the abundance of MGII in site C (8.5±10.1×10⁷ copies/L) was significantly higher than the other three sites (A: 3.5±8.8×10⁵ copies/L; B: 2.7±4.5×10⁷ copies/L; D: 2.2±4.4×10⁷ copies/L) in all seasons, indicating the perennial blooming of MGII that might be due to the optimal combination of available organic carbon and salinity at this site. We also observed that the correlation between MGI and MGII became better toward the marine water and was significant at site D (R²: A, 0.06; B, 0.1; C, 0.24; D, 0.64), indicating the potential functional relationship between them with increasing salinity. This allowed us to hypothesize that the growth of MGI in the coastal site is more dependent on release of ammonia from organic matter degradation by MGII and other heterotrophic organisms. The Pearl River estuary may be an ideal environment for testing this hypothesis, which may provide insight into the mechanisms of carbon cycle performed by different archaea in continental margin systems.