Effects of As:P Ratios on Co-cultures of a Green Alga and SAR11 Bacteria

Phosphorus (P) depletion in the surface ocean, as often found in subtropical gyres, results in microbial responses that lower P demand and elevate the expression of subcellular systems for P acquisition. An additional stress for microbes comes from naturally occurring dissolved arsenic (As) in seawater. The most stable form of dissolved As, arsenate, is a toxic analogue of phosphate that inhibits cellular metabolism. Recently it was shown that some cultures of SAR11 bacteria, the most abundant heterotrophs in the ocean, are not inhibited by high arsenate concentrations. It was demonstrated that some SAR11 are capable of utilizing methylated As, a metabolite produced by phytoplankton detoxification pathways, to meet their energy requirements. We co-cultured the green alga Dunaliella tertiolecta with the SAR11 gyre strain HTCC7211 at elevated As:P ratios to test the hypothesis that phytoplankton arsenic detoxification products can serve as a conduit that transfers energy from phytoplankton to bacteria. In co-cultures, the growth rate of HTCC7211 was 8% higher than in mono-cultures. As:P ratios of 1:1 did not suppress the growth of HTCC7211 in either mono- or co-cultures. However, the growth of D. tertiolecta was influenced by arsenate. In subtropical gyres where P-limitation and high As:P ratios are often found, they may play a role in interactions between phytoplankton and bacteria.