Adventures in the Comau Fjord: Exploring the Invisible World Beneath the Surface

ecastron@utalca.cl (Eduardo Castro) — Thu, 23 Apr 2026 00:00:00 +0000

We made this short video and illustrated poster to share our research with the general public — in Spanish, for the communities of the Patagonian coast. If you prefer a deep technical dive, see our analysis walkthrough and the GitHub repository.

A fjord full of invisible life

Nestled at 42°S in northern Chilean Patagonia, the Comau Fjord is a place of extraordinary beauty: steep walls of temperate rainforest drop directly into a narrow, 480-meter-deep channel where freshwater rivers meet the Pacific. What the eye cannot see is equally extraordinary. Every milliliter of that water contains millions of bacteria, archaea, viruses, and other microorganisms — a living community that silently regulates the carbon cycle, cycles nutrients, and shapes the health of the entire ecosystem.

These microbial communities are far more than biological background noise. They act as the ocean’s recyclers (breaking down organic matter into nutrients plants and animals can use), its thermostat (controlling greenhouse gases like methane and CO₂), and its medicine cabinet (producing compounds used in biotechnology). They are also the first to register the consequences of human activity.

Why Comau?

The Comau Fjord sits at the intersection of two worlds: a relatively pristine Patagonian wilderness and one of the most intense salmon-farming regions on Earth. Chile is the world’s second-largest producer of farmed salmon, and the coastal waters of northern Patagonia receive a disproportionate share of the antibiotics and waste that industry generates. This makes Comau an ideal natural laboratory — a gradient from undisturbed to human-impacted — to ask a fundamental question: what does industrial activity do to the invisible life that sustains the ocean?

What we did

Over three years (2016–2019), our team collected water samples at two depths — 5 m (surface) and 20 m (below the pycnocline) — across three Southern Hemisphere seasons. We used shotgun metagenomics, sequencing all the DNA in each sample without targeting any particular organism, to reconstruct 513 high-quality metagenome-assembled genomes (MAGs) from 93 individual metagenomes spanning a 35-km transect along the fjord.

We also ran a mesocosm experiment: we filled 3,500-liter tanks with fjord water and applied antibiotics at concentrations representative of salmon-farming effluent, then watched what happened to the microbial communities over 11 days.

What we found

The fjord has a microbial personality

The community composition in Comau is distinct from global ocean surveys such as the Tara Oceans project, yet it shares its dominant groups — Proteobacteria, Bacteroidetes, and Actinobacteria — with cosmopolitan marine microbes. The viral communities are equally distinctive, with up to 50% of their protein content unique to this fjord. Comau is, in short, a reservoir of untapped microbial diversity.

Season matters more than location

The strongest driver of community composition across the entire fjord is season, not geography. Samples collected in the same season from sites kilometers apart resemble each other more than samples from the same site but different seasons. pH emerged as the single most powerful continuous predictor — a chemical fingerprint of the freshwater inputs, organic matter decomposition, and biological productivity that vary seasonally.

Surface and deep communities play by different rules

At 5 m depth, microbial communities show phylogenetic overdispersion — closely related taxa avoid co-occurring, a hallmark of competitive exclusion. At 20 m, the pattern reverses: phylogenetic clustering reveals that environmental filtering selects for functionally similar organisms adapted to the colder, saltier, darker water below the pycnocline. Same fjord, same season, fundamentally different assembly mechanisms separated by just 15 meters.

Functional genes pulse with the seasons

The metabolic potential of the community follows a seasonal rhythm:

Summer: methane-oxidation genes peak, coinciding with warming surface waters.
Autumn: sulfur-cycling and vitamin B₁₂ biosynthesis genes peak, fueling the broader food web.
Winter: photosystem II genes peak alongside a conspicuous cyanobacterial bloom at 5 m depth (April–August), indicating that some phototrophs thrive under Patagonian winter light.

Antibiotic resistance is already there — and grows with pressure

Even in surface waters without direct aquaculture contact, we detected antibiotic resistance genes (ARGs) dominated by beta-lactam and tetracycline resistance, consistent with broader studies in Chilean coastal waters showing that salmon farming has already elevated ARG burdens in the regional marine environment. In our mesocosm experiment, antibiotic application triggered significant shifts in microbial diversity and community structure — and those shifts did not reverse over 11 days, suggesting that the microbial community may not quickly recover once perturbed.

This finding has direct relevance beyond ecology. Antibiotic resistance is a global public-health emergency: when resistance genes spread from environmental bacteria to human pathogens, treatments fail. Marine environments like Comau fjord act as reservoirs and conduits for that spread.

Why it matters

Patagonian fjords cover roughly 240,000 km² and are among the least-studied aquatic systems on Earth. Yet they sit downstream of rapidly expanding aquaculture, increasing maritime traffic, microplastic pollution, and climate-driven changes in rainfall and glacial melt. Our work provides the first multi-year, spatially explicit, metagenomic baseline for how microbial communities are structured in this ecosystem — and the first experimental evidence of how fragile that structure can be in the face of antibiotic pressure.

Understanding these communities is not an academic exercise. It is a prerequisite for knowing whether the ocean’s invisible workforce can continue doing its job — regulating climate, cycling nutrients, and sustaining the fisheries and coastal livelihoods that millions of people depend on.

Learn more

Video and illustrated poster (in Spanish): see the video above and download the full poster.
Technical walkthrough: step-by-step R code for all analyses → Analysis walkthrough.
Data and scripts: fully reproducible → github.com/ecastron/comau-reproducibility.

Key references

Castro-Nallar, E., Berríos-Farías, V., Díez, B., & Guajardo-Leiva, S. (2023). Spatially and temporally explicit metagenomes and metagenome-assembled genomes from the Comau Fjord (42°S), Patagonia. Microbiology Resource Announcements, 12(6), e00059-23.

Guajardo-Leiva, S., Mendez, K.N., Meneses, C., Díez, B., & Castro-Nallar, E. (2023). A first insight into the microbial and viral communities of Comau Fjord — a unique human-impacted ecosystem in Patagonia (42°S). Microorganisms, 11(4), 904.

Ortíz-Severín, J., et al. (2024). Impact of salmon farming in the antibiotic resistance and structure of marine bacterial communities from surface seawater of a northern Patagonian area of Chile. Biological Research, 57.

Buschmann, A.H., et al. (2012). Salmon aquaculture and antimicrobial resistance in the marine environment. PLoS ONE, 7(8), e42724.

Patagonia on Castro Lab