# Autecology of an oscillating population of a novel host-associated Phaeobacter species proliferating in marine bryozoans

## TL;DR

A new species of Phaeobacter bacteria lives in marine bryozoans, producing antibacterial compounds and fluctuating in population with seasons.

## Contribution

Discovery of a novel Phaeobacter species associated with bryozoans and its stable genetic capacity to produce antibacterial compounds over 12 years.

## Key findings

- Phaeobacter sp. reaches high concentrations in bryozoans during warmer months and fluctuates seasonally.
- Seawater Phaeobacter populations likely originate from bryozoan-associated bacteria, not the reverse.
- All isolates belong to a single novel species with stable TDA biosynthesis genes over 12 years.

## Abstract

Phaeobacter are marine alphaprotebacteria capable of producing a potent antibacterial compound, tropodithietic acid. Here we demonstrate that they are part of the microbiome of marine bryozoans where they during warmer months reach 105 CFU/g. The levels exhibited a bimodal fluctuation, in both bryozoans and seawater across seasons. However, the population of Phaeobacter sp. was already established in the bryozoans prior to the peak in seawater and did not accumulate as a function of filter feeding on phytoplankton biomass, suggesting that the seawater population is likely seeded from the bryozoan-associated Phaeobacter sp. population rather than the opposite. By comparing whole-genome sequences of more than 100 bryozoan-associated Phaeobacter isolates sampled over a 12-year period, we found that all belonged to the same novel species and no systematic genetic changes occurred within it over the 12 year sampling period despite the fact that the population oscillated from below the limit of detection and across five orders of magnitude to 5.2 Log10 CFU g−1 bryozoan within individual years and hence were subject to drift. All isolates had the genetic capacity to produce tropodithietic acid (TDA) and the algicidal compounds, roseobacticides. The genes encoding the enzymes for TDA biosynthesis remained stable over time, indicating a conserved phenotype important in the ecophysiology of the bacteria. TDA biosynthetic genes were actively transcribed within the bryozoan host further corroborating the notion that the secondary metabolites of this novel host-associated Phaeobacter sp. may be central to its role within the bryozoan microbiome.