SINKING PARTICULATE ORGANIC MATTER IS A MAJOR ROUTE OF CARBON EXPORT FROM THE SUNLIT SURFACE OCEAN TO THE OCEAN'S INTERIOR. BOTH SINGLECELLED MICROBES AND ZOOPLANKTON PLAY A ROLE IN SETTING THE FATE OF PARTICLES IN SUBSURFACE MESOPELAGIC WATERS. MICROBES EXPLOIT THE RAIN OF CARBON-RICH MATERIAL AS A SUBSTRATE FOR GROWTH OFTEN ATTACHING TO SINKING PARTICLES PARTIALLY DISSOLVING THEM AND RESPIRING THE CARBON AS CO2. ALTHOUGH WE KNOW THAT PARTICLE FLUX IS MARKEDLY ATTENUATED WITH DEPTH PREDICTING THE MICROBIAL CONTRIBUTION TO THIS LOSS TERM USING SURFACE MEASUREMENTS IS CURRENTLY NOT POSSIBLE. THE GOAL OF THIS PROPOSAL IS TO DEVELOP A MECHANISTIC LINK BETWEEN MICROBIAL ATTENUATION OF SINKING PARTICLE FLUX AND SURFACE OCEAN PROPERTIES WITH THE FOLLOWING OBJECTIVES: MEASURE IN SITU THE MAGNITUDE OF MICROBIAL RESPIRATION AS A SINK FOR CARBON THROUGHOUT THE UPPER MESOPELAGIC DURING THE TWO EXPORTS FIELD CAMPAIGNS REFINE THE EXISTING CONCEPTUAL MODEL OF THE RELATIONSHIP BETWEEN SURFACE ECOSYSTEMS SUBSURFACE BIOGEOCHEMICAL CHARACTERISTICS MICROBIAL RESPIRATION AND TRANSFER EFFICIENCY OF CARBON THROUGH THE MESOPELAGIC DEVELOP A PREDICTIVE SUBSURFACE PARTICLE REMINERALIZATION MODEL THAT CAN BE INCORPORATED INTO EXPORTS DATA PRODUCTS TO ACCOMPLISH THESE OBJECTIVES WE WILL DEPLOY REPLICATED SETS OF PARTICLE CAPTURE DEVICES EQUIPPED WITH OXYGEN OPTODE-BASED RESPIRATION CHAMBERS THROUGHOUT THE MESOPELAGIC DURING THE EXPORTS FIELD CAMPAIGNS. THESE SYSTEMS KNOWN AS RESPIRE TRAPS ALLOW FOR THE IN SITU CAPTURE OF SINKING PARTICLES AND SUBSEQUENT TRACKING OF OXYGEN CONSUMPTION. WE WILL COMPARE RESPIRATION RATES TO SUBSURFACE ECOSYSTEM AND BIOGEOCHEMICAL CHARACTERISTICS INCLUDING PARTICLE SINKING RATES GEOCHEMICAL CHARACTERIZATION OF PARTICLES AND MICROBIAL COMMUNITY STRUCTURE. WE WILL FURTHER COMPARE OUR RESULTS TO REMOTELY-SENSED PROPERTIES SUCH AS NET PRIMARY PRODUCTION PHYTOPLANKTON COMMUNITY COMPOSITION AND PARTICLE SIZE SPECTRA TO DETERMINE THE MECHANISTIC BASIS FOR THE RELATIONSHIP BETWEEN SURFACE OCEAN PROPERTIES AND SUBSURFACE ACTIVITY. THIS RESEARCH WILL DETERMINE THE IMPORTANCE OF MICROBIAL PROCESSES RELATIVE TO OTHER POTENTIAL SINKS SUCH AS ZOOPLANKTON PARTICLE CONSUMPTION. THE STATED GOAL OF THE EXPORTS PROGRAM IS TO CREATE A PREDICTIVE UNDERSTANDING OF BOTH THE EXPORT OF ORGANIC CARBON FROM THE UPPER OCEAN AND ITS FATE IN THE UNDERLYING 'TWILIGHT ZONE.' HERE BY QUANTIFYING POTENTIALLY THE MOST INFLUENTIAL TRANSFORMATION PATHWAY FOR POC IN THE SUBSURFACE (MICROBIAL RESPIRATION AND HENCE REMINERALIZATION) AND COMPARING IT TO OBSERVATIONS OF THE SURFACE PHYTOPLANKTON COMMUNITY (BOTH REMOTELY-SENSED AND SHIPBOARD OBSERVATIONS) WE WILL DETERMINE WHICH SURFACE ECOLOGICAL AND BIOGEOCHEMICAL PROPERTIES ENABLE THE MOST RELIABLE PREDICTION OF MICROBIAL ATTENUATION OF PARTICLE FLUX. THIS PROPOSAL RESPONDS TO SECTION 3.1 EXPORT PROCESSES IN THE OCEAN FROM REMOTE SENSING (EXPORTS) RESEARCH FIELD INVESTIGATIONS OF THE 2106 NASA ROSES CALL FOR PROPOSALS A.3 OCEAN BIOLOGY AND BIOGEOCHEMISTRY (NNH16ZDA001N-OBB).