Microbial Phosphonate Utilization Within Macroalgal Holobionts and Oligotrophic Surface Water Communities

Abstract

Pelagic Sargassum, which refers to the two species S. natans and S. fluitans within this genus of marine macroalgae, proliferate in the open ocean and have reported biomass standing stocks of up to 34 million metric tons in the subtropical and equatorial Atlantic Ocean. With standard growth rates and dispersion, rafts formed by these seaweeds serve important roles related to biodiversity and carbon sequestration in the pelagic zone. Given superblooms of this macroalgae becoming more frequent in the past decade, leading to significant environmental and economic obstacles, a firm understanding of the biogeochemical mechanisms related to pelagic Sargassum growth and decay are exceedingly important.

The essential nutrient phosphorus is biologically scarce in the Sargasso Sea, yet the pelagic macroalgae Sargassum, for which this area of the North Atlantic Ocean is named, thrives. When considering environmental adaptation and the reproductive success of macrophytes, it is important to consider the entirety of the holobiont, a term referring to all associated biota with the host, including microbes and epibionts. The discovery of widespread phosphonate degradation by bacteria has evoked questions regarding the potential for activation of this pathway to alleviate phosphorus limitation in nutrient-depleted systems. This dissertation applied questions of phosphonate utilization typically isolated to studies of free-living marine bacteria to host-symbiont interactions, investigating the role of microbially-regulated nutrient acquisition via phosphonate degradation by the epiphytic microbial community of pelagic Sargassum in both supporting host health and facilitating biomass degradation.

The first chapter of this dissertation tested the hypothesis that Sargassum holobionts utilize methylphosphonate (MPn) as an alternative source of phosphorus through timeseries measurements of methane gas production as an analog for utilization activity in incubations of enclosed bottles containing both pelagic Sargassum collected from the Sargasso Sea and MPn. We observed bacterial epibionts of pelagic Sargassum actively produced methane upon MPn amendment, with indicators for a commensal relationship between biota, and that these communities were capable of MPn lysis at realistic environmental concentrations. When expanding these incubations to include a variety of other macroalgal species, we found that phosphorus limitation was a selective pressure for phosphonate utilization activity, with ubiquitous capacity observed amongst macroalgal holobionts of the oligotrophic Sargasso Sea yet not for those from coastal Californian seaweeds.

In the second chapter of this dissertation, we asked whether degradative activity of macroalgal biomass showed similar affinity for MPn consumption. Quantification of bacterial growth and community change, the conserved C-P lyase gene phnJ required for phosphonate utilization, total organic carbon (TOC), amino acid, and nutrient concentrations determined that MPn availability enhanced carbon remineralization of Sargassum-derived DOM. Proliferations of Rhodobacterales and, to a lesser extent, SAR11 were concurrent with greatest TOC drawdown and diagenetic alteration of Sargassum-derived DOM. Bacterial response by these groups correlated to high phnJ gene abundances, establishing the catabolism of MPn to liberate phosphorus as a key process involved in the remineralization of Sargassum-derived DOM, with members of the class Alphaproteobacteria playing a significant role. 

With evidence for phosphonate utilization in both commensal epibionts and degradative bacterioplankton in the surrounding waters, the final chapter of this dissertation used genomic sequencing to investigate the ecological relationships between macroalgal host and microbial epiphyte in the incubations from Chapter 1. Through this analysis, the characteristic members of the pelagic Sargassum microbiome were defined, along with preliminary profiles for other seaweeds endemic to the Sargasso Sea. Investigation into the phosphonate degradation capacity of these communities via phnJ gene prevalence quantification revealed evidence of phosphonate-degrading Rhodobacteraceae and cyanobacterial epiphytes were associated with macroalgal hosts which displayed no visible signs of senescence. Finally, comparison to decaying macroalgal samples identified a bacterial signature for macroalgal decay that crossed algal host division, notably proliferation of taxa within Desulfovibrionia, Desulfuromonadia, Clostridia, Rhodospirillales, and Vibrionaceae paired with diminished Rhodobacteraceae populations.