Bacterial extracellular enzyme activity in a future ocean
Authors:   , Burrell Timothy James 
Published: November 2021

<p>Heterotrophic bacteria are recognised as vital components in the cycling and regulation of inorganic and organic matter in the ocean. Research to date indicates that future changes in ocean conditions may influence bacterial extracellular enzyme hydrolysis rates, which could affect the strength of the microbial loop and consequently organic matter export. The aim of this thesis was to examine how changes in ocean acidification and warming predicted to occur by the end of the century will affect extracellular enzyme activities in the near-surface ocean and below the surface mixed layer in the South West Pacific.  A series of small-scale seawater incubations were conducted under three different perturbed conditions: elevated temperature (ambient +3°C), low pH (pCO₂ 750 ppmv; pHт 7.8) and greenhouse conditions (elevated temperature and low pH), with responses compared to ambient control samples. In particular, the response of protease activity (leucine- and arginine-aminopeptidase) and glucosidase activity (β- and α-glucosidase) were examined, as these enzymes are known to degrade the two major components of organic matter in the ocean, namely proteins and carbohydrates. Bacterial secondary production rates (³H-TdR & ³H-Leu incorporation) were also examined as a proxy for carbon turnover.  To investigate spatial variability, parameter responses from near-surface open ocean seawater consisting of different phytoplankton communities were compared with coastal seawater, as well as seawater collected from below the surface mixed layer. To determine temporal variability, both direct and indirect parameter responses were investigated. Finally, responses were determined from a shallow CO₂ vent that provided a natural low pH environment in coastal waters north of New Zealand. By comparing responses derived from vent water and artificially low pH water, vent plumes were also investigated for their utility as proxies for future low pH environments.  Incubation results showed that protease activity increased in response to low pH conditions in each seawater environment tested. However, near-surface open ocean incubations showed variability in the response of protease and glucosidase activity and bacterial cell numbers between different phytoplankton communities and treatments, suggesting that parameter responses were determined by direct and indirect effects. Elevated temperature had an overall positive effect on bacterial secondary production rates between different phytoplankton communities in the near-surface open ocean. Surprisingly, although elevated temperature and low pH treatments showed independent effects, no clear additive or synergistic effect was detected in any parameter under greenhouse conditions. In contrast to the near-surface ocean, greenhouse conditions had an additive effect on protease activity in seawater collected from below the surface mixed layer (100 m depth). Bacterial secondary production rates and bacterial numbers varied in response to elevated temperature in the subsurface ocean, while bacterial secondary production rates declined under greenhouse conditions. Glucosidase and protease activities were highest in the coastal seawater, with both enzymes responding positively to low pH conditions. Coastal seawater also contained the highest bacterial secondary production rates and bacterial cell numbers, however these parameters were not significantly affected by low pH conditions. Variation in the direct response of enzyme activity to low pH between ocean environments could indicate the synthesis of different extracellular enzymes by surface and subsurface bacteria. Importantly, results from a naturally low pH vent plume indicated that pH was not the only factor influencing the response of extracellular enzymes. Other influential factors could include high concentrations of dissolved nutrients and trace metal ions. Natural low pH vents off Whale Island in the Bay of Plenty were determined not suitable as proxies for future low pH environments based on vent variability and differences in seawater biogeochemistry when compared to the ambient ocean.  Overall, the incubation results show that under conditions predicted for the end of the century, protease activity will increase in open ocean and coastal waters which could accelerate and strengthen the heterotrophic microbial loop. Bacterial secondary production rates are expected to vary in the near-surface ocean, but decline in the subsurface. The resulting increase in surface ocean protease activity could increase heterotrophic metabolic respiration and reduce organic matter export, weaken the biological carbon pump and diminish long-term carbon sequestration. An increased turnover of proteins and amino acids in each environment tested could lead to nitrogen limitation and contribute to an expansion of oligotrophic waters. This future scenario may create a positive inorganic carbon feedback that would further exacerbate acidification of the surface ocean.</p>