Abstract
Macroalgae form a diverse and ubiquitous group of photosynthetic organisms that play a key role in the structuring and function of marine communities. Through the fixation of carbon, macroalgae contribute a sizeable proportion of total coastal primary production. It is known that a considerable fraction of this production is released as dissolved organic carbon (DOC) directly into the water column. DOC is of crucial importance to the marine carbon cycle as it sustains secondary production by heterotrophic bacteria and represents the starting point for the microbial loop. This process is important as it acts to link primary production with higher order consumers in the food web. The surfaces of macroalgae contain biofilms of epibiotic bacteria that utilize macroalgae derived DOC from the algal surface. These biofilms could potentially contribute significantly to the carbon flux of marine ecosystems. The aim of this study was to quantify the level of secondary production by heterotrophic bacteria associated with macroalgal biofilms and gain a greater understanding of the interactions macroalgae have with heterotrophic bacteria. By studying the pathways and fate of macroalgae primary production we hope to better understand the ecology of the marine system as a whole. To address this, the bacterial biofilms of a variety of macroalgae species found throughout southern New Zealand were investigated. The incorporation of tritiated leucine was used to measure bacterial biomass production of their biofilm. Biolog Ecoplates were employed to further assess the bacterial biofilm communities. The results suggest that secondary production derived from macroalgae biofilms likely constitutes a significant pathway for fixed carbon within coastal systems, but that such production varies as a consequence of several key factors. Underlying host specific tissue characteristics were identified to be important in controlling the associated biofilms productivity. More specifically, productivity is linked to microbial successional factors that drive the bacterial assemblage to align with the consumption of host specific carbon products. As a whole, the contribution of macroalgae biofilms to the carbon flux of the system is tied to the overall abundance of macroalgae and is it not chiefly controlled by the productivity of biofilms specifically. The assessment of the carbon consumption profile of the giant kelp Macrocystis pyrifera biofilm provided insights into host-epiphyte interactions and elucidated factors of their greater ecology. This technique is identified to be of use in future works of algal chemical ecology. This study is the first to examine macroalgae biofilms in terms of heterotrophic biomass production across a range of species and phyla within the context of the marine ecosystem as a whole. It is also the first to detail the carbon consumption profile of a macroalgae biofilm using Biolog Ecoplates. This work extends our current knowledge on biofilm community assembly and their dynamics on living surfaces. Information garnered from this study highlights the importance heterotrophic bacteria have in the process of cycling macroalgae primary production in the marine food web.