Potential overestimation of community respiration in the western Pacific boundary ocean: What causes the putative net heterotrophy in oligotrophic systems?

Abstract

Microbial metabolism is of great importance in affecting the efficiency of biological pump and global carbon cycles. However, the metabolic state of the oligotrophic ocean, the largest biome on Earth, remains contentious. We examined the planktonic and bacterial metabolism using in vitro incubations along the western Pacific boundary during September and October 2016. The integrated gross primary production (GPP) of the photic zone exhibited higher values in the region of 2°–8°N along 130°E and the western Luzon Strait, which is consistent with the regional variability of nutrients in the different ocean provinces. Spatially, the community respiration (CR) was less variable than the GPP and slightly exceeded the GPP at most of the sampling stations. Overall, the in vitro incubation results suggest a prevailing heterotrophic state in this region. A comparison of the metabolic rates from the in vitro incubations with recently published biogeochemical model results in the same region shows that our observed GPP values were close to those predicted by the model, but the measured CR was approximately 30% higher than the modeled values. We also found that most of the in vitro CR estimates were higher than the upper range of the empirical CR estimated from the sum of the contributions of the main trophic groups. Conversely, the estimates of the empirical CR support the rationality of the CR predicted by the biogeochemical model. In general, the results indicate that systematic net heterotrophy is more likely a result of the overestimation of CR measured by the light–dark bottle incubation experiments, although the exact cause of the methodological problem remains unknown.

Global Estimates of Marine Gross Primary Production Based on Machine Learning Upscaling of Field Observations

“…The bottle incubations are also prone to errors due to confinement, which might artificially modify the growth conditions (i.e. temperature, light and nutrients) and community structure in the enclosed bottle (Huang et al., 2019). For example, Westberry et al.…”

“…The bottle incubations are also prone to errors due to confinement, which might artificially modify the growth conditions (i.e. temperature, light and nutrients) and community structure in the enclosed bottle (Huang et al, 2019). For example, Westberry et al (2012) inferred that photosynthetic rates may be inhibited because the reduced turbulence in the bottle limits nutrient regeneration and supply to autotrophic organisms.…”

Dissolved Organic Matter Dynamics in the Epipelagic Northwest Pacific Low‐Latitude Western Boundary Current System: Insights From Optical Analyses

“…Thus, it has a greater opportunity to receive humic materials from the land and shelf area. These humic materials may be responsible for the higher a CDOM (325) and FDOM H in the mixed layer of NGC(U)C. The higher levels of a CDOM (254) in the mixed layer of NGC(U)C were more likely sourced from enhanced primary production (Huang et al., 2019). Although the mixed layer of NGC(U)C was characterized as nutrient‐depleted like the NEC N (Figures 2e and 2f), its slower water mixing due to lower wind shear could lead to a greater Chl a inventory in the DCM layer located above the base of the mixed layer (Figure 2c), thus contributing to an increase of phytoplankton‐sourced components (Figures 4b and 4c).…”

Dissolved and Particulate Primary Production and Subsequent Bacterial C Consumption in the Southern East China Sea

“…However, weak coupling has also been reported (e.g., Duarte et al, 2005). Recently, there have been some debates about phytoplankton-bacteria coupling and decoupling (Fouilland and Mostajir, 2010;Morán and Alonso-Sáez, 2011;Huang et al, 2019a). In the open ocean where dissolved organic carbon (DOC) is mainly autochthonous, phytoplankton and bacteria are usually coupled (Morán et al, 2002).…”