The oligotrophic ocean is autotrophic.

Area of Science:

• Marine biology
• Oceanography
• Biogeochemistry

Background:

• Oligotrophic subtropical gyres are critical ocean regions with debated carbon cycling dynamics.
• In vitro and in situ measurements yield contradictory conclusions regarding net community production (NCP).
• In vitro observations suggest these regions are net heterotrophic, while in situ data indicate net autotrophy.

Purpose of the Study:

• To resolve the discrepancy between in vitro and in situ measurements of NCP in subtropical gyres.
• To identify potential biases in existing methodologies for assessing ocean carbon metabolism.

Main Methods:

• Evaluation of oxygen-based production estimates for potential biases.
• Analysis of (13)C enrichment in surface dissolved inorganic carbon (DIC).
• Assessment of lateral, vertical, and atmospheric organic carbon inputs.
• Comparison of observed DIC isotopic signatures with predictions from heterotrophic and autotrophic models.

Main Results:

• In situ oxygen-based estimates are unlikely to produce false-positive net autotrophy.
• Observed (13)C enrichment of DIC strongly supports net autotrophy (positive NCP).
• Organic carbon inputs (lateral, vertical, atmospheric) are insufficient to support net heterotrophy.
• Atmospheric organic matter input is too small and would cause (13)C depletion, contrary to observations.

Conclusions:

• In vitro measurements implying net heterotrophy are likely biased.
• The bias stems from an underestimation of photosynthesis and/or an overestimation of respiration in laboratory incubations.
• Subtropical gyres are likely net autotrophic, consistent with in situ observations and DIC isotopic data.