Wenley, J. | Search bioRxiv

bioRxiv2021

Microbial rhodopsins are increasingly favored over chlorophyll in High Nutrient Low Chlorophyll waters

Microbial rhodopsins are simple light-harvesting complexes that, unlike chlorophyll photosystems, have no iron requirements for their synthesis and phototrophic functions. Here we report the first environmental concentrations of rhodopsin along the Subtropical Frontal Zone off New Zealand, where Subtropical waters encounter the iron-limited Subantarctic High Nutrient Low Chlorophyll (HNLC) region. Rhodopsin concentrations were highest in HNLC waters where chlorophyll-a concentrations were lowest. Furthermore, while the ratio of rhodopsin to chlorophyll-a photosystems was on average 20 along the transect, this ratio increased to over 60 in HNLC waters. We further show that microbial rhodopsins are abundant in both picoplankton (0.2-3m) and in the larger (>3m) size fractions of the microbial community containing eukaryotic plankton and/or particle-attached prokaryotes. These findings suggest that rhodopsin phototrophy could be critical for microbial plankton to adapt to resource-limiting environments where photosynthesis and possibly cellular respiration are impaired. Originality-Significance statementHigh Nutrient Low Chlorophyll (HNLC) regimes cover approximately 30% of the global ocean surface and play a crucial role in the Earths carbon cycle. Here we show that microbial rhodopsins are particularly abundant in a HNLC region of the Subantarctic ocean, where chlorophyll abundance is relatively low and photosynthesis and respiration might be impaired due to iron limitation. These data suggest that rhodopsin phototrophy can contribute significantly to the energy budgets of HNLC regions, capturing meaningful amounts of light that cannot be channeled through photosynthesis.

ecology↗

bioRxiv2020

Predation impacts late but not early community assembly in model marine biofilms.

Bottom-up selection plays an important role in microbial community assembly but is unable to account for all observed variance. Other processes like top-down selection (e.g. predation) may be partially responsible for the unexplained variance. However, top-down processes often remain unexplored, especially in interaction with bottom-up selective pressures. We utilised an in situ marine biofilm model system to test the effects of bottom-up (i.e. substrate properties) and top-down (i.e. predator exclusion via 100 {micro}m mesh) selective pressures on community assembly over time (56 days). Community compositions were monitored using 16S and 18S rRNA amplicon sequencing. Wooden substrates promoted heterotrophic growth, while the inert substrates (i.e., plastic, glass, tile) lack of degradable material selected for autotrophs. Early wood communities contained 9-50% more mixotrophs and heterotrophs (e.g. Proteobacteria and Euglenozoa) compared to inert substrates. Inert substrates instead showed twice the autotrophic (e.g. Cyanobacteria and Ochrophyta) abundance. Late communities differed mainly due to exclusion status, as large predators preferably pruned heterotrophs. This resulted in the autotrophic domination of native communities, while high heterotrophic abundance characterised exclusive conditions. Top-down control through exclusion increased explainable variance by 18-53%, depending on community age, leading to increased understanding of the underlying ecological framework that guides microbial community assembly.

ecology↗