On the use of sunlight by the ocean's most abundant inhabitants

Abstract

The two most abundant organisms in the ocean, the microbes Prochlorococcus (Pro) and SAR11, can harvest and use sunlight in a way that deviates from our conventional understanding of light use, i.e. for photosynthesis. Pro can not only photosynthesise but can also enhance the uptake of dissolved organic material using sunlight through a process called photoheterotrophy. SAR11 is also a photoheterotroph but cannot perform photosynthesis. This thesis aims to explore the importance of photoheterotrophy for Pro and SAR11 growth.
Models of individual Pro and SAR11 cells have been built and parameterised that allow for an investigation into the benefits that photoheterotrophic light use may confer to the growth of these organisms.
The results from the model suggest that the ability to partition harvested solar energy between photosynthesis and photoheterotrophy in Pro can increase growth rate by up to -50 % relative to an equivalent cell that cannot perform photoheterotrophy. Photoheterotrophy increases Pro growth rate over a broad irradiance range and allows it to grow in nitrogen limiting conditions that are characteristic of the ecosystems it dominates. SAR11 also benefits from the ability to enhance nutrient uptake through photoheterotrophy. The growth rate of SAR11 increases by up to ~20 % relative to an equivalent heterotroph, with the effect being critically dependent on ambient conditions. Photoheterotrophy also significantly influences the processing of carbon and nitrogen by Pro and SAR11.
Despite the significant increases in growth rate that photoheterotrophy provides for Pro, it is of secondary importance for growth compared to the ability to acquire carbon through photosynthesis. Although photoheterotrophy results in significant increases to SAR11 growth rate in certain conditions, the advantages that being small and irregular in shape confer on nutrient harvesting ability and growth outweigh the benefits of photoheterotrophy.
Nevertheless, the results from this thesis suggest that future ecosystem models based on systems dominated by Pro and SAR11 would benefit by including photoheterotrophy.

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