Deans, F. | bioRxiv Science

bioRxiv2022

Salp blooms increase carbon export 5-fold in the Southern Ocean

The Southern Ocean (SO) contributes substantially to the global biological carbon pump (BCP). Salps in the SO, in particular Salpa thompsoni, are keystone grazers that produce large, fast-sinking fecal pellets with high export potential. In a first study of this kind, we conducted Lagrangian experiments to quantify the salp bloom impacts on export pathways by contrasting locations differing in salp bloom presence/absence. We show that blooms increased particle export by ~5-fold, and exported up to 46% of net primary production out of the euphotic zone. BCP efficiency increased from 5% in non-salp areas to up to 28% in salp areas, which is among the highest recorded in the global ocean. Using SO salp abundances from KRILLBASE, we estimate they can consume ~ 13% of regional production, mediating 13-40% of the SO BCP. Consideration in models forecasting the SO BCP is recommended considering long-term increases in SO salp abundances.

ecology↗

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↗