by Linda Wegley, a Ph.D. student in the Rohwer Lab, SDSU
No matter how remote the coral reef, sooner or later some ship is apt to go aground there, leaving a carcass of wood and steel slowly disintegrating on the sea floor. The Line Islands are no exception. Shipwrecks have been documented on Fanning, Palmyra, Starbuck, and Millennium (as well as at several atolls in the nearby Phoenix Islands). The reefs surrounding these shipwrecks decline into a ghastly scene known as a black reef. Here turf algae and cyanobacterial mats flourish and there is a dramatic loss of corals.
In 2009, we investigated a black reef on Millennium Atoll in the Southern Line Islands where an 85 foot steel tug had run aground in 1993 while towing a sailing vessel out through a narrow reef passage. Instead of the 66% live coral cover found elsewhere on Millennium, near the wreck coral cover was reduced to less than 17% of the substrate. Similar declines in coral and increases in turf algae have been observed on black reefs at Fanning and Starbuck.
How are these shipwrecks causing such devastating effects? One clue comes from the observation that the black reef on Millenium extends for about 2500 feet downcurrent from the ship debris, thus suggesting that some substance has leached from the wreck and is dispersing with the reef water. The prime suspect was iron. Iron is normally scarce in the oceans. The main source of supply is dust carried in from distant lands by the wind. The lack of iron limits the growth of some organisms and helps to maintain a healthy reef. We postulated that the iron leaching from shipwreck debris had initiated a shift from a healthy coral-dominated state to the algae-dominated state characteristic of a black reef. To test this idea, we needed to obtain several types of evidence.
First step was to determine whether iron levels are indeed higher at the black reef than at other sites on Millenium. Using a sophisticated instrument named for its energy source, an Inductively Coupled Plasma (ICP), we measured the concentration of iron present in the tissues of marine organisms collected from the black reefs and from control sites away from the wreckage. Indeed, the concentration of iron in mixed algal samples collected from reefs adjacent to the shipwreck debris was 6-times higher than in algae from the control sites.
The next question was whether this increase in iron correlated with a change in the microbial community. We were particularly interested to determine whether the black reef community contained more disease-causing pathogens. To answer this we used metagenomics. This means that we collected all the microbes from water samples at the different locations, extracted and sequenced the microbial DNA, and then analyzed the data to find out what types of microbes are present and what metabolic capabilities the community as a whole possesses. We found that the community at the algal-dominated shipwreck site differed significantly from that at nine pristine sites both in terms of the types of microbes present and the community’s metabolic potential.
Previous studies had found more pathogen-like microbes and more virulence-related genes on degraded reefs such as Christmas Island. To determine if this was also true at the black reef on Millennium, we took a closer look at our metagenomic data. The results? The microbes from the black reef had between 2 and 10 times as many virulence-related genes as did the microbes at the other sites. We made a further comparison since the competition for iron is known to play a role in many pathogen-host interactions. Interestingly, 12% of the virulence genes at the black reef are related to the use of iron, compared to 5% or less at the other sites. Combined, our results suggest that iron enrichment at a shipwreck site selects for a subset of potentially virulent microbes that could be killing the corals and/or inhibiting coral recovery.
This research is continuing during the current expedition to the Northern Line Islands. We plan to carry out some experiments at the shipwreck site on Fanning Atoll to determine whether algae can: (1) utilize iron compounds released by shipwreck debris; (2) grow faster because of the iron enrichment; and (3) kill corals. Small samples of resident coral and algae will be placed in aquaria on board the ship, along with pieces of shipwreck debris. The effect, if any, of the debris will be assessed by measuring the rate of growth of the algae and monitoring coral health over time. Antibiotics will be added to some aquaria to determine whether microbial activity is involved in any of the observed changes. Other experiments will be done on site on the black reef to determine whether algae near the shipwreck incorporate more iron than at other sites. For this, pieces of algae collected at other locations will be transplanted close to the wreckage. Five days later, the concentration of iron present in their tissues will be measured using ICP.
Our hope is that, armed with the data we have gathered from these black reefs, we can convince government agencies to take action and remove the wreckage from coral reefs. Inaction means more coral death.