Calorimetry for the Uncultured: Probing Microbial Activity in Situ

Committee Members

• Dr. Forest Rohwer (Chair), Biology
• Dr. Arlette Baljon, Physics
• Dr. Scott Kelley, Biology
• Dr. David Lipson, Biology
• Dr. Farooq Azam (UCSD)
• Dr. Terrance Hwa (UCSD)

Abstract

Coral reef health is deeply affected by microbial metabolism, yet in situ methods for quantifying this impact have been lacking. Microorganisms in coral reefs rapidly respond to changes in physiological factors such as temperature, nutrient levels, organic carbon availability, and ocean acidification. Traditional methods often underestimate the energy flux attributed to microbial activity by focusing solely on growth rates and specific substrate fluxes, neglecting intermediate metabolism.

To address this gap in knowledge, a custom isoperibol calorimeter was developed to measure the heat generated by bacterial communities in seawater. Microcalorimetry presents a promising solution by directly measuring the heat released through metabolic processes. 

This research examined waste heat, the lesser-explored aspect of energetics, to determine if heat signatures correlate with the microbialization state of coral reefs. Microcalorimetry was used to evaluate microbialization at various locations; data revealed that more efficient microbial communities in Caribbean samples–those that generated less power per gram of biomass–resulted from higher virus-to-microbe ratios (VMR), indicative of a healthier reef. Pacific reef samples were “viralized”, oligotrophic, and overall pristine.

Despite the lack of accumulated organic matter in these pristine sites, cellular power outpaced that found on the more degraded reefs from Curacao, due to elevated viral activity, and perhaps due to high nutrient content in the water column allowing for rapid-but-efficient turnover of organic matter. 

Caribbean sample results showed that metabolic efficiency from microbialized systems were significantly lower than more pristine environments. The increased biomass supported by power in samples with lower virus-to-microbe ratios (VMR) indicates that microbial communities relying on less efficient metabolic pathways disproportionately impact the local energy budget on degraded coral reefs. In both Caribbean and Pacific samples, microbial power was significantly correlated with viral abundance, in agreement with research showing that viral activity contributes to elevated microbial metabolism. 

Therefore, microbial communities on pristine reefs exhibit rapid turnover, freeing cellular material to maintain rapid growth, and leading to higher heat production. This finding underscores the method’s effectiveness in measuring microbialization. Moreover, this research marks the first successful in situ use of microcalorimetry to monitor microbial activity on coral reefs. The subsequent chapters will further explore the challenges and successes encountered with this approach.