Microbial Rhodopsins are Increasingly Favored over Chlorophyll in High Nutrient Low Chlorophyll waters

Abstract

Microbial rhodopsins are simple light-harvesting complexes that, unlike chlorophyll photosystems, have no iron requirements for their synthesis and phototrophic functions. Here we report 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 (0.2-3μm) and in the larger (>3μm) 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.

Proteorhodopsin Phototrophy in Antarctic Coastal Waters

“…The presence of the gene and protein was demonstrated through metagenomics and metaproteomics in the coastal surface seawater of Palmer Station ( 18 , 19 ). Finally, concentrations of rhodopsin-based photosystems were determined using retinal as a proxy in the subantarctic waters of the Subtropical Frontal Zone off New Zealand, showing that the abundance of rhodopsin-based photosystems was, on average, 20 times higher than that of Chl a -based photosystems ( 20 ). However, in Antarctica’s coastal waters, the relative contribution of proteorhodopsin-bearing bacteria to the microbial community remains elusive, as the effect of environmental parameters on the transcription of the proteorhodopsin gene.…”