Does Symbiosis with Bacteria Influence the Transition to Multicellularity in Algae?

Abstract

The transition from unicellular to multicellular life remains one of evolution’s biggest mysteries. In this study we introduce symbioses with bacteria as a possible player. We use metagenomic analysis to investigate bacterial communities associated with Volvocine green algae, a model system for multicellularity. We compared bacterial communities of 34 algal species (21 unicellular and 13 multicellular). We developed a new workflow for metagenomic profiling (MBO100) that significantly reduces false positives produced by popular tools in the field. Our results reveal that the composition of bacterial communities is broadly similar between multicellular and unicellular species, but some bacterial families showed specificity to unicellular algae, such as Roseobacteraceae. However, many bacteria were specific to certain algae species making it challenging to separate the effect of host identity from associations with multicellularity. To solve this issue, further studies are required to assess functional interactions between bacteria and algal hosts.

Popular Abstract

Bacterial Communities in Unicellular and Multicellular Algae

Multicellular organisms contribute to the vast majority of Earth’s biomass. It is estimated that around 84% of the total 550 Gigatons of carbon on Earth (total biomass) comes from multicellular life forms: plants, animals and fungi. In addition to dominating the biosphere, the transition from unicellular to multicellular life has played a significant role in increasing organismal complexity and innovation. It is therefore not surprising that this transition is regarded by some researchers as “perhaps, the most spectacular of the major transitions during the evolutionary history of life”

Our Hypothesis

Several challenges must be overcome for a group of cells to become a multicellular organism. Examples of such challenges are maintaining cell to cell adhesion, nutrient delivery to all cells and communication between cells. It is not clear how unicellular organisms were able to come up with solutions to such challenges. As bacteria has been successful in forming symbiotic relationships with many organisms, it is conceivable that bacteria helped in making such a transition possible. In this study, bacterial communities of Volvocine Green Algae were identified and analysed. Volvocine Green Algae are a group of organisms that comprise of both unicellular and multicellular species and are considered a model system for studying multicellularity. We hypothesised that, if bacteria play a role in the evolution of multicellularity, then unicellular and multicellular algae will have different bacterial communities.

How Did We Approach the Problem?

For this study we genome sequenced 40 samples of algae from 34 species (13 multicellular and 27 unicellular). From these samples we were able to identify bacterial species associated with each sample. We started by using available tools for metagenomic analysis such as Kraken, Bracken and Metaphlan. However, we noticed that such tools produce too many false positives — that is, they detect bacterial species that are not actually present, as confirmed using test samples.. Therefore, we developed and tested a workflow where we take the overlap of the bacterial species detected by Braken and Metaphlan and applied this workflow to our samples. This workflow (MBO100), minimized the false positives by more than 77%.

Results

We examined three attributes of the bacterial communities of our algal samples: species composition, species diversity (Fig.1) and specificity. Our analyses showed that, unicellular and multicellular species have similar overall bacterial communities. Nevertheless, there were some bacterial families that were specific to unicellular algae, such as Roseobacteraceae. It was, however, challenging to separate the effect of host identity from associations with multicellularity as many bacteria were specific to particular algae species.

This work provides information on the bacteria associated with 34 algal species. Such data was not available for most of these algae as most of the studies in the literature focused on the model organism Chlamydomonas reinhardtii and few others. The Proteobacteria phylum was the most abundant among bacterial phyla in our Volvocine green algae samples making up to 95.9% of all bacteria.

Our results don’t necessarily show that bacterial symbiosis has no effect on multicellularity, but rather proposes that, if it does, it is likely to involve specific bacterial partners that require more data to reliably identify. Another way to investigate this is to look at the services provided by bacteria in different multicellular algae to see if there are common functions that bacterial partners assist with that are not present in unicellular algae. Investigations into the genes bacteria express with different hosts may also be insightful.


Master’s Degree Project in Bioinformatics, 45 credits, 2025
Department of Biology, Lund University

Advisors: Charlie Cornwalis and Homa Yazdi
Functional Ecology Division /Department of Biology

• author: Almurad, Tamim
• supervisor: Charlie Cornwallis; Homa Papoli Yazdi
• organization: Degree Projects in Bioinformatics
• course: BINP51 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9188387
• date added to LUP: 2025-05-14 14:07:31
• date last changed: 2025-05-14 14:07:31