The role of kelp-bryozoan symbiosis in the carbon cycleBiogeochemical Cycling

Ethan Quah^1,2, Luka Seamus Wright³, Rosemary Cottell¹, Andrew Foggo¹

¹University of Plymouth, UK - ²Marine Biological Association, UK - ³University of Western Australia, Australia

Kelp forests play an important role in the marine carbon cycle and have been proposed to be major carbon sinks. However, these forests also provide habitat for a plethora of calcifying organisms, some as epibionts. Encrusting organisms can affect carbon budget by reducing (1) the irradiance available for photosynthesis and (2) seawater alkalinity via the formation of CaCO3. To date, productivity estimates are almost exclusively based on epibiont-free kelp and their influence on host photosynthesis is poorly understood. Laminaria digitata, a dominant northeast Atlantic kelp species, often has most of its blades colonised by the bryozoan Membranipora membranacea. Using this symbiosis as a model system, we aim to predict the effect of bryozoan cover on kelp photosynthesis by closed oxygen evolution. We found a clear negative correlation between gross photosynthesis and bryozoan cover. Instantaneous and daily net photosynthesis are negative above a bryozoan cover of 94% and 84%, marking the transition from carbon sink to source. Our results align with previous research on brown macroalgae (Fucus serratus) but disagree with some evidence from red macroalgae (Gelidium spp.). Mutualism between seaweeds and bryozoans may be restricted to macroalgae without a carbon concentrating mechanism and bryozoan cover should be incorporated into kelp forest carbon budgets.