NOAA Teacher at Sea
Aboard R/V Tiglax
April 26 – May 9, 2019
Mission: Northern Gulf of Alaska Long-Term Ecological Research project
Geographic Area of Cruise: Northern Gulf of Alaska – currently in transit from ‘Seward Line’ to ‘Kodiak Line’
Date: May 5, 2019
Weather Data from the Bridge
Latitude: 57o 34.6915’
Longitude: 150o 06.9789’
Wind: 18 knots, South
Seas: 4-6 feet
Air Temperature: 46oF (8oC)
Air pressure: 1004 millibars
Cloudy, light rain
Science and Technology Log
I was going to just fold the information about mixotrophs into the phytoplankton blog, but this is so interesting it deserves its own separate blog!
On land, there are plants that photosynthesize to make their own food. These are called autotrophs – self-feeding. And there are animals that feed on other organisms for food – these are called heterotrophs – other-feeding. In the ocean, the same is generally assumed. Phytoplankton, algae, and sea grasses are considered autotrophs because they photosynthesize. Zooplankton, fish, birds, marine mammals, and benthic invertebrates are considered heterotrophs because they feed on photosynthetic organisms or other heterotrophs. They cannot make their own food. But it turns out that the line between phytoplankton and zooplankton is blurry and porous. It is in this nebulous area that mixotrophs take the stage!
Mixotrophs are organisms that can both photosynthesize and feed on other organisms. There are two main strategies that lead to mixotrophy. Some organisms, such as species of dinoflagellate called Ceratium, are inherently photosynthetic. They have their own chloroplasts and use them to make sugars. But, when conditions make photosynthesis less favorable or feeding more advantageous, these Ceratium will prey on ciliates and/or bacteria. Bacteria are phosphorous, nitrogen, and iron rich so it is beneficial for Ceratium to feed on them at least occasionally. Microscopy work makes it possible to see the vacuole filled with food inside the photosynthetic Ceratium.
Other organisms, including many ciliates, were long known to be heterotrophic. They feed on other organisms, and it is particularly common for them to eat phytoplankton and especially cryptophyte algae. Recent research has revealed, however, that many ciliates will retain rather than digest the chloroplasts from the phytoplankton they’ve eaten and use them to photosynthesize for their own benefit. Viewing these mixotrophs under blue light with a microscope causes the retained chloroplasts to fluoresce. I saw photos of them and they are just packed with chloroplasts!
Mixotrophs are an important part of the Gulf of Alaska ecosystem. They may even help to explain how a modestly productive ecosystem (in terms of phytoplankton) can support highly productive upper trophic levels. Mixotrophy can increase the efficiency of energy transfer through the trophic levels, so more of the energy from primary productivity supports the growth and reproduction of upper trophic levels. They also may increase the resiliency of the ecosystem, since these organisms can adjust to variability in light, nutrients, and phytoplankton availability by focusing more on photosynthesis or more on finding prey. Yet little is known about mixotrophs. Only about one quarter of the important mixotroph species in the Gulf of Alaska have been studied in any way, shape or form!
Researchers are trying to determine what kinds of phytoplankton the mixotrophic ciliates and dinoflagellates are retaining chloroplasts from. They are also curious whether this varies by location, season or year. Understanding the conditions in which mixotrophic organisms derive energy from photosynthesis and the conditions in which they choose to feed is another area of research focus, especially because it has important ramifications for carbon and nutrient cycling and productivity across trophic levels. And it is all very fascinating!
Did you know?
Well over half of the oxygen on earth comes from photosynthetic organisms in the ocean. So next time you take a breath, remember to thank phytoplankton, algae, and marine plants!
Tonight was likely our last full night of work, as we expect rough seas and high winds will roll in around midnight tomorrow and persist until the afternoon before we head back to Seward. We were able to get bongo net sampling completed at 6 stations along the Kodiak Line, and hope that in the next two nights we can get 2-4 stations done before the weather closes in on us and 2-4 nets on the last evening as we head back to Seward.
Despite our push to get 6 stations finished tonight, we took time to look more closely at one of the samples we pulled up. It contained a squid as well as a really cool parasitic amphipod called Phronima that lives inside of a gelatinous type of zooplankton called doliolids. Check out the photos and videos below for a glimpse of these awesome creatures (I couldn’t figure out how to mute the audio, but I would recommend doing that for a less distracting video experience).